In order increase the ability of Solr to be used as a NoSql database (lots of concurrent inserts, updates, deletes and queries in the entire lifetime of the index) instead of just a search index (first: everything indexed (in one thread), after: only queries), I would like Solr to support versioning to be used for optimistic locking.
When my intent (see SOLR-3173) is to update an existing document, I will need to provide a version-number equal to "the version number I got when I fetched the existing document for update" plus one. If this provided version-number does not correspond to "the newest version-number of that document at the time of update" plus one, I will get a VersionConflict error. If it does correspond the document will be updated with the new one, so that "the newest version-number of that document" is NOW one higher than before the update. Correct but efficient concurrency handling.
When my intent (see SOLR-3173) is to insert a new document, the version number provided will not be used - instead a version-number 0 will be used. According to SOLR-3173 insert will only succeed if a document with the same value on uniqueKey-field does not already exist.
In general when talking about different versions of "the same document", of course we need to be able to identify when a document "is the same" - that, per definition, is when the values of the uniqueKey-fields are equal.
The functionality provided by this issue is only really meaningfull when you run with "updateLog" activated.
This issue might be solved more or less at the same time as SOLR-3173, and only one single SVN patch might be given to cover both issues.