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Under construction, please come back soon! |
JMS-based messaging for the Fedora Commons repository which is based on sending messages notifying consumers that content items within a Fedora Repository have been modified, identifying with URIs the item that has changed and the location of the modified content.
Important Note: This is pre-alpha code published to encourage discussion on this topic to work towards a consensual agreement on a generic messaging and Fedora implementation pattern suitable for all kinds of potential consumers of Fedora messages. Particularly see the list of known issues.
Rationale
Fedora includes JMS-based messaging capabilities. Messages are dispatched on every Fedora API-M method invocation, and contain details of the method name and parameters. Messages are therefore closely coupled to Fedora's API-M.
There exists a class of potential consumers of Fedora messages with responsibilities for indexing or otherwise manipulating and persisting the content within Fedora. These clients in general are interested in knowing when content within Fedora changes, and what those changes are.
These consumers currently have to "decode" the API-M messages received via JMS to identify what the message means to them in terms of modifications to the content within Fedora.
Hence Fedora Content Change Messaging. The messages are designed to be deliberately agnostic to Fedora's API, and instead represent, using a generic message format (encapsulated within AtomPub), additions, deletions and modifications to content within Fedora; identifying the item using an HTTP URI, and providing a "callback" HTTP URI from which the new or modified content can be accessed.
Installing and using
The source code is hosted on Github. See the README file for instructions on installing and using.
Consumers of messages can make use of the following classes:
net.acuityunlimited.fedora.messaging.AtomContentChangeMessage which can be used to deserialize the AtomPub message to net.acuityunlimited.gen.messaging.ContentObjectModification.
Code overview
This is a module that utilises Fedora's decorator pattern, in the same way as the current JMS messaging module. It can either be used alongside the existing messaging plug-in, or as an alternative to it.
API-M methods are handed off to an implementation of org.fcrepo.server.management.Management which constructs the appropriate message, and then passes the method down the decorator chain.
(In the future other architectural patterns might be considered, eg there may be more appropriate places to hook in notification with High-Level Storage.)
The Messages
A message represents a concurrent set of changes to a Fedora digital object. Changes may be either modifications of content (create, update, delete) or modifications of state.
In essence it encapsulates one or more datastream updates and/or state changes for a single object, corresponding to a single API-M method invocation.
For example, for an ingest operation, it represents a set of Add operations for each datastream created on ingest; for a modifyDatastream operation it represents an Update for the datastream if the API-M method parameters include updated content.
Messages are dispatched based on modifications to the latest versions of datastreams; it is assumed that consumers are only interested in the current state. So for example if the latest version of a datastream is purged, this is represented as a modification, with the new content being the now-latest version (the version preceding the one being purged).
Data model
A ContentObjectModification is used to encapsulate a set of ContentItemModifications
A ContentObjectModification represents:
- The User making the change
- The date/time of the change
- The URI identifier of the Fedora digital object (the REST API endpoint of getObjectProfile)
- The URI identifier of the parent Fedora digital object (not implemented - see Issues)
- The type(s) of the Fedora digital object (not implemented - see Issues)
- A textual description of the modification (used for information/debugging only)
- A set of ContentItemModifications
Each ContentItemModification represents:
- The URI identifier of the datastream (the REST API endpoint of getDatastreamDissemination)
- A URL for the modified content location (the datastream version) - present for Add and Update operations, not present for Delete operations and state changes
- The internet media type (MIME type) of the content - present for Add and Update operations, not present for Delete operations and state changes
- The type of modification (Add, Update, Delete) - not present for state-only changes
Wiki Markup The state change of the datastream, specifying the previous and new states \[1\]- For Add operations the previous state is not specified
| Wiki Markup |
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\[1\] So the consumer of the messages doesn't have to track the state of individual items but can for instance remove an item from an index if the state has changed from Active to Deleted or Inactive, but can ignore a state change from Inactive to Deleted (but see Issues on modelling state) |
The above are represented as:
- net.acuityunlimited.gen.messaging.ContentObjectModification
- net.acuityunlimited.gen.messaging.ContentItemModification
ContentObjectModification implements Iterable<ContentItemModification>
AtomPub serialisation
Content changes using the above POJOs are serialised to AtomPub as follows:
An AtomPub Feed for the ContentObjectModification with:
- a Category for the server version
- a Category for the message format
- an Author for the user
- a Title for the textual description
- an Updated item for the date/time of modification
- a Link with a relationship REL_VIA for the object URI
- a Link with a relationship REL_RELATED for the parent object URI
- Category elements for the object type(s)
An AtomPub Entry for each ContentItemModification:
- an Author for the user (same as Feed)
- a Title (same as Feed)
- an Updated entry (same as Feed)
- a Content item with the URL of the content location and the MIME type (for Add and Update)
- a Link with a relationship REL_VIA for the item's URI
- a Summary item with a textual description (mandatory, but not used for anything)
- Category items for the before and after states (for state changes)
- a Category item for the type of modification (not present for state-only changes)
Example messages
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<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:changes="http://www.acuityunlimited.net/fedora/messaging#">
<category term="3.5-SNAPSHOT" scheme="info:fedora/fedora-system:def/view#version"></category>
<category term="http://www.acuityunlimited.net/fedora/messaging#content-change-message-0.1" scheme="http://www.fedora.info/definitions/1/0/types/formatURI"></category>
<author>
<name>fedoraAdmin</name>
</author>
<title type="text">ingest</title>
<updated>2011-08-03T08:14:25.943Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass"></link>
<id>urn:uuid:c4bf3a5c-69e7-4a1a-941b-dfd328bb4a48</id>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:296f93e1-7f22-430a-99f6-256cf13d1fa3</id>
<title type="text">ingest</title>
<updated>2011-08-03T08:14:25.943Z</updated>
<content type="text/xml" src="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/DC/content?asOfDateTime=2008-07-02T05:09:42.921Z"></content>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/DC/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:CONTENT:ITEM:STATEAFTER" term="ACTIVE">Fedora datastream state after modification</category>
<category scheme="changes:ITEM:CHANGE:TYPE" term="ADD">Fedora datastream modification type</category>
</entry>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:aabed436-70b7-4e21-aec7-bde3c70d8ba7</id>
<title type="text">ingest</title>
<updated>2011-08-03T08:14:25.943Z</updated>
<content type="application/rdf+xml" src="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/RELS-EXT/content?asOfDateTime=2008-07-02T05:09:42.921Z"></content>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/RELS-EXT/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:CONTENT:ITEM:STATEAFTER" term="ACTIVE">Fedora datastream state after modification</category>
<category scheme="changes:ITEM:CHANGE:TYPE" term="ADD">Fedora datastream modification type</category>
</entry>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:3931f264-2a58-4f20-b177-39fb2a4e49ce</id>
<title type="text">ingest</title>
<updated>2011-08-03T08:14:25.943Z</updated>
<content type="image/jpeg" src="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/FULL_SIZE/content?asOfDateTime=2008-07-02T05:09:42.921Z"></content>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/FULL_SIZE/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:CONTENT:ITEM:STATEAFTER" term="ACTIVE">Fedora datastream state after modification</category>
<category scheme="changes:ITEM:CHANGE:TYPE" term="ADD">Fedora datastream modification type</category>
</entry>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:70f98231-301f-4f5e-a1d3-14d57f2a7322</id>
<title type="text">ingest</title>
<updated>2011-08-03T08:14:25.943Z</updated>
<content type="image/jpeg" src="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/MEDIUM_SIZE/content?asOfDateTime=2008-07-02T05:09:42.921Z"></content>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/MEDIUM_SIZE/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:CONTENT:ITEM:STATEAFTER" term="ACTIVE">Fedora datastream state after modification</category>
<category scheme="changes:ITEM:CHANGE:TYPE" term="ADD">Fedora datastream modification type</category>
</entry>
</feed> |
| Code Block | ||||
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<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:changes="http://www.acuityunlimited.net/fedora/messaging#">
<category term="3.5-SNAPSHOT" scheme="info:fedora/fedora-system:def/view#version"></category>
<category term="http://www.acuityunlimited.net/fedora/messaging#content-change-message-0.1" scheme="http://www.fedora.info/definitions/1/0/types/formatURI"></category>
<author>
<name>fedoraAdmin</name>
</author>
<title type="text">modifyDatastreamByValue</title>
<updated>2011-08-03T08:18:48.092Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass"></link>
<id>urn:uuid:babb08b1-46dd-407c-859d-f2f355634e3a</id>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:0bcf71cb-620b-453a-9ffa-443f414595b7</id>
<title type="text">modifyDatastreamByValue</title>
<updated>2011-08-03T08:18:48.092Z</updated>
<content type="text/xml" src="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/DC/content?asOfDateTime=2011-08-03T08:18:48.092Z"></content>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/DC/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:ITEM:CHANGE:TYPE" term="UPDATE">Fedora datastream modification type</category>
</entry>
</feed> |
| Code Block | ||||
|---|---|---|---|---|
| ||||
<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:changes="http://www.acuityunlimited.net/fedora/messaging#">
<category term="3.5-SNAPSHOT" scheme="info:fedora/fedora-system:def/view#version"></category>
<category term="http://www.acuityunlimited.net/fedora/messaging#content-change-message-0.1" scheme="http://www.fedora.info/definitions/1/0/types/formatURI"></category>
<author>
<name>fedoraAdmin</name>
</author>
<title type="text">setDatastreamState</title>
<updated>2011-08-03T08:20:30.460Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass"></link>
<id>urn:uuid:be983aa5-43fd-4330-b8bb-127bac00c3cb</id>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:841b1023-52c7-4322-b10a-daa2037bb3e7</id>
<title type="text">setDatastreamState</title>
<updated>2011-08-03T08:20:30.460Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/DC/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:CONTENT:ITEM:STATEBEFORE" term="ACTIVE">Fedora datastream state before modification</category>
<category scheme="changes:CONTENT:ITEM:STATEAFTER" term="INACTIVE">Fedora datastream state after modification</category>
</entry>
</feed> |
| Code Block | ||||
|---|---|---|---|---|
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<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:changes="http://www.acuityunlimited.net/fedora/messaging#">
<category term="3.5-SNAPSHOT" scheme="info:fedora/fedora-system:def/view#version"></category>
<category term="http://www.acuityunlimited.net/fedora/messaging#content-change-message-0.1" scheme="http://www.fedora.info/definitions/1/0/types/formatURI"></category>
<author>
<name>fedoraAdmin</name>
</author>
<title type="text">purgeObject</title>
<updated>2011-08-03T08:21:32.169Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass"></link>
<id>urn:uuid:9733fc70-67e0-49ef-aa2b-e8d9c95a5db7</id>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:2d769c36-baba-47be-8716-524a58542063</id>
<title type="text">purgeObject</title>
<updated>2011-08-03T08:21:32.169Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/DC/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:ITEM:CHANGE:TYPE" term="DELETE">Fedora datastream modification type</category>
</entry>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:4164cd78-1fa3-4292-bf7c-47f62bb672b7</id>
<title type="text">purgeObject</title>
<updated>2011-08-03T08:21:32.169Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/RELS-EXT/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:ITEM:CHANGE:TYPE" term="DELETE">Fedora datastream modification type</category>
</entry>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:796c0612-171f-46ad-bcb7-e2a9a6166ae7</id>
<title type="text">purgeObject</title>
<updated>2011-08-03T08:21:32.169Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/MEDIUM_SIZE/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:ITEM:CHANGE:TYPE" term="DELETE">Fedora datastream modification type</category>
</entry>
<entry>
<author>
<name>fedoraAdmin</name>
</author>
<id>urn:uuid:64571aac-58cc-47d2-a9b6-3bf0ac870101</id>
<title type="text">purgeObject</title>
<updated>2011-08-03T08:21:32.169Z</updated>
<link rel="via" href="http://localhost:8080/fedora/objects/demo:SmileyBeerGlass/datastreams/FULL_SIZE/content"></link>
<summary type="text">Datastream modification/state change, see content src for new content</summary>
<category scheme="changes:ITEM:CHANGE:TYPE" term="DELETE">Fedora datastream modification type</category>
</entry>
</feed> |
Code overview
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