There is a synchronize block that blocks other update requests whose IDs fall in the same hash bucket. The update waits forever until it gets the lock at the synchronize block, this can be a problem in some cases.
Some add/update requests (for example updates with spatial/shape analysis) like may take time (30+ seconds or even more), this would the request time out and fail.
Client may retry the same requests multiple times or several minutes, this would make things worse.
The server side receives all the update requests but all except one can do nothing, have to wait there. This wastes precious memory and cpu resource.
We have seen the case 2000+ threads are blocking at the synchronize lock, and only a few updates are making progress. Each thread takes 3+ mb memory which causes OOM.
Also if the update can't get the lock in expected time range, its better to fail fast.
We can have one configuration in solrconfig.xml: updateHandler/versionLock/timeInMill, so users can specify how long they want to wait the version bucket lock.
The default value can be -1, so it behaves same - wait forever until it gets the lock.