Internet Draft: IMAP Extension for Conditional STORE A. Melnikov Document: draft-melnikov-imap-condstore-03.txt S. Hole Expires: March 2002 ACI WorldWide/MessagingDirect September 2001 IMAP Extension for Conditional STORE operation 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 Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet- Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society 2001. All Rights Reserved. 0.1. Open issues 1). Should conditional STORE be atomic accross message set (i.e. either all messages in message set weren't changed since and condtiontal STORE succeeds or operation fails for all messages)? This can be very expensive for some servers. 2). How specify different UNCHANGESINCE for different flags in the same STORE? Do we want such granularity anyway? 3). The document assumes that each flag has a corresponding ANNOTATE entry. This has to be synchronized with ANNOTATE draft. 4). Add support for SORT extension? 0.2. Change History Changes from -02 to -03: 1. Changed MODTIME untagged response to MODTIME response code. 2. Added MODTIME response code to the tagged OK response for SEARCH. Updated examples accordingly. 3. Changed rule for sending untagged FETCH response as a result of STORE when .SILENT prefix is used. If .SILENT prefix is used, server doesn't have to send untagged FETCH response, because MODTIME response code already contains modtime. 4. Renamed MODTIME to MODSEQ to make sure there is no confusion between mod-sequence and ACAP modtime. 5. Minor ABNF changes. 6. Minor language corrections. Changes from -01 to -02: 1. Added MODTIME data item to STATUS command. 2. Added OK untagged response to SELECT/EXAMINE. 3. Clarified that MODIFIED response code contains list of UIDs for conditional UID STORE and message set for STORE. 4. Added per-message modtime. 5. Added PERFLAGMODTIME capability. 6. Fixed several bugs in examples. 7. Added more comments to ABNF. Changes from -00 to -01: 1. Refreshed the list of Open Issues. 2. Changed "attr-name" to "entry-name", because modtime applies to entry, not attribute. 3. Added MODTIME untagged response. 4. Cleaned up ABNF. 5. Added "Acknowledgments" section. 6. Fixed some spelling mistakes. Table of Contents <<To be completed later>> 1. Abstract There is a class of applications that wish to have multiple IMAP clients coordinate activities with a single shared mailbox. These applications need some mechanism to synchronize state changes for messages within the mailbox. They must be able to guaranty that only one client can change message state (e.g. message flags or annotations) at any time. An example of such an application is use of an IMAP mailbox as a message queue with multiple dequeueing clients. The Conditional Store facility provides a protected update mechanism for message state information that can detect and resolve conflicts between multiple writers. 2. Conventions Used in This Document 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 RFC 2119 [KEYWORDS]. Formal syntax is defined using ABNF [ABNF] as modified by [IMAP4]. In examples, "C:" and "S:" indicate lines sent by the client and server respectively. The term metadata or metadata item will be used throughout this document. It references any system or user defined flag or an annotation [ANNOTATION]. 3. Introduction and Overview The Conditional STORE extension is present in any IMAP4 implementation which returns "CONDSTORE" as one of the supported capabilities in the CAPABILITY command response. Every read-write metadata of an IMAP message has an associated unsigned 64-bit value called modification sequence (mod-sequence). This is an opaque value updated by the server whenever metadata item is modified. The value is intended to be used only for comparisons within a server. However the server MUST guaranty that each STORE command (including simultaneous stores from different connections on different attributes) will use different mod-sequence values. Also, for any two successfull consequent conditional store operations performed in the same session, mod-sequence of the second MUST be greater than mod-sequence of the first. Note, that the latter rule disallows the use of system clock as mod-sequence, because if system time changes (e.g. NTP client adjusting time), next generated value can be less than previous. Mod-sequence allows the client that supports CONDSTORE extension to track whether the value of particular flag has changed since some known moment. Whenever the state of a flag change (i.e. the flag is added and before it wasn't set or the flags is removed and before it was set) the value of modification sequence for that flag MUST be updated. Adding the flag when it is already present of removing when it is not SHOULD NOT change mod-sequence. Change to any flag MUST also update per message mod-sequence. Each flag SHOULD have separate mod-sequence, for example change to \Draft flag SHOULD NOT affect mod-sequence for \Deleted flag. Server that supports per flag per message mod-sequences (i.e. satisfies the latter SHOULD) MUST also report "PERFLAGMODSEQ" in CAPABILITY command response. When message is appended to mailbox (via APPEND command or using external mechanism) the server assigns the current server modification sequence to every flag or annotation specified in the APPEND command. When an annotation is removed mod-sequence SHOULD be preserved. This extension makes the following changes to the IMAP4 protocol: a) extends syntax of the STORE command for allowing to specify STORE modifiers b) adds MODIFIED response code that should be used with NO response to STORE c) adds a new MODSEQ message data item for use in the FETCH command d) adds a new MODSEQ message data item for use in the SEARCH command e) adds a new MODSEQ response code f) adds a new OK untagged response for SELECT and EXAMINE g) adds HIGHESTMODSEQ status data item The rest of this document describes protocol changes more rigorously. 4. IMAP Protocol Changes 4.1. OK untagged response for SELECT and EXAMINE This document adds a new OK untagged response for SELECT and EXAMINE commands. A server supporting CONDSTORE extension MUST send the following OK untagged response with any successful SELECT or EXAMINE command. OK [HIGHESTMODSEQ <mod-sequence-value>] The highest mod-sequence value for any metadata item of any message in the mailbox. Disconnected client can use the value of HIGHESTMODSEQ to check if it has to refetch flags from the server. If the value stored in clients cache is less than the value returned by the server, this means that some metadata items on the server changed since last synchronization and client has to update its cache. Client MAY use SEARCH MODSEQ as described in 4.4 to find out exactly what metadata items has changed. Example: C: A142 SELECT INBOX S: * 172 EXISTS S: * 1 RECENT S: * OK [UNSEEN 12] Message 12 is first unseen S: * OK [UIDVALIDITY 3857529045] UIDs valid S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited S: * OK [HIGHESTMODSEQ 20010715194045007] S: A142 OK [READ-WRITE] SELECT completed 4.2. STORE and UID STORE Commands Arguments: message set OPTIONAL store modifiers message data item name value for message data item Responses: untagged responses: FETCH OK untagged responses: MODSEQ (See section 4.5) Result: OK - store completed NO - store error: can't store that data BAD - command unknown or arguments invalid This document extends syntax of the STORE (and UID STORE respectively) command (see section 6.4.6 of [IMAP]) to include optional STORE modifiers. The document defines the following modifier: UNCHANGEDSINCE If the mod-sequence of any metadata item specified in STORE operation for any message in the message set is greater than the unchangedsince value, then the store fails with a MODIFIED response code that includes message set (for STORE) or set of UIDs (for UID STORE) of all messages that failed UNCHANGESINCE test. Example: C: a101 STORE 7,5,9 (UNCHANGEDSINCE 20000320162338) +FLAGS.SILENT (\Deleted) S: a101 NO [MODIFIED 7,9] Conditional STORE failed In spite of failure of conditional STORE operation for message 7 server continues to perform conditional STORE to find all messages for which operation will fail. Use of UNCHANGEDSINCE with a modification sequence of 0 will always fail if the metadata item exists. Example: C: a102 STORE 12 (UNCHANGEDSINCE 0) +FLAGS.SILENT ($MDNSent) S: a102 NO [MODIFIED 12] Conditional STORE failed If operation is successful the server MUST update the mod-sequence attribute for every metadata item that was changed. Untagged FETCH response MUST be sent (unless .SILENT is specified) and it MUST include MODSEQ message data item as described in 4.3. Also server MUST send MODSEQ response code to indicate that client has received all updates to metadata items which have mod-sequence values less than or equal to the indicated mod-sequence value. Example: C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE 200012121230045) +FLAGS.SILENT (\Deleted) S: * 1 FETCH (UID 4 MODSEQ (200012121231000 "/message/flags/system/\\Deleted" 200012121231000)) S: * 2 FETCH (UID 6 MODSEQ (200012101230852 "/message/flags/system/\\Deleted" 200012101230852)) S: * 4 FETCH (UID 8 MODSEQ (200012121130956 "/message/flags/system/\\Deleted" 200012121130956)) S: a103 OK [MODSEQ 4,6,8 200012121231000] Store completed Example: C: a104 STORE * (UNCHANGEDSINCE 200012121230045) +FLAGS.SILENT (\Deleted $Processed) S: * 50 FETCH (MODSEQ (200012111230045 "/message/flags/system/\\Deleted" 200012111230045 "/message/flags/system/$Processed" 200012111230045)) S: a104 OK [MODSEQ 50 200012111230045] Store completed In the latter example UNCHANGEDSINCE value is checked against mod-sequences for both flags. Note: If the message is specified multiple times in the message set and the server doesn't internally eliminate duplicates from the message set it MUST NOT fail conditional STORE operation for the second occurrence of the message in the message set if operation completed successfully for the first occurrence. For example, if the client specifies: a100 STORE 7,3:9 (UNCHANGEDSINCE 200012121230045) +FLAGS.SILENT (\Deleted) the server must not fail operation for the message 7 as a part of processing "3:9" if it succeeded when the message 7 was processed the first time. 4.3 MODSEQ message data item in FETCH Command This extension adds an MODSEQ message data item to the FETCH command. This allows clients to retrieve mod-sequence for various metadata items for a range of messages in the currently selected mailbox. Syntax: MODSEQ <entry-names> The MODSEQ message data item, when used by the client in the FETCH command, takes a list of metadata items. For a flag <flagname> the corresponding entry-name has a form "/message/flags/system/<flagname>". Empty list requests server to return only per-message mod-sequence. MODSEQ message data item causes server to return MODSEQ fetch response data item. Syntax: MODSEQ ( <permsg-modsequence> <entry-name> <mod-sequence-value> ... ) MODSEQ response data item contains per-message mod-sequence and possible empty list of requested metadata items and their corresponding mod-sequences. Example: C: a FETCH 1 (MODSEQ ("/message/comment" "/message/flags/system/$MDNSent")) S: * 1 FETCH (MODSEQ (20000624140000 "/message/comment" 20000624140000 "/message/flags/system/$MDNSent" 20000624140000)) S: a OK Fetch complete 4.4 MODSEQ criterion in SEARCH The MODSEQ criterion for the SEARCH command allows a client to search for the specified mod-sequence of a metadata item in a message. Syntax: MODSEQ <entry-name> <mod-sequence-value> Messages that have modification counter for metadata item <entry-name> with value equal or greater than <mod-sequence-value>. This allows a client, for example, to find out which messages contain metadata items that have changed since the last time it updated its disconnected cache. If client specifies MODSEQ creterion in SEARCH command and server returns non empty SEARCH result, server MUST also return MODSEQ response code in the tagged OK response for all messages returned in untagged SEARCH. See also 4.5. Example: C: a SEARCH MODSEQ "/message/flags/system/\\draft" 20010320162338 ANNOTATION "/message/comment" "value" "IMAP4" S: * SEARCH 2 5 6 7 11 12 18 19 20 23 S: a OK [MODSEQ 2,5:7,11:12,18:20,23 20010917162338] Search complete In the above example, the message numbers of any messages containing the string "IMAP4" in the "value" attribute of the "/message/comment" entry and having mod-sequence 20010320162338 for flag \Draft are returned in the search results. Example: C: a SEARCH OR NOT MODSEQ "/message/flags/system/$MDNSent" 20010320162338 LARGER 50000 S: * SEARCH S: a OK Search complete, nothing found 4.5 MODSEQ Response code for successful FETCH, STORE and SEARCH Data: message set mod-sequence value MODSEQ response code can be sent in the following three cases: 1) Successful STORE UNCHANGEDSINCE results in MODSEQ response code being sent in tagged OK response to the client to inform of the latest mod-sequence of all metadata items specified in the STORE command. MODSEQ response code contains message set mod-sequence applies to if is caused by STORE command or UID set it is caused by UID STORE. Example: C: a103 STORE 9 (UNCHANGEDSINCE 200012121230045) +FLAGS.SILENT ($Forwarded) S: * 9 FETCH (MODSEQ (200012121231000 "/message/flags/system/$Forwarded" 200012121130046)) S: a103 OK [MODSEQ 9 200012121231000] Store completed 2) MODSEQ response code in untagged OK response MUST be sent by the server following a group of one or more unsolicited FETCH responses to indicate that the client has received all updates to metadata items which have mod-sequence values less than or equal to the indicated mod-sequence value. MODSEQ response code terminating a group of unsolicited FETCH responses always contain message set mod-sequence applies to. Example: C: a103 UID FETCH 20138:* (FLAGS INTERNALDATE) S: * 101 FETCH (UID 20140 FLAGS (\Seen \Answered $Forwarded) INTERNALDATE "28-Oct-2001 15:44:25 +0300") S: * 12 FETCH (FLAGS (\Flagged) MODSEQ (200109121231000 "/message/flags/system/\\Flagged" 200109121231000)) S: * 16 FETCH (FLAGS (\Answered \Flagged) MODSEQ (200109121231000 "/message/flags/system/\\Flagged" 200109121231000 "/message/flags/system/\\Answered" 200109121231000)) S: * OK [MODSEQ 12,16 200109121231000] S: a103 OK Store completed 3) If client specifies MODSEQ creterion in SEARCH command and server returns non empty SEARCH result, server MUST also return MODSEQ response code in the tagged OK response for all messages returned in untagged SEARCH response. MODSEQ response code contains message set mod-sequence applies to if is caused by SEARCH command or UID set it is caused by UID SEARCH. 4.6 HIGHESTMODSEQ status data items This document defines a new status data item: HIGHESTMODSEQ The highest mod-sequence value for any metadata item of any message in the mailbox. This is the same value that is returned by server in HIGHESTMODSEQ response code in Ok untagged response (See 4.1). Example: C: A042 STATUS blurdybloop (UIDNEXT MESSAGES HIGHESTMODSEQ) S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292 HIGHESTMODSEQ 200201011231777) S: A042 OK STATUS completed 5. Formal Syntax The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in [ABNF]. Non-terminals referenced but not defined below are as defined by [IMAP4]. Except as noted otherwise, all alphabetic characters are case- insensitive. The use of upper or lower case characters to define token strings is for editorial clarity only. Implementations MUST accept these strings in a case-insensitive fashion. capability =/ "CONDSTORE" / "PERFLAGMODSEQ" mailbox-data =/ "STATUS" SP mailbox SP "(" [status-att SP status-value *(SP status-att SP status-value)] ")" status-value = number / mod-sequence-value store = "STORE" SP set store-modifiers SP store-att-flags store-modifiers = [ SP "(" 1*store-modifier ")" ] store-modifier = "UNCHANGEDSINCE" SP mod-sequence-value fetch-att =/ fetch-mod-sequence ;; modifies original IMAP4 fetch-att fetch-mod-sequence = "MODSEQ" SP entry-names fetch-mod-resp = "MODSEQ" SP "(" permsg-modsequence *(entry-name SP mod-sequence-value) ")" search-key =/ search-modsequence ;; modifies original IMAP4 search-key search-modsequence = "MODSEQ" SP entry-name SP mod-sequence-value resp-text-code =/ "HIGHESTMODSEQ" SP mod-sequence-value / "MODIFIED" SP set / "MODSEQ" SP set SP mod-sequence-value ;; set of message numbers for STORE/FETCH or ;; set of UIDs for UID STORE/UID FECTH entry-names = "(" *entry-name ")" ;; empty list means that only per-message MODSEQ should be returned entry-name = '"' "/message/flags/system/" attr-flag '"' ;; each system or user defined flag <flag> is mapped to ;; "/message/flags/system/<flag>". ;; system IMAP flags must have two leading "\", ;; because "\" is an escape character. permsg-modsequence = mod-sequence-value ;; per message mod-sequence, if server supports per flag mod-sequences, ;; this is the highest mod-sequence between all metadata items mod-sequence-value = 1*DIGIT ;; Unsigned 64-bit integer (mod-sequence) ;; (0 <= n < 18,446,744,073,709,551,615) <<Borrowed from IMAP4rev1 and modified accordingly:>> attr-flag = "\\Answered" / "\\Flagged" / "\\Deleted" / "\\Seen" / "\\Draft" / attr-flag-keyword / attr-flag-extension ;; Does not include "\Recent" attr-flag-extension = "\\" atom ;; Future expansion. Client implementations ;; MUST accept flag-extension flags. Server ;; implementations MUST NOT generate ;; flag-extension flags except as defined by ;; future standard or standards-track ;; revisions of this specification. attr-flag-keyword = atom 6. Security Considerations There are no known security issues with this extension. 7. References [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd, November 1997. [IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 2060, University of Washington, December 1996. [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, Harvard University, March 1997. [ACAP] Newman, Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, Innosoft, Netscape, November 1997. <ftp://ftp.isi.edu/in-notes/rfc2244.txt> [ANNOTATION] Gellens, R., Daboo, C., "IMAP ANNOTATE Extension", work in progress. <http://www.ietf.org/internet-drafts/draft-ietf-imapext-annotate-xx.txt> [SORT-EXT] Crispin, M., "Internet Message Access Protocol -- SORT Extension", work in progress. <http://www.ietf.org/internet-drafts/draft-crispin-imapext-sort-xx.txt> 8. Acknowledgments Some text was borrowed from "IMAP ANNOTATE Extension" by Randall Gellens and Cyrus Daboo and "ACAP -- Application Configuration Access Protocol" by Chris Newman and John Myers. Many thanks to Randall Gellens for his comments on how CONDSTORE should interact with ANNOTATE extension. Authors also acknowledge the feedback provided by Cyrus Daboo and Larry Greenfield. 8. Full Copyright Statement Copyright (C) The Internet Society 2001. All Rights Reserved. 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