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Troubleshoot Sharded Clusters

This section describes common strategies for troubleshooting sharded cluster deployments.

Config Database String Error

Start all mongos instances in a sharded cluster with an identical configDB string. If a mongos instance tries to connect to the sharded cluster with a configDB string that does not exactly match the string used by the other mongos instances, including the order of the hosts, the following errors occur:

could not initialize sharding on connection


mongos specified a different config database string

To solve the issue, restart the mongos with the correct string.

Cursor Fails Because of Stale Config Data

A query returns the following warning when one or more of the mongos instances has not yet updated its cache of the cluster’s metadata from the config database:

could not initialize cursor across all shards because : stale config detected

This warning should not propagate back to your application. The warning will repeat until all the mongos instances refresh their caches. To force an instance to refresh its cache, run the flushRouterConfig command.

Avoid Downtime when Moving Config Servers

Use CNAMEs to identify your config servers to the cluster so that you can rename and renumber your config servers without downtime.