91 ShutdownInProgress: The server is in the process of shutting down

What does it mean?

The operation could not complete because the server (mongod/mongos) is shutting down. A node that is shutting down rejects new requests and terminates in-progress operations.

Why does it happen?

A planned restart/maintenance, a shutdown preceding a primary stepdown in a replica set, a termination signal sent by the container/orchestrator, or the process being stopped due to resource exhaustion. It is expected during rolling upgrades.

How to fix it

This is usually transient; use retry logic in the application and a replica set connection so that requests are routed to another healthy member. If unexpected, check the mongod logs for the shutdown reason (signal, OOM killer, resources). During rolling maintenance, step down the primary cleanly before shutting it down (rs.stepDown()).