FlushNotifications

Called after one or more segments were flushed to disk.

Called when files were written to disk that are not used anymore. It's the implementation's responsibility to clean these files up

Called when a segment failed to flush.

Called once deletes have been applied either after a flush or on a deletes call

Called once the DocumentsWriter ticket queue has a backlog. This means there is an inner thread that tries to publish flushed segments but can't keep up with the other threads flushing new segments. This likely requires other thread to forcefully purge the buffer to help publishing. This can't be done in-place since we might hold index writer locks when this is called. The caller must ensure that the purge happens without an index writer lock being held.

Should be called if a flush or an indexing operation caused a tragic / unrecoverable event.