Thing-to-Thing Research Group K. Hartke
Internet-Draft Ericsson
Intended status: Experimental October 22, 2018
Expires: April 25, 2019
Thing-to-Thing Data Hub
draft-hartke-t2trg-data-hub-02
Abstract
The Thing-to-Thing Data Hub is a RESTful, hypermedia-driven Web
application that can be used in Thing-to-Thing communications to
share data items such as thing descriptions, configurations, resource
descriptions, or firmware updates at a central location.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Data Model . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Interaction Model . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Related Work . . . . . . . . . . . . . . . . . . . . 8
A.1. CoAP Publish-Subscribe . . . . . . . . . . . . . . . . . 8
A.2. CoRE Resource Directory . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
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 cases and
don't accept any other kinds of data. Creating 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
to 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].
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Features:
o General
The data hub generalizes the concept of a directory or repository
to data items of 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].
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2. Data Model
The data model consists of three elements: the _data hub_, a
_data collection_, and a number of shared _data items_ (Figure 1).
Data Hub
+-----------------------------------------------+
| ___ |
| Data / \ |
| Collection \___/ |
| | ___ |
| |________/ \ Data Item |
| | \___/ |
| | ___ |
| |________/ \ Data Item |
| | \___/ |
| | . |
| | . |
| | . |
| | ___ |
| |________/ \ Data Item |
| \___/ |
| |
+-----------------------------------------------+
Figure 1: A Data Collection with a Number of Shared Data Items Hosted
at a Data Hub
Data Hub
A data hub is a Web application running at a Web server. It hosts
the data collection and the data items.
Data Collection
A data collection is a collection resource that contains the data
items.
Representations of data collections MUST have 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 the data items. Forms contained in the representation enable
interactions with the collection and the 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
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that references an alternate representation of the data collection
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 collection resource. This could be extended to
multiple levels in a future revision of this document.
Data Item
A data item is a resource that is a member of the data collection
resource.
Data item representations MAY have any media type. However, a
data collection MAY restrict the media types it accepts for
publication. In this case, the form in the data collection
representation for creating data items MUST list the acceptable
media types using form fields with name <urn:TBD#accept>.
The representations of the data items MAY link back to the data
collection resource using the "collection" link relation type
[RFC6573].
3. Interaction Model
The interaction model consists of eight possible interactions with a
data collection: discovering and reading the data collection, and
creating, reading, observing, updating, deleting, and finding shared
data items in the data collection.
Discovering a Data Hub
For a start, this revision of the document assumes that clients
are pre-configured with a link to a data collection at some data
hub.
Reading a Collection
A client can retrieve a representation of a data collection by
following the pre-configured link. The representation of the data
collection includes links to (and, optionally, representations of)
the data items in the data collection. The representation of the
data collection also includes forms for creating, updating,
deleting, and finding data items.
Creating an Item
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The representation of a data collection MAY contain a form with
the <urn:TBD#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 collection. The acceptable media
types are indicated by <urn:TBD#accept> form fields.
Data hubs implementing this specification MUST offer the POST
method [RFC7252] in this form. The location of the created data
item is conveyed in the 2.01 (Created) response by the Location-
Path and Location-Query options [RFC7252].
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 collection representation.
Observing an Item
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 collection representation and observing the target
resource as specified in RFC 7641 [RFC7641].
Updating an Item
For each data item in a data collection, the representation of the
data collection MAY include a nested form with the
<urn:TBD#update> form relation type. Submitting this form updates
the data item 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 collection, the representation of the
data collection MAY include a nested form with the
<urn:TBD#delete> form relation type. Submitting this form deletes
the data item from the data collection.
Data hubs implementing this specification MUST offer the DELETE
method [RFC7252] in this form.
Searching for Items
The representation of a data collection MAY contain a form with
the <urn:TBD#search> form relation type. This form can be used to
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find data items in the data collection. 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 has no IANA actions.
6. References
6.1. Normative References
[I-D.hartke-t2trg-coral]
Hartke, K., "The Constrained RESTful Application Language
(CoRAL)", draft-hartke-t2trg-coral-06 (work in progress),
October 2018.
[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>.
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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-05 (work in
progress), July 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-15 (work in progress), October 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
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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
collection resource (Figure 3). Hypermedia controls in the data
collection representation enable publishers to create, update, and
delete topics as well as subscribers to read or observe these topics.
Data Hub
+--------------------------------+
| ___ |
| / \ Data Collection |
| \___/ |
| | ___ |
| |________/ \ Topic A |
| | \___/ |
| | ___ |
| |________/ \ Topic B |
| | \___/ |
| | ___ |
| |________/ \ Topic C |
| \___/ |
| |
+--------------------------------+
Figure 3: A Data Hub Acting as a Publish-Subscribe Broker
+-------------+--------------------------+
| Interaction | Mapped to |
+-------------+--------------------------+
| DISCOVERY | Discovering a Data Hub / |
| | Reading a Collection / |
| | 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]. The links are annotated with a
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variety of link attributes providing the type of and hints about the
linked resources.
Data Hub
+-------------------------------------+
| ___ |
| / \ Data Collection |
| \___/ |
| | ___ |
| |________/ \ Data Item in | +----------------------+
| | \___/ Link Format | | ___ |
| | |___________________|____|____/ \ Resource |
| | | | | \___/ |
| | | | | ___ |
| | |___________________|____|____/ \ Resource |
| | | | \___/ |
| | | | |
| | ___ | +----------------------+
| |________/ \ Data Item in | +----------------------+
| \___/ Link Format | | ___ |
| |___________________|____|____/ \ Resource |
| | | \___/ |
| | | |
+-------------------------------------+ +----------------------+
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 a data collection
(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
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Acknowledgements
Thanks to Christian Amsuess and Jaime Jimenez for helpful comments
and discussions that have shaped the document.
Author's Address
Klaus Hartke
Ericsson
Torshamnsgatan 23
Stockholm SE-16483
Sweden
Email: klaus.hartke@ericsson.com
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