Q:

How to fix ElasticSearch conflicts on the same key when two process writing at the same time

I have multiple processes to write data to ES at the same time, also two processes may write the same key with different values at the same time, it caused the exception as following:
"error" : "VersionConflictEngineException[[website][2] [blog][1]:
             version conflict, current [2], provided [1]]",
"status" : 409

How could I fix the above problem please, since I have to keep multiple processes. 

A:

VersionConflictEngineException is thrown to prevent data loss.  Every document in elasticsearch has a _version number that is incremented whenever a document is changed.  
When you query a doc from ES, the response also includes the version of that doc.  When you update the same doc and provide a version, then a document with the same version is expected to be already existing in the index.  
If the current version is greater than the one in the update request, What we would get now is a conflict, with the HTTP error code of 409 and VersionConflictEngineException
In your current scenario, 

version conflict, current 2, provided 1

The current version in ES is 2 whereas in your request is 1 which means some other thread has already modified the doc and your change is trying overwrite the doc. 
In case of VersionConflictEngineException, you should re-fetch the doc and try to update again with the latest updated version. 
Whether or not to use the versioning / Optimistic Concurrency Control, depends on the application.  If you can live with data-loss, you may avoid passing version in the update request. 