Number of results per shard

Elastic Stack Elasticsearch
1

We have a basic ES instance, and I'm considering an index that has only one primary shard and one replica shard.

Doing a basic bool query, hits.total turns out to depend on which shard we hit. I get consistently different numbers specifying ?preference=_primary or ?preference=_replica.

The shards are different in some sense, because with _cat I see:

index       shard prirep state   docs   store ip            node
admin_ch-v1 0     r      STARTED 3220 295.2mb x.x.x.x  JO8tqXw
admin_ch-v1 0     p      STARTED 3220   294mb x.x.x.x aCqEzYQ

However, the document count is the same. I also wrote a script to get all the documents specifically from each shard (using preference=f"_only_nodes:xxx") and comparing them, and, modulo a bug in my script, everything is identical.

So... what is going on?

2

Merging of segments is not coordinated across shards, so even if primary and replica shards hold exactly the same contents their size may differ as they may have merged differently.

3

That's perfectly fine. My question is why, if the contents are exactly the same, the counts for the same query are different.

4

Are you making changes to these indices? What is the refesh_interval set to?

5

Sorry I didn't notice your reply earlier.

I don't see refresh_interval in GET <index>/_settings, so I assume it's the default 1s.

And I don't think it's a matter of heavy write usage: the index receives an average of 2 new documents per day.

Also we got the same two counts, say N and M, for the same query on the two shards, trying it hours apart.

6

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