Confusion of elasticsearch

Hi , i am study the source of elk.
As all know, there are two types of write operation in elasticseasrch , sync and unsync.
I wonder. What does a sync operation really mean,
If i write a document to primary and its replica in sync model, the writing operation on replica will return success signel on which point.
The write operation record into translog ?
or the translog flush to lucene index?

Looking forward your reply


The ability to specify async replication for indexing operations is deprecated and removed in version 2.0.0 (see for reference). Indexing operations with sync will return after they have been written to disk to the translog and are also written to the lucene in-memory buffer on primary and all replica shards. For reference:


Thanks, Christoph
It helps me a lot to understand elk