INTERNET DRAFT Mandatory LDAP Replica Management March 2003
Internet-Draft Ryan Moats
LDAP Duplication/Replication/Update Lemur Networks, Inc.
Protocols WG Rick Huber
Expires September 2003 AT&T Laboratories
John McMeeking
IBM
March 2003
Mandatory LDAP Replica Management
Filename: draft-ietf-ldup-mrm-02.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
The goal of standards for LDAP replication is to allow interoperable
replication among products from many different vendors. Defining the
mechanism to move data among replicas is a necessary part of this work,
but management of the replicated environment must also be standardized
for replication to be truly interoperable.
This document presents the replication management functions that must
be performed. Whenever possible, these functions are defined in terms
of existing LDAP functionality using existing LDAP operations and
existing data definitions. In some cases, changes or additions to the
existing model are required, and specifications for these changes are
included in this document.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Table of Contents
Status of this Memo...................................................1
Abstract..............................................................1
Table of Contents.....................................................2
1 Introduction .......................................................3
2 Administrative Precursors ..........................................4
3 Operational "Atoms" ................................................4
3.1 Add area of replication to a server ............................5
3.2 Delete area of replication from a server .......................5
3.3 Copy base of area of replication between servers ...............6
3.4 Create server entry for an area of replication .................6
3.5 Delete server entry from an area of replication ................6
3.6 Modify replica .................................................7
3.6.1 Change Replica Type .........................................7
3.6.2 Change Between Full/Partial Replica .........................7
3.6.3 Change Replica URI for one server for one area of replication
8
3.7 Add Replication Agreement ......................................8
3.8 Delete Replication Agreement ...................................9
3.9 Modify Replication Agreement ...................................9
3.10 Suspend Replication ..........................................9
3.11 Resume Replication ..........................................10
3.12 Trigger an Immediate Replica Cycle ..........................10
3.13 Immediately Terminate a Replica Cycle .......................11
3.14 Search with Meta-Data .......................................11
3.15 Changing Replication Meta-Data ..............................12
3.15.1 Add with Meta-Data .......................................12
3.15.2 Delete with Meta-Data ....................................13
3.15.3 Modify with Meta-Data ....................................13
3.16 Write-Unwriteable Control ...................................13
4 Common Tasks ......................................................14
4.1 Add a new replica to an existing replica group ................14
4.1.1 Large area of replication support ..........................15
4.2 Verify replication information is present between two servers .16
4.2.1 Verify that replication objects exist ......................17
4.2.2 Verify that it is valid for replication to occur between
these servers .....................................................18
4.3 Start replication between two servers .........................18
4.4 Suspend Replication activity on one area of a server ..........19
4.5 Suspend Replication activity on all areas of a server .........19
4.6 Restart Replication activity on one area of a server ..........19
4.7 Restart Replication activity on all areas of a server .........19
4.8 Halt replication ..............................................19
4.9 List status of a particular area of replication on a given
server .............................................................20
4.9.1 Local Replica On-Line ......................................20
4.9.2 Remote Replicas On-Line ....................................20
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4.9.3 Check Status of Outward Replication ........................20
4.9.4 Check Status of Incoming Replication .......................20
4.10 List all areas of replication defined on a given server and
their status .......................................................21
4.11 Restore a server and replication agreements after a server
crash 21
4.11.1 Recent Backup is Available ...............................21
4.11.2 Backup is Not Available ..................................22
4.12 Split an Area of Replication ................................22
4.13 Move an existing area of replication to a new server ........23
4.14 Join two Areas of Replication ...............................23
4.14.1 Preconditions ............................................23
4.14.2 Procedure ................................................24
4.14.3 Server requirements ......................................24
4.15 Stop Replicating an Area of Replication. ....................24
4.16 Convert a read-only replica to an updateable replica ........25
4.17 Convert an updateable replica to a read-only replica ........25
4.18 Postpone a Replica Cycle to a Later Time ....................26
4.19 Examine Replication Audit History on a Server ...............26
4.20 Compare Two Replicas on Two Servers for Differences .........26
4.21 Fix an Entry Without Triggering Replication .................27
4.22 Check Reported Schema Mismatches Discovered During Replication
27
4.23 Adding a new directory server to a replica group and
initializing the contents ..........................................28
4.24 Restore from the master failure in a single-master system ...28
5 Formal Specifications .............................................29
5.1 New/Modified Object Classes ...................................29
5.2 New/Modified Attributes .......................................29
5.3 New/Modified Extended Operations ..............................29
5.4 New/Modified Replication Primitives ...........................29
5.5 New/Modified Controls .........................................29
6 Security Considerations ...........................................30
7 Acknowledgements ..................................................30
8 References ........................................................31
Authors' Addresses:..................................................31
Full Copyright Statement.............................................32
1 Introduction
In the LDAP replication architecture [Arch], the LDAP servers and
replication agreements between them are represented by entries in the
directory tree, as part of the replicated naming context. The LDAP
replication information model [InfoMod] describes the contents of these
entries.
Replication management entries, such as replicaSubentries or
replication agreements, can be altered on any updateable replica using
standard LDAP operations [RFC2251]. These entries are explicitly
included in the directory entries governed by any agreement associated
with this area of replication. As a result, all servers with a replica
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of an area of replication will have access to information about all
other replicas of that area of replication and associated agreements.
The deployment and maintenance of a replicated directory network
involves the creation and management of the replicas themselves and
associated replication control information (e.g. replicationSubentries
and replication agreements). This document outlines the administrative
actions necessary to create and maintain replication agreements.
Typically, administrative tools will guide the administrator and
facilitate these actions.
2 Administrative Precursors
In this document the term "administrative user" refers to an identity
that will be performing replication configuration by binding to and
invoking operations on directory servers. Most LDAP server
implementations have the concept of a superuser or power user, however
this need not be the same as the administrative user, so long as the
administrative user has been granted appropriate privileges. The
administrative user MAY be running as an autonomous process, and MUST
be capable of securely maintaining its own credentials.
Deployments SHOULD create an administrative user identity that is
granted access to all servers holding a copy of a replicated area to
perform the procedures described below, in particular to read the root
DSE, the replicationContext prefix entry and all subordinate
subentries. The administrative user who will be viewing or modifying
the replication status MUST have already been provided with and
established in the directory server or servers appropriate
authentication credentials and authorization rights to retrieve
attributes and invoke DIT modification operations that are beyond the
ability of the 'average' directory user.
3 Operational "Atoms"
The following operational atoms are used to build up more complex tasks
in section 4.
Most of these operational "atoms" make the following assumptions:
Through prior LDAP or out-of-band means the administrative user MUST
have been granted the following access control permissions to the
directory in order to establish replication:
- Create the naming context prefix entry
- Create subentries immediately below the naming context prefix
entry
In several sections below, we refer to "source" and "target" servers.
The "source" server is a server that already holds a copy of the area
of replication. It may already be replicating that area with other
servers. The "target" server does not currently hold a copy of the
area of replication. The "target" is being added to the replica-group.
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The LDUP Architecture [Arch] allows for read-only replicas. Setting up
and maintaining a read-only replica will sometimes require that the
administrator create or modify data on the read-only replica. In this
case, the Write-Unwriteable control (Section 3.16) should be used.
In the remainder of the document, any numbered list of steps is
intended to be performed in the order given. Un-numbered (dash) lists
will be used where order is unimportant.
3.1 Add area of replication to a server
The client SHOULD invoke a ModifyRequest in which the object field is
the context prefix of the replication context, and the modification
list consists of a single item to add the value replicationContext to
the attribute objectClass. If the server responds with the resultCode
attributeOrValueExists, then the value is already there. If the server
responds with a resultCode other than attributeOrValueExists or
success, then this is an error. Should an error occur at this point,
the server is in an inconsistent state and needs to be fixed.
After this step is completed, the server will begin storing change
information for this area of replication.
3.2 Delete area of replication from a server
On the target server, the client SHOULD invoke a SearchRequest to find
all replicaSubentry objects which refer to the area of replication:
1. Retrieve all the values of the replicaContextRoots attribute in the
root DSE of the server. Each value is the distinguished name of
the base entry of a Replication Context held on the server.
2. Perform a one-level LDAP search in each replicaContextRoot for
objects of class replicaSubentry whose replicaURI attribute points
to the given server (e.g. use a filter of
"(&(objectclass=replicaSubentry-2)(replicaURI=ldap://thisserver))")
where "thisserver" is the fully-qualified DNS name of the given
server with the associated port if it is not port 389.
The client then examines the subtreeSpecifications to determine which
one it wants and then deletes all replicaSubentries with that
subtreeSpecification.
If any of the DelRequests fail, the area of replication has not been
completely removed.
After this step is completed, the target server will no longer
replicate this area of replication nor will it record changes for later
replication. However, the other servers in the replica group for this
area will still have a replicaSubentry for the given server, and this
should be cleaned up on all servers that hold the replica.
WG Issue: This is a reason for the subtreeSpecification issue. If it
were a DN, this would be much easier. Otherwise, we're going to need a
matching rule for subtreeSpecification.
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3.3 Copy base of area of replication between servers
In this section, the "target server" is the server on which the client
has just modified the root DSE.
The client MUST separately contact another server, one that already
holds a copy of this replication context, and issue a SearchRequest on
that server in which the baseObject is the DN of the base of the area
of replication, the scope baseObject, the filter "(objectClass=*)" and
the attributes list {"*", entryUuid}. If the client cannot obtain the
single entry at this point, the procedure will fail.
Now that it has the entry, the client SHOULD invoke an AddWithMetaData
(see 3.15.1) on the target server with entry set to the DN of the base
of the area of replication and attributes the same list as obtained in
the previous search.
If the server returns a resultCode other than success, it is an error,
and the server will be in an inconsistent state.
3.4 Create server entry for an area of replication
Each server needs to have in its copy of the area of replication a
replicaSubentry for each of the servers involved in replicating that
area before replication can be started. These entries MUST have the
following attributes:
- objectclass, with values top, Subentry and replicaSubentry
- cn
- replicaURI
- replicaType
and MAY contain other attributes, as described in the Information Model
[InfoMod].
WG Issue: This means that InfoMod needs to explicitly define
replicaOnline as DSA specific with a default value of FALSE.
3.5 Delete server entry from an area of replication
On the target server, the client SHOULD invoke a SearchRequest to find
all replicaSubentry objects which refer to the area of replication:
1. Retrieve all the values of the replicaContextRoots attribute in
the root DSE of the server. Each value is the distinguished
name of the base entry of the base entry of a Replication
Context held on the server.
2. Perform a one-level LDAP search in each replicaContextRoot for
objects of class replicaSubentry whose replicaURI attribute
points to the given server (e.g. use a filter of
"(&(objectclass=replicaSubentry-2)
(replicaURI=ldap://thisserver))") where "thisserver" is the
fully-qualified DNS name of the given server. The subentry
control MUST be included on this operation.
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The client then examines the subtreeSpecifications and the replicaURI
to determine which one(s) it wants and then deletes all
replicaSubentries with that proper subtreeSpecification and replicaURI.
If any of the DelRequests fail, the area of replication has not been
completely removed.
WG Issue: This is another reason for the subtreeSpecification issue.
If it were a DN, this would be much easier. Otherwise, we're going to
need a matching rule for subtreeSpecification.
3.6 Modify replica
3.6.1 Change Replica Type
Note: This section covers only the simple protocol operation to change
the replica type. Section 4.16 covers the full set of operations for
converting from a ReadOnly to an Updateable replica.
The client SHOULD invoke a ModifyRequest with the Write-Unwriteable
control (Section 3.16) in which the object field is the
replicationSubentry, and the modification list consists of a single
item to change the value of the attribute replicaType. If the server
responds with the resultCode attributeOrValueExists, then the value is
already there. If the server responds with a resultCode other than
attributeOrValueExists or success, then this is an error.
3.6.2 Change Between Full/Partial Replica
Note: This section currently discusses how to change between fractional
and full replication. Once the information model supports sparse
replication, it will need to be added here.
This section only discusses the simple LDAP protocol operations to
change between full and partial replicas. Additional actions are
discussed in Section 4.15.
To switch from fractional to full replication, a client SHOULD invoke a
ModifyRequest with the Write-Unwriteable control (Section 3.16) in
which the object field is the replicaSubentry, and the modification
list consists of two items: one to remove the attribute
attributeExclusionFilter and the other to remove the attribute
attributeInclusionFilter. If the server responds with a resultCode
other than noSuchAttribute or success then an error has occurred.
To switch from full to fractional replication, a client SHOULD invoke a
ModifyRequest with the Write-Unwriteable control in which the object
field is the replicaSubentry, and the modification list consists of one
or two items: one to add the attribute attributeExclusionFilter and/or
the other to add the attribute attributeInclusionFilter. If the server
responds with a resultCode other than attributeOrValueExists or
success, then an error has occurred.
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3.6.3 Change Replica URI for one server for one area of replication
Note: This section covers only the simple protocol operation to change
the replica URI. Replication will carry this change to other servers.
The client SHOULD invoke a ModifyRequest in which the object field is
the replicationSubentry, and the modification list consists of a single
item to delete the old value of the attribute replicaURI and add the
new value. If the server responds with the resultCode
attributeOrValueExists, then the value is already there. If the server
responds with a resultCode other than attributeOrValueExists or
success, then this is an error.
3.7 Add Replication Agreement
Each server that acts as a supplier in replication sessions needs to
have in its copy of the area of replication a replicaAgreementSubentry
for each server that it replicates to. ReplicaAgreementSubentry
objects are created as subordinates of the replicaSubentry representing
the supplier server. These entries MUST have the following attributes:
- objectclass, with values top, Subentry and replicaAgreementSubentry-
2, and
- cn
and MAY contain other attributes, as described in the Information Model
[InfoMod]. The cn attribute value has no significance to replication.
The cn attribute SHOULD have a value that is meaningful to the
replication administrator.
These objects MUST have the following attributes (defined as optional
in the Information Model) for replication to occur:
- replicaDN
If replicaDN is absent, no replication occurs under this agreement.
A client AddRequest creates the replicaAgreementSubentry. The entry
field of the AddRequest is the DN of the agreement (formed by using the
cn attribute as the RDN naming attribute in combination with the DN of
the parent replicaSubentry). The attributes field of the AddRequest
contains the required attributes for the agreement and any optional
attributes that are to be defined at this time.
Note: while replicationAgreementSubentries could be replicated, since
the replicationCredentialsDN are contained in the
replicationAgreementSubentry, there is a bootstrapping issue with
replicating via anonymous replication. To avoid this, clients MAY
create the replicationAgreementSubentries on all servers. If they do
this, they should use the AddWithMetaData operation (Section 3.15.1) to
ensure that all replicationAgreementSubentries have the same entryUUID.
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3.8 Delete Replication Agreement
To delete a replication agreement a LDAP client SHOULD invoke a
DeleteRequest naming the replicaAgreementSubentry to be deleted.
The termination of replication agreements should be done with caution
as it can easily result in a partition of the directory servers if
performed incorrectly.
Once all replication agreements have been terminated between a server
and others for a naming context, then that copy of the context on the
server will be divergent, and any updates made there will not be
propagated to any other server.
3.9 Modify Replication Agreement
To modify a replication agreement, the client SHOULD invoke a
ModifyRequest naming the replicaAgreementSubentry. The modification
list MAY include any of the following attributes:
- attributeExclusionFilter
- description
- replicaDN
- replicationMechanismOID
- replicationCredentialsDN
- replicationScheduleDN
No further administrative action is required for these changes to take
affect.
Changing the replicaDN attribute has the effect of ending the existing
agreement with the replica named by the old replicaDN value (if a value
was present). Similary, changing the replicationMechanismOID attribute
to specify ietf-ldup-full-update has the effect of ending incremental
replication relationship. In this respect, these changes are
equivalent to deleting a replication agreement and have the same
considerations (see section 3.8)
The replicationStatus attribute cannot be modified.
3.10 Suspend Replication
The client SHOULD invoke a ModifyRequest in which the object field is
the DN of the target replicationSubentry, and the modification list
consists of a single item to change the value of replicaOnline
attribute to false. If the server responds with the resultCode
attributeOrValueExists, then the value is already there. If the server
responds with a resultCode other than attributeOrValueExists or
success, then this is an error.
If replicaOnline is FALSE for a replicaSubentry that represents the
server containing the instance of the replicaSubentry, the server MUST
NOT initiate or accept any new incremental replication sessions. If
replicaOnline is FALSE for a replicaSubentry that represents a replica
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other than the server containing the instance of the replicaSubentry,
the server MUST NOT initiate or accept any new incremental replication
sessions with that replica.
3.11 Resume Replication
The client SHOULD invoke a ModifyRequest in which the object field is
the DN of the target replicationSubentry, and the modification list
consists of a single item to change the value of replicaOnline
attribute to true. If the server responds with the resultCode
attributeOrValueExists, then the value is already there. If the server
responds with a resultCode other than attributeOrValueExists or
success, then this is an error.
If replicaOnline is TRUE for a replicaSubentry that represents the
server containing the instance of the replicaSubentry, the server MAY
initiate or accept new incremental replication sessions. If
replicaOnline is TRUE for a replicaSubentry that represents a replica
other than the server containing the instance of the replicaSubentry,
the server MAY initiate or accept new incremental replication sessions
with that replica.
3.12 Trigger an Immediate Replica Cycle
An immediate replication cycle can be triggered using an LDAP extended
operation - "Trigger Replication". This operation takes 4 or 5
arguments:
. The distinguished name of the replicaSubentry for the target replica.
If there is no instance of this entry on the server where the
extended operation is executed, the operation is not performed and an
error is returned.
. The LDAP URL of the target server. (Note: Per [Req] M8, a single
Replication Agreement may accommodate more than one pair of servers.
Thus this argument is necessary.)
. Type of replication session to be performed (full update or
incremental update).
. Flags indicating whether the replication session is online or
offline, and if offline whether the server should create the offline
file or read the offline file. See Section 4.1.1 for more details of
offline replication.
. If the replication session is offline, the name of the file to be
used.
All other required information (connection information, authentication
information, etc.) is obtained from the Replication Agreement.
The server on which the operation is executed immediately initiates a
replica cycle with the target server. Updates are transferred in both
directions if allowed by the replication agreement.
If either the target server or the server executing the extended
operation has a currently-active replica cycle for the replication
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context specified by the extended operation, the extended operation
will fail ([Arch] Section 10).
Since replication is normally an asynchronous operation, the Trigger
Replication extended operation completes and returns status when
replication is started. It does not wait for completion of the replica
cycle and the returned status indicates whether the cycle was started
successfully, not whether it completed successfully. Further progress
of the replica cycle can be checked using other mechanisms (e.g.
Section 4.9).
The detailed syntax of the operation and associated response are
presented in Section 5.3.
3.13 Immediately Terminate a Replica Cycle
Note: this section discusses just the operation for terminating a
replica cycle. See section 4.18 for a discussion of suspending
replication or changing the replication schedule.
An in-progress replication cycle can be terminated immediately using an
LDAP extended operation - "Terminate Replication". This operation
takes 2 arguments:
1. The distinguished name of the replicaSubentry for the target
replica. If there is no instance of this entry on the server
where the extended operation is executed, the operation is not
performed and an error is returned.
2. The LDAP URL of the target server. (Note: Per [Req] M8, a single
Replication Agreement may accommodate more than one pair of
servers. Thus this argument is necessary.)
All other required information is obtained from the Replication
Agreement.
The server on which the operation is executed immediately terminates
its current replica cycle with the target server. Termination will not
interrupt transmission of a Replication Update [Arch], it will occur
only between Replication Updates.
The detailed syntax of the operation and associated response are
presented in Section 5.3.
Note: If data is queued awaiting replication between a pair of servers
and if replication is set to trigger on updates, the replication system
may automatically start a new replica cycle shortly after a cycle is
terminated. To avoid this, the replication schedule should be adjusted
to suspend replication before the Terminate Replication extended
operation is issued.
3.14 Search with Meta-Data
Many pieces of replication meta-data are used by LDUP. In some cases
(entryUUID, createdEntryCSN) they are stored as operational attributes
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and can be read using the LDAP search operation. Other meta-data
(deleted entries, CSNs related to changes of attribute values) are not
visible via normal LDAP operations but must be obtainable by
administrative users attempting to deal with replication problems (e.g.
dealing with Lost+Found entries).
"Search with Meta-Data" is an extended operation that is modeled on the
LDAP search operation [RFC2251]. There is an additional parameter that
indicates what additional data should be returned be the search. The
following values are permitted:
- deletedEntries - Any deleted entries in the scope of the search
are returned. Deleted entries can be distinguished from non-
deleted entries because the deleted entries will have a value in
their deletedEntryCSN attribute. If the deletedEntries
controlValue is given, the deletedEntryCSN attribute is
automatically added to the AttributeDescriptionList of the search.
- attributeChangeState - Each attribute value returned as part of
the search will be returned as a triple consisting of the
attribute value, the deleted flag associated with the value, and
the modificationCSN associated with the value.
The formal specification of the Search with Meta-Data extended
operation is given in Section 5.3.
Note: Search with Meta-Data is designed as an extended operation
rather than a control because the format of the returned data (sets of
triples) is different from a normal LDAP search operation. Other than
this, Search with Meta-Data operates the same as search.
3.15 Changing Replication Meta-Data
When fixing a replication problem, administrators need a way to modify
meta-data values that are normally treated as operational attributes
(createdEntryCSN, entryUUID) or as completely hidden data
(deletedEntryCSN, deleted flag, modificationCSN).
This set of extended operations mirrors the normal LDAP operations that
allow modification, but they allow modification of the associated meta-
data as well.
3.15.1 Add with Meta-Data
The Add with Meta-Data extended operation is modeled on the LDAP Add
operation [RFC2251].
The differences from the standard LDAP Add operation are:
- Each value is specified as a triple consisting of the value, the
deleted flag to be associated with that value, and the
modificationCSN to be associated with that value.
- A value may be supplied for the createdEntryCSN attribute.
- A value may be supplied for the entryUUID attribute.
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- If the Add with Meta-Data extended operation is executed on a
readonly replica it will be executed locally rather than returning
a referral to an updateable replica.
The formal specification of Add with Meta-Data is in Section 5.3.
3.15.2 Delete with Meta-Data
The Delete with Meta-Data extended operation is modeled on the LDAP
Delete operation [RFC2251]. The differences from the standard LDAP
Delete operation are:
- A deletedEntryCSN may be supplied with the Delete with Meta-Data
extended operation. In this case the delete operation is
performed as in [RFC2251], except that the value of the
deletedEntryCSN is taken from the controlValue.
- It may be flagged as a noRecordDelete. In this case the targeted
entry is deleted with no meta-data left behind (no deletedEntry or
"tombstone").
- If the Delete with Meta-Data extended operation is executed on a
readonly replica it will be executed locally rather than returning
a referral to an updateable replica.
The formal specification of the Delete with Meta-Data extended
operation is given in Section 5.3.
3.15.3 Modify with Meta-Data
The Modify with Meta-Data extended operation is modeled on the LDAP
Modify operation [RFC2251].
The differences from the standard LDAP Modify operation are:
- Each value is specified as a triple consisting of the value, the
deleted flag to be associated with that value, and the
modificationCSN to be associated with that value.
- The createdEntryCSN attribute may be modified.
- The entryUUID attribute may be modified.
- If the Modify with Meta-Data extended operation is executed on a
readonly replica it will be executed locally rather than returning
a referral to an updateable replica.
The formal specification of Modify with Meta-Data is in Section 5.3.
3.16 Write-Unwriteable Control
There are a number of attributes defined in [InfoMod] which are marked
"NO-USER-MODIFICATION". When fixing replication problems, there may be
times when these values need to be changed. In addition, when
administering readonly replicas it may be necessary for administrators
to modify values on readonly replicas. The Write-Unwriteable control
handles these cases.
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The control can be attached to an LDAP modify operation. If the
control is present, it allows the modify operation to change values
that are marked "NO-USER-MODIFICATION" (the only exception is
attributes which are locally calculated by the implementation). Also,
if the control is present, modify operations can be executed on
readonly replicas. In this case the readonly replica will not return a
referral to an updateable replica and the operation will be performed
on the readonly replica.
The formal specification of the control is given in Section 5.5.
The current list of NO-USER-MODIFICATION attributes in [InfoMod] is:
attributeExclusionFilter (Section 8.2.4)
attributeInclusionFilter (Section 8.2.5)
replicationStatus (Section 8.2.7)
replicaType (Section 8.2.8)
updateVector (Section 8.2.9)
secondsToWaitDefault (Section 8.2.18)
secondsToWait1 (Section 8.2.19)
secondsToWait2 (Section 8.2.21)
4 Common Tasks
There are many tasks that administrators need to perform in a
replicated environment. This section describes many typical tasks and
describes how they are performed in terms of the atoms defined above.
4.1 Add a new replica to an existing replica group
Assume there is an existing replica group for a given area of
replication. To add one new server (which does not already hold a copy
of the area) to the replica group, perform the following steps:
1. Copy the base entry of the area of replication from one of the
current servers to the new server (Section3.3).
2. On one of the existing servers in the replica group, create the
replicaSubentry for the new server with the replicaOnline
attribute set to "false" (Section 3.4). Replication will copy
this to all the existing replicas (trigger immediate replication
per Section 3.12 if necessary). Since replicaOnline does not
replicate, it needs to be set "false" on all existing servers.
3. On the new server, create a replicaSubentry for one of the other
servers in the replica group with the replicaOnline attribute set
to "false". The UUID of this replicaSubentry MUST be identical
to the UUID it has on the existing replicas, so Add with Meta-
Data (Section 3.15.1) should be used to create the entry.
4. If the authentication mechanism used for replication requires
credentials, make appropriate entries on all existing servers for
the new server and make appropriate entries for all the existing
servers on the new server. If any of the credential data is in
the current area of replication, use Add with Meta-Data on the
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new server and make the UUIDs the same as those on the existing
servers.
5. Create the replication agreement(s) on one of the existing
servers and let it replicate to the other existing servers
(Section 3.7).
6. Set the replicaOnline attribute to true for the new server on
both the source and target servers (Section 3.11).
7. On the source server, trigger an immediate "full update" replica
cycle (Section 3.12). The data replicated will include the
replicaSubentries and replication agreements for all other
servers in the replica group since they are in the area of
replication.
8. On the new server, set the replicaOnline attributes in the
replicaSubentries for all the other servers to "true".
9. On the servers in the replica group other than the source server,
set the replicaOnline attribute in the replicaSubentry for the
target server to "true".
4.1.1 Large area of replication support
In some cases, an area of replication is so large or available
bandwidth so small that out-of-band mechanisms (e.g. mailing a tape)
need to be used to transport the initial copy from the source to the
target. The target then needs to be updated with changes made to the
source since the copy was made.
While LDIF is typically used to transport bulk LDAP data, it is not
suitable here because it does not transport replication meta-data
(CSNs, GUIDs, etc.). To ensure that all needed data is available; bulk
replication data is stored as a stream of protocol elements from
[Proto] in network byte order. There is an LDAP extended operation
that will either cause a server to generate or read a stream of
protocol elements (see Section 3.12). Use of the extended operation to
generate or a read a file is OPTIONAL, but if the file is generated it
MUST be a stream of protocol elements.
The following restrictions are imposed on the data stream:
- The BIND operation is unauthenticated. The extended operation
that causes the destination server to read protocol elements from
a local file should be restricted to privileged users only (See
Section 6); this provides authentication for the operation.
- The "supplier initiated" protocol flow is used.
- A "full update" replication session is assumed.
- Since input is coming from a file, there are no responses. The
stream of protocol elements should assume that all response codes
are "REPL_SUCCESS".
- Since this is a "full update" session, [Proto] specifies that "the
consumer's update vector should be assumed to be set to times
'earlier than' the oldest times known to the supplier server."
Thus the protocol stream can be generated without receiving a
createGroupingResponse extended operation from the consumer.
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- The value of the groupCookie in the endGroupingRequest extended
operation is ignored.
Once the file has been loaded on the destination server, the
destination server should initiate a replication session with the
source server (Section 3.12). This will pick up changes that have
occurred since the original file was created.
In summary, to add a new server to an existing replica group for a
large replica, follow these steps (steps 1-7 are the same as steps 1-7
of Section 4.1):
1. Copy the base entry of the area of replication from one of the
current servers to the new server (Section3.3).
2. On one of the existing servers in the replica group, create the
replicaSubentry for the new server with the replicaOnline
attribute set to "false" (Section 3.4). Replication will copy
this to all the existing replicas. Since replicaOnline does not
replicate, it needs to be set "false" on all existing servers.
(Trigger immediate replication per Section 3.12 if necessary.)
3. On the new server, create a replicaSubentry for one of the other
servers in the replica group with the replicaOnline attribute set
to "false". The UUID of this replicaSubentry MUST be identical
to the UUID it has on the existing replicas, so Add with Meta-
Data (Section 3.15.1) should be used to create the entry.
4. If the authentication mechanism used for replication requires
credentials, make appropriate entries on all existing servers for
the new server and make appropriate entries for all the existing
servers on the new server. If any of the credential data is in
the current area of replication, use Add with Meta-Data on the
new server and make the UUIDs the same as those on the existing
servers.
5. Create the replication agreement(s) on one of the existing
servers and let it replicate to the other existing servers
(Section 3.7).
6. Set the replicaOnline attribute to "true" for the new server on
both the source and target servers (Section 3.11).
7. Generate an update file on the source server (Section 3.12)
8. Transport the file to the destination site by any appropriate
means (FTP, express parcel, postal mail, etc.)
9. Load the file onto the destination server (create a disk file
from tape if necessary)
10. Import the file into LDAP (Section 3.12)
11. Set the replicaOnline attribute to true for the new server
12. Trigger a replica session between the source and destination
machine to pick up changes since the file was generated (Section
3.12)
4.2 Verify replication information is present between two servers
This section describes steps that verify the proper replication
information is present for replication to occur between two replicas.
There are two parts to this process:
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1. Verify that the necessary objects have been created on both
replicas. These objects include replicaSubentry objects
representing the replicas, replication agreements describing
consumer-supplier relationships between the replicas, and the
credential and schedule objects named in the agreements.
2. Verify that it is valid to replicate between the replicas. For
example, a fractional replica cannot act as a supplier to a full
replica.
4.2.1 Verify that replication objects exist
This section describes how to verify that the necessary replication
objects exist. Within this section the term replica refers to one of
the two replicas for which information is being verified. It is
assumed that an administrator has identified the replicas (both the IP
address/host name and the replicaId), the area of replication, and
which of the replicas are expected to act as suppliers. One or both of
the replicas must be a supplier to the other replica.
1. The client SHOULD invoke an object scope search specifying the DN
of the root entry of the area of replication. This entry MUST
exist on both servers, and the objectclass attribute values MUST
include the value replicationContext.
2. On the replicas which the administrator has indicated is a
supplier, the client SHOULD invoke a baseObject scope search of
the root DSE requesting the replicationSubentries attribute. The
value for this attribute MUST name exactly one replicaSubentry
object that is a child of the root entry of the area of
replication. The distinguished name of the entry MUST be of the
form:
cn=<replicaId>,<DN of root entry of area of replication>
3. On each supplier replica, the client SHOULD invoke a baseObject
scope search specifying the DN of replicaSubentry that was
identified in the replicaSubentries attribute of the root DSE in
the previous step. This object MUST exist for replication to
occur, and MUST include the value replicaSubentry in the list of
objectclass values. The client SHOULD invoke an object scope
search for the other server. The base DN for this search is of
the form:
cn=<replicaId>,<DN of root entry of area of replication>
where <replicaId> is the replicaId of the other replica. This
object MUST exist, and MUST include the value replicaSubentry in
the list objectclass values.
4. On each supplier replica, the client SHOULD invoke a oneLevel
search, specifying the DN of replicaSubentry for that replica as
the baseObject, and a search filter like:
(&(objectclass=replicationAgreementSubentry-2)
(replicaDN=<consumer-replica-subentry-DN>))
where <consumer-replica-subentry-DN is the DN of the
replicaSubentry representing the other server, which is the
consumer in this agreement. There MUST be at least one entry in
the search results. For incremental update replication to occur at
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least one of the entries in the search results MUST have a
replicationMechanismOID attribute which is either absent or has
the value ietf-ldup-incremental-update [Proto].
5. On each supplier replica, the client SHOULD invoke a baseObject
for each object named in the replicationCredentialsDN and
replicationScheduleDN attribute values of the replicationAgreement
entries discovered in the previous step. These attributes are
optional, but if the values are present, the objects named by the
values of these attributes MUST exist.
If all the above steps are successful, the objects necessary for
replication to occur have been created.
4.2.2 Verify that it is valid for replication to occur between
these servers
For a replica to act as a supplier to another replica, the set of
entries and attributes specified in the consumer replica's fractional
entry specification MUST also be present in the supplier's fractional
entry specification. The fractional entry specification is defined by
the attributeExclusionFilter and attributeInclusionFilter attributes of
the replicaSubentry object for the replica. If these attributes are
not present the replica is a full replica.
The client SHOULD perform a baseObject scope search on the supplier
replica specifying the replicaSubentry objects for both replicas to
obtain the fractional entry specification for the replicas. The
following conditions MUST be satisfied:
1. The attributeExclusionFilter of the supplier MUST NOT contain
attributes that are not present in the attributeExclusionFilter of
the consumer. This condition is also satisfied if the
attributeExclsionFilter attribute is absent from either the
supplier replicaSubentry or both the supplier and consumer
replicaSubentry objects.
2. The attributeExclusionFilter of the supplier MUST NOT contain
attributes that are present in the attributeInclusionFilter of the
consumer. This condition is also satisfied if the
attributeExclusion filter attribute is absent from the supplier
replicaSubentry, or if the attributeInclusionFliter attribute is
absent from the consumer replicaSubentry.
3. The attributeInclusionFilter of the consumer MUST NOT contain
attributes that are not present in the attributeInclusionFilter of
the supplier. This condition is also satisfied if the
attributeInclusionFilter attribute either is absent from the
consumer replicaSubentry or is absent from both the supplier and
consumer replicaSubentry objects.
4.3 Start replication between two servers
For this operation, the client SHOULD follow the steps in Section 4.1
followed by starting replication as specified in Section 3.11.
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4.4 Suspend Replication activity on one area of a server
For this operation, the client SHOULD issue a search request to find
the replicationSubentry of the target area on the target server. Then
the client SHOULD suspend replication for this area as specified in
Section 3.10.
Note: As replicaOnline is DSA-specific, changing the value to false on
one server will not change the value on other servers. Those servers
will keep trying to initiate replication and failing. To avoid this,
replicaOnline must be set to false on all servers.
4.5 Suspend Replication activity on all areas of a server
The client SHOULD retrieve all the values of the replicaSubentries
attribute in the root DSE of the server. The client SHOULD then
suspend replication on the replicationSubentry for the target server by
following Section 3.10.
Note: As replicaOnline is DSA-specific, changing the value to false on
one server will not change the value on other servers. Those servers
will keep trying to initiate replication and failing. To avoid this,
replicaOnline must be set to false on all servers.
4.6 Restart Replication activity on one area of a server
For this operation, the client SHOULD issue a search request to find
the replicationSubentry of the target area on the target server. Then
the client SHOULD resume replication for this area as specified in
Section 3.11.
Note: if the client turned off replicaOnline on all servers in an area
of replication (as discussed in sections 4.4 and 4.5 above), the client
will need to turn on replicaOnline on all servers to resume
replication.
4.7 Restart Replication activity on all areas of a server
The client SHOULD retrieve all the values of the replicaSubentries
attribute in the root DSE of the server. Then the client SHOULD take
each area in turn and contact each server in that area's replication
group. The client SHOULD then resume replication for each
replicationSubentry for the target server by following Section 3.11.
Note: if the client turned off replicaOnline on all servers in an area
of replication (as discussed in sections 4.4 and 4.5 above), the client
will need to turn on replicaOnline on all servers to resume
replication.
4.8 Halt replication
To halt replication (i.e. terminate a current replication session and
then prevent further replication occurring), the client follows the
appropriate steps from sections 4.4 and 4.5 above, inserting the step
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of issuing a termination operation (as specified in Section 3.13) after
replicaOnline is set to FALSE. The reason for terminating after setting
replicaOnline to FALSE is to avoid a new replication session starting
immediately after the terminating operation is complete.
Note: the difference between Halt and Suspend replication is that
suspend allows a currently ongoing replication session to finish, while
halt specifically invokes the operation of 3.13 to immediately
terminate an ongoing replication session.
4.9 List status of a particular area of replication on a given server
There are many pieces of information that could be considered "status".
A number of them are listed below, and a description of how to read
them is provided.
An area of replication is defined by its replicaSubentry (the
replicaSubentry that refers to the local server in its replicaURI
attribute). The DN of this subentry will be called SDN in the rest of
this section.
4.9.1 Local Replica On-Line
To determine whether the given area of replication on the local server
is on-line, check the replicaOnline attribute of SDN.
4.9.2 Remote Replicas On-Line
To determine which other servers have the same replica on-line, do a
one-level LDAP search in the parent container of SDN. The filter
should be set to find entries of objectclass replicaSubentry-2 which
have subtreeSpecification identical to the subtreesSpecification of SDN
The replicaURI and replicaOnline attributes of the objects that match
the filter will show what other servers hold this replica and whether
they are on-line or off-line. Since replicaOnline is DSA-specific,
this search MUST be performed on each server that holds the replica.
4.9.3 Check Status of Outward Replication
If the area of replication in question uses replication agreements,
there is an optional replicationStatus attribute in the replication
agreement. If it exists, it holds a human-readable text message
containing the status of the most recent attempt at replication for the
replication agreement which contains it. To get the status for all
outgoing replication from the local server, do a one-level LDAP search
with base SDN, filter of objectclass=replicaAgreementSubentry-2, and
retrieve the replicationStatus attribute.
4.9.4 Check Status of Incoming Replication
To get the status for incoming replication to the local server, locate
the replicaSubentries for other replicas of the area of replication as
described in Section 4.9.2. Then for each replicaSubentry, do a one-
level LDAP search with base being that replicaSubentry using a filter
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of (objectclass=replicaAgreementSubentry-2), and retrieve the
replicationStatus attribute.
To be safe, replicationStatus SHOULD always be checked on the supplier
server. The copy on the consumer server may not be correct if there
was a replication problem. In the multi-master case, any server may
act as a supplier and all servers that hold the replica SHOULD be
checked.
4.10 List all areas of replication defined on a given server and their
status
To find all the areas of replication on a given server, do the
following on that server:
1. Retrieve all the values of the replicaContextRoots
attribute in the root DSE of the server. Each value is the
distinguished name of the base entry of a Replication Context
held on the server.
2. Perform a one-level LDAP search in each replicaContextRoot for
objects of class replicaSubentry whose replicaURI attribute
points to the given server (e.g. use a filter of
"(&(objectclass=replicaSubentry-
2)(replicaURI=ldap://thisserver))") where "thisserver" is the
fully-qualified DNS name of the given server.
This will provide a list of all replicas held on the given server.
Section 4.9 describes how to determine the status of each area.
4.11 Restore a server and replication agreements after a server crash
To restore a server and replication agreements after a server crash,
the following steps SHOULD be performed.
4.11.1 Recent Backup is Available
If a sufficiently recent backup is available, each area of replication
MAY be recovered by doing the following:
Note that the backup must contain UUIDs and CSNs.
1. Suspend replication to the server from all supplier servers (see
Section 3.10).
2. Restore the directory data and replication meta-data from the
backup.
3. If replication agreements to the server have been deleted,
recreate the desired replication agreements.
4. Compare the update vector for this replica, to the update vectors
for other replicas (on the servers that hold the replicas). If
the update vector for this server is greater than the minimum of
the CSNs present in the other update vectors, resume replication
to this server. No further action is necessary, as the other
servers contain sufficient replication updates to recover all
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subsequent updates via incremental replication. Otherwise,
containue to the next step.
5. Suspend replication to a replica that acts as a supplier to this
server. This is done so that no changes occur on the supplier
while recovery is in progress.
6. Compare the DITs for each area of replication on this server with
the DIT on one of the servers acting as a supplier to this server
(section 4.20). For each difference found, repair the entry
(section 4.21).
7. Set the recovered server's update vector on the recovered server
to the value of update vector for the area of replication on the
supplier server.
8. Resume replication to the supplier replica and to the recovered
server.
4.11.2 Backup is Not Available
If a sufficiently recent backup is not available, the following steps
SHOULD be performed:
1. Suspend replication to the server from all supplier servers (see
Section 3.10).
2. Clear the contents of the server. The mechanism for doing this is
implementation specific.
3. Initialize the areas of replication on the server as if adding a
new server to a replica group (section 4.1).
4. Start replication.
4.12 Split an Area of Replication
To split an area of replication, the atoms are:
1. Add the replicationContext objectclass to the root entry of the
new area of replication (Section 3.1)
2. Create replicaSubentry objects under the new area of replication
for each replica of the parent area of replication (Section 3.4)
3. Create any replication credentials objects and
replicaEventSchedule objects that will be referenced by the
replicaAgreement objects to be created in step 4.
JAM - I put the above there in case of any referential integrity
issues.
4. Create replicaAgreement objects under the new replicaSubentries,
where agreements are created that correspond to each agreement
defined for the parent (Section 3.7).
WG Issue: Schedules are referenced by DN. RVH - Can the schedules
of a different area of replication be referenced? Is there
something that says that the schedules must be IN the area of
replication they control? JAM - I think that schedules and
credentials need only be accessible to the replicas that use them.
That said, they are defined as subentries, and ought at least be
under a replicaSubentry. RDM - If this means that there are no
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reusable schedules and credentials, I think that is a Bad Thing.
TJH- Thinks nothing in infomod says that schedules are not
reusable. RVH- But infomod requires schedule to be under the
replicaSubentry, so they can't be reused because they can only be
under one replicaSubentry. Maybe we will settle this when we talk
to the infomod authors. JAM - infomod says it is thought these
objects will be placed below replicaAgreements but this is not
required. Maybe we can drop this issue? Or maybe we want infomod
to explicitly say that schedules can be placed elsewhere (so they
can be shared/reused?). Also, I think we decided that schedules
did not need to be subentries, or at least that
subtreespecification had no meaning for schedules.
5. Modify the subtreespecification of the superior replica's
replicaSubentry to exclude the new subordinate area of
replication.
JAM - Doesn't the scope of a subentry exclude subordinate
subentries governing the same kind of administrative area? So we
could remove this step?
These operations must be performed on each server containing a replica
of the parent area of replication.
WG Issue: RVH - Why? - Aren't all of the items changed WITHIN the
original area of replication? So won't they replicate as long as step
4 is done last? JAM - Once the new area of replication is defined on
one server, it quite likely defines the bounds of the superior. That
means that any further activity in the new area is replicated according
to the subentries and agreements defined for it - initially none. It
would be cleaner, in some respects, though, it would be nice if
defining the new area of replication cloned the subentries, agreements,
etc. from the superior and was replicated. RDM - If that isn't covered
in info-mod, I think it probably should be.
4.13 Move an existing area of replication to a new server
First add the area of replication to the new server as described in
Section 4.1. Then delete the area of replication from the old server
as described in Section 3.2.
Note that the "atom" in Section 3.2 will remove the old server from the
replica group but it will not delete the entries in the area of
replication from the old server. If the entries are to be deleted,
this can be done using standard LDAP operations after the old server is
removed from the replica group.
4.14 Join two Areas of Replication
This section describes how to join two areas of replication.
4.14.1 Preconditions
Before joining two areas of replication, certain preconditions need to
be satisfied:
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1. Any server that contains a replica of one area of replication must
also contain a replica of the other area of replication. This may
require copying either area of replication to additional servers,
or deleting either area of replication from servers.
2. The replicas on any given server MUST be of the same type. Both
replicas must be updateable, both-readonly, or both primary.
Furthermore, if the replicas are readonly, they must both be full
replicas, or must both be fractional replicas with identical
fractional entry specifications.
3. One area of replication must be directly subordinate to the other.
4.14.2 Procedure
1. In each of the superior area's replicaSubentries, change the
subtreespecification attribute to include the subordinate area.
2. Remove the replicationContext object class from the root of the
subordinate area (this will replicate, so it only needs to be done
on one server).
3. To clean up, remove the replicaSubentry entries (and any
subordinate replication agreements) from the subordinate area of
replication.
4.14.3 Server requirements
When the replicationContext objectclass is removed from the root of an
area of replication, the server MUST immediately treat entries within
the area of replication as belonging to the parent area of replication
(if there is any). This includes replicating any pending replication
updates (those not yet replicated to other replicas) as if they
occurred under the parent area of replication, as well as preserving
any Lost and Found entries.
The replicaSubentries have a subtreespecification attribute which
defines the "bottom" of the area of replication. At some point this
has to be changed. If the subtreespecification is changed BEFORE the
subordinate replicationContext is removed, we should be OK. Depending
on the implementation, some changes may be sent twice (once in each of
the overlapping areas of replication), but that shouldn't matter -
conflict resolution can sort things out.
If a server receives a request to delete the replicationContext from an
area of replication, and there is a parent area of replication, the
Server MUST verify that these replicas are of the same type, and if
fractional, that the fractional entry specifications are identical. If
the replicas are not of the same type, the request MUST fail with
resultCode unwillingToPerform.
4.15 Stop Replicating an Area of Replication.
This section describes how to stop replicating an area of replication.
At the end of the procedure, the subtree represented by the area of
replication will exist on one server, all replication agreements will
have been deleted, and the root of the area of replication will no
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longer be an area of replication. The server on which the subtree will
remain is referred to as the surviving replica.
To stop replicating an area of replication, a client with
administrative authority should perform the following operations:
1. Change the replica type of the non-surviving replicas to readonly
(see section 3.6.1)
2. Halt replication by changing the replicaOnline attribute of all
replicaSubentries on all servers to "false".
After halting, the client MAY optionally delete information by:
3. Delete all replication agreements (Section 3.8).
4. Delete all replicaSubentries under the area of replication
(section 3.5)
5. Issue a modifyRequest to the surviving server where the object
field is the DN of the area of replication, and the modifications
list consists of a single item, delete the attribute objectclass
with value replicationContext (Section 3.2).
4.16 Convert a read-only replica to an updateable replica
To convert a read-only replica to an updateable replica, the client
SHOULD send a single request that follows section 4.8.1 to change the
replica type in the replicaSubentry on the target server to '2'
(Updatable) using the write unwritable control (3.16).
If the read-only replica is not a full replica, this request SHOULD
also follow section 3.6.2 to make it a full replica before making it
updateable.
As noted in [InfoMod], "The consequences of having incomplete
updateable replicas are not fully understood. LDAP DSAs MAY require
updateable replicas to be complete replicas." If the DSA requires the
updatebale replica to be complete and the client sends a single request
that follows section 3.6.1 *only* and the readOnly replica is not
currently complete, the DSA may respond with an unwillingToPerform
error.
The DSA upon receiving and processing the request should trigger an
immediate replica cycle to receive necessary data to make the target
replica complete.
Note: single-master implementations may not support the concept of two
updateable masters being simultaneously active. In this case, the
client must convert the original master to read only via the following
section before converting the new master to updateable via these steps.
4.17 Convert an updateable replica to a read-only replica
To convert an updateable replica to a read-only replica, the client
SHOULD send a single request that follows section 3.6.1 to change the
replica type in the replicaSubentry to '3' (Read-Only).
The server, upon receiving and processing such a request SHOULD:
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1. Accept only LDAP search requests for that replica.
2. Finish replicating changes that had been accumulated.
4.18 Postpone a Replica Cycle to a Later Time
To temporarily halt replication to a particular server see Section 4.4.
To temporarily halt replication on all servers for a particular area of
replication see Section 4.5. To resume replication after these
temporary halts, see Section 4.6 and 4.7 respectively.
To change the schedule for scheduled replication, find the
replicaAgreementSubentry for the given replication (see Section 4.9 and
4.10). The replicationScheduleDN attribute of the
replicaAgreementSubentry contains the DN of the scheduling information.
Information on how to set the scheduling information can be found in
[Infomod], [RFC3060], and [Policy].
If a replica cycle is already in progress, it can be terminated as
described in Section 3.13.
Note that there are several events that may trigger a replica cycle,
and schedule is only one of them. If, for example, a given replication
agreement triggers replication whenever a change is made in the area of
replication, a new cycle may be triggered as soon as the current cycle
is terminated.
4.19 Examine Replication Audit History on a Server
While whether DSAs store replication audit history in the directory is
outside the scope of this document, DSAs MUST store audit history in a
file available to users of the underlying operating system (OS). A
person wanting to examine the replication audit history should make use
of the underlying OS. Whether they are required to have special
permissions is outside the scope of this document.
The reason for storing this information outside the directory is so
that the administrator may still have access to it in cases of
directory failure or inaccessibility.
4.20 Compare Two Replicas on Two Servers for Differences
It may be desirable to compare replicas of an area of replication on
two servers for differences. The process for doing this is to do a
recursive singleLevel search starting at the root entry of the area of
replication. The search SHOULD specify an attribute list that includes
the value ?*? (all non-operational attributes), as well as the
following operational attributes: entryUuid. Each search is performed
on both servers, and the results compared as follows:
1. If the DN of an entry matches the DN of a subordinate area of
replication identified in the replicationContexts root DSE
attribute for that server, exclude that entry from further
processing.
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2. Compare the set of RDNs from the search on each server to
determine if there are entries present on one server that are not
present on the other server.
3. For each entry that exists in both servers, compare the set of
attribute values returned from each server to determine if there
attribute values present in the entry on one server that are not
present in the entry on the other server.
4. For each entry that is not the root of a subordinate area of
replication, form the search and comparisons described above.
4.21 Fix an Entry Without Triggering Replication
When conflicts cause entries to be put in the Lost+Found area, the
administrator needs a mechanism to make appropriate changes. These
changes may include fixes to replication meta-data (UUIDs, CSNs, etc.)
that cannot be changed using normal LDAP operations. Once the revised
entries have been stored, any future replication operations will be
based on the modified meta-data.
The "atoms" of Section 3.14 can be used to read meta-data that is not
readable through normal LDAP operations. entryUUID and createdEntryCSN
are available as operational attributes so they can be read with a
normal LDAP "search". The atoms of Section 3.15 can be used to write
meta-data.
Typically, an administrator would resolve a replication conflict using
the following steps:
1. Read (using Section 3.14) and temporarily store all the meta-data
associated with a conflicting set of entries (where some of the
entries may be in Lost+Found)
2. Decide what the final result should be (including all associated
meta-data)
3. Depending on the complexity of the change and on whether the
entry to be changed has children:
a. Delete (using Delete with Meta-Data defined in Section 3.15.2)
the conflicting entry or entries, and
b. Build the "correct" new entry or entries (with all appropriate
meta-data) on all affected nodes using Add with Meta-Data from
Section 3.15.1; or
c. Use Modify with Meta-Data (Section 3.15.3) to make the change.
Changes may need to be made on several replicas. In all cases, care
should be taken to keep the UUIDs of the entries consistent across
replicas.
4.22 Check Reported Schema Mismatches Discovered During Replication
DSAs MAY store the result of reported schema mismatches in the
directory. They SHOULD store the schema mismatch and any resulting
action in the Audit History. The record SHOULD include the type of
mismatch (some examples may be found in [USAGE]) as well as the
resulting action: items moved to lost+found, items not added, etc.
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4.23 Adding a new directory server to a replica group and initializing
the contents
In this case, the client:
1. Copies the base entry for the area of replication from a source to
the target (section 3.3)
2. Create the entries for the new server on all servers in the replica
group (section 3.4)
3. Create the entries for the existing replica group servers on the new
(section 3.4)
4. Create the replication agreement on the new server and one other
server (section 3.7)
5. Client issues a "Initiate Full Update" request to a full replica for
the new replica -- or new replica requests consumer initiated full
update (section 3.12).
4.24 Restore from the master failure in a single-master system
To provide a fast fail-over mechanism for the failure of the master
server in a single-master system, the following steps SHOULD be
performed:
1. When the system is initially set up, at least one server SHOULD be
designated as the fail-over server. This does not reflect a
special replica type for the server, rather it reflects an
administrative decision. This server is referred to as the fail-
over replica in the following steps.
2. For each area of replication, replication agreements SHOULD be
created between the fail-over server and each of the replicas to
which the master server acts as a supplier. The fail-over server
SHOULD be defined as the supplier in these agreements. These
agreements serve two purposes: The fail-over server will not
purge replication updates until all replicas have received the
change. And the server is pre-configured to take over as master
with minimal action.
3. An agreement MAY also be created to act as a supplier to the
master. This allows the master server to be updated via
replication if the failure does not involve a loss of data on the
master server.
4. In the event of a failure of the master server, change the
replicaType of the fail-over server to updateable or primary (see
section 5.17). The fail-over server is now ready to act as a
master server.
WG Issue: The above steps imply the need to ensure that the master
replicates changes to the fail-over server before replicating a change
to other servers. Otherwise, a replica may have changes that have not
been replicated to the fail-over server. This can be accomplished by
requiring that if writable replica notes that there is only one other
replica with replication agreements defined, the supplier should
replicate to that server first. TJH- More generally, replicate first
to servers that are suppliers.
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WG Issue: Step 2 implies that a replica that acts as a supplier to no
other server need only keep sufficient state information to satisfy
idempotency and conflict resolution.
If the above steps are not taken, a full read-only replica can be
promoted to a master by following these steps:
1. Designate one replica as the new master server. This master
SHOULD be a replica that has an update vector at least as recent
as any other replica. Do not change the replicaType at this time.
2. Perform pair-wise DIT comparisons between the new master and each
other replica. Record the discrepancies and ensure that the
affected entries are removed or fixed on all servers (see section
4.21 - fix entries)
3. On the new master, change the replicaType to updateable or primary
and mark the replicaSubentry for the new master as offline. This
ensures that the server will hold client updates for future
replication but not replicate them at this time.
4. Add replication agreements, replication credentials entries, and
replication schedule entries as needed between the new master and
all other replicas.
5. On the new master, mark the replicaSubentry online. Replication
of the replication agreements and other client updates will start.
5 Formal Specifications
The Replica Management features depend heavily on defined LDAP and LDUP
structure, operations, and data formats. But some changes will be
needed to accommodate Replica Management. All these changes are pulled
together in this section for easy reference.
5.1 New/Modified Object Classes
TBD
5.2 New/Modified Attributes
TBD
5.3 New/Modified Extended Operations
Trigger Replica Operation
TBD
5.4 New/Modified Replication Primitives
TBD
5.5 New/Modified Controls
TBD
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6 Security Considerations
For security purposes it is important to be able to limit the number of
individuals with administrative access and to track the actions
performed by each administrator. Thus, servers SHOULD allow multiple
administrative users, and they SHOULD allow each administrative user to
have distinct rights. It SHOULD be possible to log all of the
administrative actions discussed in this document and the log entry
SHOULD include the identity of the administrator performing the action.
In all cases, it is assumed that the client establishes a connection to
the LDAP server and SHOULD authenticate using a recommended
authentication method [RFC2829] that establishes the identity of the
client user and SHOULD provide for connection integrity. In
deployments where the underlying network service is vulnerable to
eavesdropping and clients are intending to retrieve sensitive server
credentials, the chosen method SHOULD also provide for encryption of
data in transit.
In general, where the client is unaware of any network level protection
services, it is RECOMMENDED that the client immediately after
connection establishment invoke Start TLS to establish connection
integrity and confidentiality, and follow this by authentication by one
of:
- the "DIGEST-MD5" SASL mechanism,
- the "simple" authentication choice, or
- the "EXTERNAL" SASL mechanism if the client provided its
certificate during TLS establishment.
The client MAY determine the supported authentication mechanisms of the
server from the supportedSASLMechanisms attribute of the root DSE after
Start TLS has been invoked, and use this to decide whether to use
DIGEST-MD5 or EXTERNAL. See [RFC2830] for more information on TLS.
Some of the controls/extended operations defined in this document allow
modification of the data that controls replication document (Modify
with Meta-Data) or modification of data in the DIT (Trigger Immediate
Replica Cycle from a file). Unauthorized use of these features can
destroy a directory. Directories which support these features MUST
also provide a mechanism to restrict their use to authorized users.
7 Acknowledgements
Thanks to Mark Wahl and Ed Reed for providing a lot of the initial
text.
This document is a product of the LDUP Working Group of the IETF. The
contributions of its members are greatly appreciated.
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8 References
[Arch] J. Merrells, E. Reed, U. Srinvasan, "LDAP Replication
Architecture", draft-ietf-ldup-model-01.txt.
[InfoMod] E. Reed, "LDAP Replication Information Model", draft-ietf-
ldup-infomod-00.txt.
[Policy] J. Strassner, A. Westerinen, E. Ellesson, B. Moore, R. Moats,
"Policy Core LDAP Schema", Internet draft, draft-ietf-policy-core-
schema-13.txt, November 2001.
[Proto] E. Stokes, G. Good, R. Harrison, T. Hahn, "The LDUP Replication
Update Protocol", Internet Draft, draft-ietf-ldup-protcol-03.txt,
November 2001.
[Req] E. Stokes, R. Weiser, R. Moats, R. Huber, "LDAPv3 Replication
Requirements", Internet Draft, draft-ietf-ldup-replica-req-10.txt, July
2001.
[RFC2119] S. Bradner, "Key Words for Use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2251] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC 2251, December 1997.
[RFC2829] M. Wahl, H. Alvestrand, J. Hodges, RL Morgan, "Authentication
Methods for LDAP", RFC 2829, May 2000.
[RFC2830] J. Hodges, R. Morgan, M. Wahl, "Lightweight Directory Access
Protocol (v3): Extension for Transport Layer Security", RFC 2830, May
2000.
[RFC3060] B. Moore, E. Ellesson, J. Strassner, A. Westerinen, " Policy
Core Information Model -- Version 1 Specification", RFC 3060, February
2001.
[Usage] R. Huber, et al. "General Usage Profile for LDAPv3 Replication,"
draft-ietf-ldup-usage-profile-02, November 2001.
Authors' Addresses:
Ryan Moats
Lemur Networks, Inc.
Email: rmoats@lemurnetworks.net
Rick Huber
AT&T Laboratories
Email: rvh@att.com
John McMeeking
IBM
Email: jmcmeek@us.ibm.com
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Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Moats, et al Expires September 2003 [Page 32]
