what are the risks with deleting Cosmos Replica?

Yep - your data is safe. Deleting a read-only replica region in a single-write Azure Cosmos DB account does not delete the data in the remaining primary region. Each Cosmos DB region maintains its own complete copy of the container, with the data internally replicated within that region. When you remove the secondary region, you are removing that region's copy, while the primary write region and its data remain intact.

Note that the main consequence is loss of regional redundancy. Once you have a single region, a regional outage can no longer be handled by failing over to the second Cosmos DB region. This is the primary risk to consider, rather than data loss caused by removing the replica.

There is no instantaneous undo or rollback button, but the change is reversible in the sense that you can add the removed region back to the Cosmos DB account later. Because your database is large, however, this would not be instantaneous. Cosmos DB must replicate the complete dataset from the remaining region into the newly added region before that region is fully available. Therefore, adding it back is a re-replication operation, not restoration of the old replica.

Operationally, avoid making other account-level changes at exactly the same time as the region removal. For example, active asynchronous throughput-scaling operations can be temporarily paused while the region change completes. Also monitor the account if the portal remains in an Updating or Deleting state for an unusually long period. The removal itself should not affect the integrity of the data in the surviving region.

Also check your application configuration. If your Cosmos DB SDK has the region you are removing explicitly configured as a preferred location, update that configuration so the application does not unnecessarily attempt to connect to the removed region. The SDK can generally fall back to the remaining region, but removing a no-longer-valid preferred location avoids unnecessary connection attempts.

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hth

Marcin