We are trying to debug a problem where indexing a document halts the ingestion pipeline on that particular shard
Taking into account the way sequence numbers are implemented, do documents get processed sequentially? I don't think so since ES has a write threadpool. At what point does the indexing fan out to leverage parallelism within a shard? What I'm trying to understand is, can a document A which is undergoing indexing and has been assigned with seqno=1 blocks indexing of document B with seqno=2 if it's taking time to index document A ? if not, then B with seqno=2 can be indexed before A gets indexed (and likewise visible in queries)? in that case what happens if A fails, what happens to the seqno=1? There has to be some sequential processing going on at some point right?