Entity versioning and consensus

[erikhofer]

As partly discussed in #6 every user should be able to edit entities like products. We thus need a versioning system that

• tracks all changes made to an entity trasparently
• allows to discuss changes
• allows to reset an entity to a previous state
• solves conflicting concurrent changes
• allows locking entities in case of "edit wars"
• achieves consensus in Wikipedia style - everyone can edit entities, changes are public immiadately. Changes are not voted upon, they can only be overriden. Maybe there should be some rate-limiting. Also changes could be flagged as malicious and removed by mods.
• requires approval by a mod for new contributors? If implemented, auto-approve after a number of manual approvals.

Proposal: Whenever an entity of this kind is updated, a diff is created and saved as a "changeset". Also a snapshot of the old version is saved to the database.

The basic entity structure could look like this (optimistic locking not included):

Considerations:

• Changes must be internationalizable (e.g. use category serials instead of names)

[erikhofer]

Hibernate Envers can be used to automatically create the "snapshots" (revisions).

Envers can flag properties as changed but these flags can only be used to filter queries. They can't be easily retrieved and Envers can't generate a diff of those properties. ChangeSets could be built using onFlushDirty() of a Hibernate interceptor.