Major Description
The existing optimistic concurrency features of Solr can be very handy for ensuring that you are only updating/replacing the version of the doc you think you are updating/replacing, w/o the risk of someone else adding/removing the doc in the mean time – but I've recently encountered some situations where I really wanted to be able to let the client specify an arbitrary version, on a per document basis, (ie: generated by an external system, or perhaps a timestamp of when a file was last modified) and ensure that the corresponding document update was processed only if the "new" version is greater then the "old" version – w/o needing to check exactly which version is currently in Solr. (ie: If a client wants to index version 101 of a doc, that update should fail if version 102 is already in the index, but succeed if the currently indexed version is 99 – w/o the client needing to ask Solr what the current version)
The idea Yonik brought up in SOLR-5298 (letting the client specify a _new_version_ that would be used by the existing optimistic concurrency code to control the assignment of the _version_ field for documents) looked like a good direction to go – but after digging into the way _version_ is used internally I realized it requires a uniqueness constraint across all update commands, that would make it impossible to allow multiple independent documents to have the same _version_.
So instead I've tackled the problem in a different way, using an UpdateProcessor that is configured with user defined field to track a "DocBasedVersion" and uses the RTG logic to figure out if the update is allowed.
