[SLING-3993] Avoiding loops when syncing via coordinate agents

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: Content Distribution 0.1.0
• Component/s: Content Distribution
• Assignee: Tommaso Teofili
• Reporter: Tommaso Teofili
• Created: 02/Oct/14 10:31
• Updated: 31/Mar/15 15:22
• Resolved: 28/Oct/14 08:45

Description

In the case of a 'coordinate agent', sitting on 'author' instance, responsible for synchronizing 2+ 'publish' instances the simplest setup would consist of a 'queueing agent' (local exporter, no importer) sitting on each of the 'publish' instances with a trigger responsible of signaling changes of resources under a certain path so that when, for example, a resource 'bar' gets created under '/foo' the trigger will cause the creation of a package for '/foo/bar' which will be queued locally, the 'coordinate agent' will remotely fetch such a package from the queue of e.g. 'pub1' instance and remotely import it into 'pub2' instance.
If both 'pub1' and 'pub2' have the same trigger this may result into a looping scenario when instances keep exchanging packages for the same changed content.

Possible solutions to this can be, conceptually, based on:
1. identifying the source of the event that generates the replication so that events on changed resources generated by the package builder (upon package installation) are filtered and do not trigger a replication request.
2. filtering packages before they get transported if a same/similar package has been already sent by the target instance, keeping something a cache-like structure for the communication (in the coordinate agent).