#10032 introduced the notion of sealing an index by marking it with a special read only marker, allowing for a couple of optimization to happen. The most important one was to speed up recoveries of shards where we know nothing has changed since they were online by skipping the file based sync phase. During the implementation we came up with a light notion which achieves the same recovery benefits but without the read only aspects which we dubbed synced flush. The fact that it was light weight and didn't put the index in read only mode, allowed us to do it automatically in the background which has great advantage. However we also felt the need to allow users to manually trigger this operation.
The implementation at #11179 added the sync flush internal logic and the manual (rest) rest API. The name of the API was modeled after the sealing terminology which may end up being confusing. This commit changes the API name to match the internal synced flush naming, namely `{index}/_flush/synced'.
On top of that it contains a couple other changes:
- Remove all java client API. This feature is not supposed to be called programtically by applications but rather by admins.
- Improve rest responses making structure similar to other (flush) API
- Change IndexShard#getOperationsCount to exclude the internal +1 on open shard . it's confusing to get 1 while there are actually no ongoing operations
- Some minor other clean ups
b376a3fbfb