Unique Index and Concurrency


If I define an unique index on a field (say “field1”) and multiple truly parallel threads (i.e. exact same time up to milliseconds) tries to insert the same <field1: same-value> in the same collection, is it guaranteed that only ONE of these threads will actually succeed and others will fail with “Duplicate Key Error”?

Does the server-side ensure this?

How does the behavior change based on if the field is scalar or array?

We are using java, spring-boot stack to integrate with MongoDB 4.x.

NOTE - Posting on an Urgency. It will be really helpful, if we can receive a kind response soon.


Each insert or update to a single document is atomic in MongoDB and therefore each one takes an exclusive lock on the index and collection documents.

Read more here:

So only a single concurrent thread will be able to set the value whithin a single replica set Primary. (Sharding must have the unique index as the shard key to enforce uniqueness).


Sincere Thanks for the timely response. It was indeed helpful, I missed to timely update the comment.

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