Thing-to-Thing Research Group K. Hartke
Internet-Draft Universitaet Bremen TZI
Intended status: Experimental March 2, 2018
Expires: September 3, 2018
Thing-to-Thing Data Hub
draft-hartke-t2trg-data-hub-01
Abstract
The Thing-to-Thing Data Hub is a RESTful, hypermedia-driven Web
application that can be used in Thing-to-Thing communication to share
data items such as thing descriptions, configurations, resource
descriptions, or firmware updates at a central location.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on September 3, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hartke Expires September 3, 2018 [Page 1]
Internet-Draft Thing-to-Thing Data Hub March 2018
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Data Model . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Interaction Model . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Related Work . . . . . . . . . . . . . . . . . . . . 8
A.1. CoAP Publish-Subscribe . . . . . . . . . . . . . . . . . 8
A.2. CoRE Resource Directory . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
In Thing-to-Thing communication, there is often a need to share data
items of common interest at a central location. For example, the
CoRE Resource Directory [I-D.ietf-core-resource-directory] aggregates
descriptions of resources held on other servers, which enables things
to easily discover these resources. Similarly, a W3C Web-of-Things
Thing Description Repository [WOT] stores semantic metadata of things
as well as functional descriptions of their interfaces, making this
data available to Web dashboards, commissioning tools and other
things.
As more and more thing-to-thing applications are implemented, it
becomes increasingly important to be able to share not only resource
and thing descriptions but also many other kinds of data, such as
default configurations for new devices, service locations, firmware
updates, or certificate revocation lists. The existing resource
directories and thing description repositories are not a good fit for
these kinds of data, as they're specialized to their use case and
don't accept other kinds of data. And defining a new specialized
application for each use case is not practical in the long term.
This document defines a simple "data hub" application, a RESTful Web
application with a hypermedia API that is suitable for constrained
environments and that generalizes the concept of a central repository
for sharing any kinds of data. A data hub enables clients to share
data items in any format and provides means for creating, reading,
observing, updating, deleting and finding data items at a data hub
server.
Data hubs are intended to be used primarily with CoAP [RFC7252].
Hartke Expires September 3, 2018 [Page 2]
Internet-Draft Thing-to-Thing Data Hub March 2018
Features:
o General
The data hub generalizes the concept of a directory or repository
to data items to any Internet media type. This means applications
using the data hub aren't stuck forever with the same media types
or limited to resource descriptions and thing descriptions.
o Searchable
Clients can retrieve a subset of data items from a data hub based
on item metadata.
o Observable
Data items published to a data hub are exposed as resources. As
such, they can be observed for changes [RFC7641]. This allows
clients to stay informed of information that other clients update
over time. As a result, the data hub functions similar to a CoAP
Publish-Subscribe Broker [I-D.ietf-core-coap-pubsub], although
this isn't its primary use case.
o Evolvable
The key differentiator of the data hub compared to CoRE Resource
Directory and CoAP Publish-Subscribe Broker is the evolvability --
the ability to respond effectively to the need for changes without
negatively impacting existing and new clients. Data hubs enable
fine-grained evolvability by driving all interactions by machine-
readable hypermedia elements. Features can be added, changed or
removed in a safe, backwards-compatible way simply by updating the
data hub representation to expose appropriate links and forms.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
Hartke Expires September 3, 2018 [Page 3]
Internet-Draft Thing-to-Thing Data Hub March 2018
2. Data Model
The data model consists of two elements: the _data hub_ and a number
of shared _data items_ (Figure 1).
___
Data Hub / \
\___/
| ___
|________/ \ Data Item
| item \___/
| ___
|________/ \ Data Item
| item \___/
| .
| .
| .
| ___
|________/ \ Data Item
item \___/
Figure 1: A Data Hub with a Number of Shared Data Items
Data Hub
A data hub resource is a collection of shared data items.
Data hub representations MUST be formatted in the "application/
coral+cbor" or "text/coral" media type [I-D.hartke-t2trg-coral].
They primarily consist of links to the data items using the "item"
link relation type [RFC6573]. To reduce the number of round-
trips, they MAY also embed (complete or partial) representations
of data items. Forms contained in the representation enable
interactions with the hub and data items, as described in the
following section. The representations MAY additionally contain
other links and forms that are not described in this document,
such as a link with the "alternate" link relation type that
references an alternate representation of the data hub resource.
For a start, a data hub is defined to have a depth of only one
level; i.e., all data item resources are organized directly under
the top-level data hub resource. This could be extended to
multiple levels in a future revision of this document.
Data Item
A data item is a member of the collection.
Hartke Expires September 3, 2018 [Page 4]
Internet-Draft Thing-to-Thing Data Hub March 2018
Data item representations MAY be formatted in any media type.
However, a data hub instance MAY restrict the media types it
accepts for publication. The form in the data hub representation
for creating data items MUST list the acceptable media types in
this case using form fields with name <urn:ietf:rfc:XXXX#accept>.
The representations of data items MAY link back to a data hub
resource using the "collection" link relation type [RFC6573].
3. Interaction Model
The interaction model consists of eight possible interactions with a
data hub: discovering and reading the data hub, and creating,
reading, observing, updating, deleting, and finding shared data items
in the data hub.
Discovering a Data Hub
For a start, this revision of the document assumes that clients
are pre-configured with a link to a data hub.
Reading a Data Hub
A client can retrieve a representation of a data hub by following
the pre-configured link. The representation of the data hub
includes links to (and, optionally, representations of) the data
items in the data hub. The data hub representation also includes
forms for creating, updating, deleting, and finding data items.
Creating an Item
The representation of a data hub MAY contain a form with the
<urn:ietf:rfc:XXXX#create> form relation type. Submitting this
form with a representation in one of the acceptable media types
creates a new data item in the data hub. The acceptable media
types are indicated by <urn:ietf:rfc:XXXX#accept> form fields.
Data hubs implementing this specification MUST offer the POST
method [RFC7252] in this form.
Reading an Item
A client can retrieve a representation of a data item by following
a link with the <http://www.iana.org/assignments/relation/item>
link relation type in the data hub representation.
Observing an Item
Hartke Expires September 3, 2018 [Page 5]
Internet-Draft Thing-to-Thing Data Hub March 2018
A client can observe a data item by following a link with the
<http://www.iana.org/assignments/relation/item> link relation type
in the data hub representation and observing the target resource
as specified in RFC 7641 [RFC7641].
Updating an Item
For each data item in a data hub, the representation of the data
hub MAY include a nested form with the <urn:ietf:rfc:XXXX#update>
form relation type. Submitting this form updates the data item in
the data hub to the submitted representation.
Data hubs implementing this specification MUST offer the PUT
method [RFC7252] in this form.
Deleting an Item
For each data item in a data hub, the representation of the data
hub MAY include a nested form with the <urn:ietf:rfc:XXXX#delete>
form relation type. Submitting this form deletes the data item
from the data hub.
Data hubs implementing this specification MUST offer the DELETE
method [RFC7252] in this form.
Searching for Items
The representation of a data hub MAY contain a form with the
<urn:ietf:rfc:XXXX#search> form relation type. This form can be
used to find data items in the data hub. Submitting this form
with a search query returns the subset of data items that match
the query.
Data hubs implementing this specification MUST offer the FETCH
method [RFC8132] in this form.
4. Security Considerations
TODO.
5. IANA Considerations
This document includes no request to IANA.
Hartke Expires September 3, 2018 [Page 6]
Internet-Draft Thing-to-Thing Data Hub March 2018
6. References
6.1. Normative References
[I-D.hartke-t2trg-coral]
Hartke, K., "The Constrained RESTful Application Language
(CoRAL)", draft-hartke-t2trg-coral-04 (work in progress),
October 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6573] Amundsen, M., "The Item and Collection Link Relations",
RFC 6573, DOI 10.17487/RFC6573, April 2012,
<https://www.rfc-editor.org/info/rfc6573>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
6.2. Informative References
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-pubsub-03 (work in
progress), February 2018.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-13 (work in progress), March 2018.
Hartke Expires September 3, 2018 [Page 7]
Internet-Draft Thing-to-Thing Data Hub March 2018
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[WOT] Kovatsch, M., Ed. and D. Peintner, Ed., "WoT Current
Practices", February 2018, <http://w3c.github.io/wot/
current-practices/wot-practices.html>.
Appendix A. Related Work
The data hub is an instance of the well-known collection pattern. As
such, it might be used in places where a more specialized instance of
the collection pattern is currently used, such as the CoAP Publish-
Subscribe Broker [I-D.ietf-core-coap-pubsub] or the CoRE Resource
Directory [I-D.ietf-core-resource-directory]. This section shows how
these two applications might be implemented with a data hub (without
trying to replicate all of their features in detail).
A.1. CoAP Publish-Subscribe
CoAP Publish-Subscribe [I-D.ietf-core-coap-pubsub] provides means for
resource-constrained sensor and actuator nodes to publish and receive
data without having to be available at the same time. The basic
operation involves clients called "publishers" updating "topic"
resources at a server called the "broker" and clients called
"subscribers" observing these resources (Figure 2).
____________ ____________ ____________
| |--------->| | | |
| Publisher |<---------| Broker | | Subscriber |
| (Client) | | (Server) |<---------| (Client) |
|____________| |____________|--------->|____________|
Figure 2: CoAP Publish-Subscribe
A broker might be implemented as a data hub by creating the topics as
resources on the data hub server and linking to these from the data
hub resource (Figure 3). Hypermedia controls in the data hub
representation enable publishers to create, update, and delete topics
as well as subscribers to read or observe these topics.
Hartke Expires September 3, 2018 [Page 8]
Internet-Draft Thing-to-Thing Data Hub March 2018
+----------------------------+
| ___ |
| / \ Data Hub |
| \___/ |
| | ___ |
| |________/ \ Topic A |
| | item \___/ |
| | ___ |
| |________/ \ Topic B |
| | item \___/ |
| | ___ |
| |________/ \ Topic C |
| item \___/ |
| |
+----------------------------+
Figure 3: A Data Hub Acting as a Publish-Subscribe Broker
+-------------+--------------------------+
| Interaction | Mapped to |
+-------------+--------------------------+
| DISCOVERY | Discovering a Data Hub / |
| | Reading a Data Hub / |
| | Searching for Items |
| CREATE | Creating an Item |
| PUBLISH | Updating an Item |
| SUBSCRIBE | Observing an Item |
| UNSUBSCRIBE | Observing an Item |
| READ | Reading an Item |
| REMOVE | Deleting an Item |
+-------------+--------------------------+
Table 1: Mapping of Pub/Sub Interactions to Data Hub
A.2. CoRE Resource Directory
A CoRE Resource Directory [I-D.ietf-core-resource-directory] hosts
descriptions of resources held on other servers, allowing lookups to
be performed for those descriptions. The descriptions are encoded as
links in CoRE Link Format [RFC6690] annotated with a variety of link
attributes providing the type of and hints about the linked
resources.
Hartke Expires September 3, 2018 [Page 9]
Internet-Draft Thing-to-Thing Data Hub March 2018
+---------------------------------+
| ___ |
| / \ Data Hub |
| \___/ |
| | ___ |
| |________/ \ Data Item in | +------------------+
| | item \___/ Link Format | | ___ |
| | |_________________|____|__/ \ Resource |
| | | hosts | | \___/ |
| | | | | ___ |
| | |_________________|____|__/ \ Resource |
| | hosts | | \___/ |
| | | | |
| | ___ | +------------------+
| |________/ \ Data Item in | +------------------+
| item \___/ Link Format | | ___ |
| |_________________|____|__/ \ Resource |
| hosts | | \___/ |
| | | |
+---------------------------------+ +------------------+
Figure 4: A Data Hub Storing Link Format Items
A data hub might be used to store these resource descriptions. Each
resource description becomes a data item in the data hub (Figure 4).
A specialized interface for querying the cumulative set of stored
links might be provided separately.
+-----------------------+------------------------+
| Interaction | Mapped to |
+-----------------------+------------------------+
| Discovery | Discovering a Data Hub |
| Registration | Creating an Item |
| Registration Update | - |
| Registration Removal | Deleting an Item |
| Read Endpoint Links | Reading an Item |
| Update Endpoint Links | Updating an Item |
+-----------------------+------------------------+
Table 2: Mapping of Resource Directory Interactions to Data Hub
Hartke Expires September 3, 2018 [Page 10]
Internet-Draft Thing-to-Thing Data Hub March 2018
Author's Address
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Email: hartke@tzi.org
Hartke Expires September 3, 2018 [Page 11]