Will 3 replicas in Azure Cognitive Search mean index rebuilds do not interrupt querying?

Question

Hi Azure Cognitive Search experts,

In Azure Cognitive Search, assuming we have 3 replicas, and the data source is Azure SQL:

Will an index 'reset' (or complete reindex/rebuild), on the indexer be applied to the replicas sequentially, so the index querying continues to work without interruption?

(This is not mentioned in either the replicas docs or index rebuild documentation)

Thanks.

Answer 1

@ca , Thanks for the good question.

Having multiple replicas allows Azure Cognitive Search to do machine reboots and maintenance against one replica, while query execution continues on other replicas.

When a service is upgraded, each replica is updated at the same time. This is actually protection to internal updates.
When you have 3 replicas, 2 will remain available and that is enough to allow both querying and indexing to continue to be available.

Please see this doc for more details. high-availability

If you rebuild the index, all the replicas will get rid of it and will start to rebuild.

During development, the index schema changes frequently. You can plan for it by creating indexes that can be deleted, recreated, and reloaded quickly with a small representative data set.
For applications already in production, it is recommend to create a new index that runs side by side an existing index to avoid query downtime. Your application code provides redirection to the new index.

See this doc for more info: How to rebuild an index

Just note that, ,in Free tier you cannot add replicas, you would need higher tier.