How do I know I need to retry when use mongo C driver API?

Question

Considering working with a shard replica-set cluster, some emergency such as re-pick primary node will happen and maybe we can retry the command several times for waiting for recover of mongo.

Is there anyone know when use c driver api such as mongoc_collection_update, mongoc_collection_commond, mongoc_collection_insert, which error codes tell me that I can retry command for making the application more robust.

Answer 1

Generally, you can retry idempotent operations once. Please see my MongoDB World talk on how MongoDB drivers implement high availability and failover:

https://www.mongodb.com/presentations/mongodb-drivers-and-high-availability-deep-dive

The talk applies equally to how mongos implements high availability and failover, which is the relevant question in a sharded cluster. Towards the end of the talk I give a reasonable strategy for implementing retries of queries, inserts, updates, and deletes.

For the C Driver specifically, the MONGOC_ERROR_QUERY and MONGOC_ERROR_COMMAND domains imply a server-side rejection of your operation which cannot be successfully retried. However, since you're connecting through mongos, mongos may experience a network disconnect or an unavailable primary when talking to your replica sets—in this case, the error will appear to you as a QUERY / COMMAND error even though it's really a network error between mongos and mongod. I'll wait for someone from the MongoDB server team to chime in with a strategy for distinguishing those errors.

MONGOC_ERROR_COMMAND with a code of 11000 means a duplicate key error, which you can use to determine if the first try of an insert operation succeeded before a network error.

MONGOC_ERROR_CLIENT means a network error. Idempotent operations can be usefully retried one time after a network error. Starting in version 1.2, MONGOC_ERROR_SERVER_SELECTION will also be retryable.