wtimeout

Wtimeout is a write concern option used in MongoDB to control how long the client will wait for a write operation to be acknowledged by the configured number of nodes. It works in conjunction with the w setting, which specifies how many replica set members must acknowledge the write for it to be considered successful. The value of wtimeout is expressed in milliseconds.

In practice, wtimeout can be set as part of the write concern sent with a write operation. Depending on the driver and version, it may be specified as wtimeout: <milliseconds> or as wtimeoutMS: <milliseconds>. Common patterns include w: 1 with a small timeout for low latency, or w: majority with a longer timeout to ensure durability across the replica set.

If the required acknowledgments are not received within the specified timeout, the operation fails on the client side with a write concern error (often referred to as a WriteConcernError). This means the write may not be durable as requested, even if some nodes have recorded the operation. In standalone deployments where there is a single node, the write concern requirement is effectively ignored, and wtimeout has no practical effect.

Implications include a trade-off between durability and latency. Higher replication guarantees (e.g., majority) increase the chance of timeouts under load or network lag, while lower guarantees reduce latency but may risk data loss on failures. Administrators should choose w and wtimeout values based on durability requirements, network stability, and application latency goals.

Best practices advise aligning wtimeout with realistic replication latency, monitoring replica health, and testing failover scenarios to understand how timeouts affect write behavior in production.