Sets the _version_ field (or other user configurable field) value to 0 (zero) to cause the document to be added if it does not exist or overwrite any previous version if it does exist.

Sends documents to Solr without includng any _version_ field at all. Depending on the Solr server config, this may result in similar behavior to LastWriteWins, or it may result in updates being discarded entirely.

Uses the _version_ field (or other user configurable field) as a token to ensure that the document being replaced/updated has not been concurrently modified by another client.

This is the safest mode to ensure writes from multiple clients do not collide which can manifest in updates appearing temporarily and then appearing to revert to an older value.