Config Servers

Config servers store the metadata for a sharded cluster.


If the config servers become inaccessible, the cluster is not accessible. If you cannot recover the data on a config server, the cluster will be inoperable.

MongoDB also uses the config servers to manage distributed locks.

Replica Set Config Servers

Changed in version 3.2.

Starting in MongoDB 3.2, config servers for new sharded clusters can be deployed as a replica set. This change improves consistency across the config servers, since MongoDB can take advantage of the standard replica set read and write protocols to replicate the data across the config servers. In addition, using a replica set for config servers allows a sharded cluster to have more than 3 config servers since a replica set can have up to 50 members.

The following restrictions apply to a replica set configuration when used for config servers:

  • Must have zero arbiters.
  • Must have no delayed members.
  • Must build indexes (i.e. no member should have buildIndexes setting set to false).

Earlier versions of MongoDB required exactly three mirrored mongod instances to act as the config servers. MongoDB 3.2 deprecates the use of three mirrored mongod instances for config servers.

Each sharded cluster must have its own config servers. Do not use the same config servers for different sharded clusters.

Read and Write Operations on Config Servers

Config servers store the cluster’s metadata in the config database. The mongos instances cache this data and use it to route reads and writes to shards.

MongoDB only writes data to the config servers when the metadata changes, such as

When writing to the replica set config servers, MongoDB uses a write concern of "majority".

MongoDB reads data from the config server in the following cases:

  • A new mongos starts for the first time, or an existing mongos restarts.
  • After change in the cluster metadata, such as after a chunk migration.

When reading from the replica set config servers, MongoDB uses a Read Concern level of "majority".

Config Server Availability

If the config server replica set loses its primary and cannot elect a primary, the cluster’s metadata becomes read only. You can still read and write data from the shards, but no chunk migration or chunk splits will occur until the replica set can elect a primary. If all config databases become unavailable, the cluster can become inoperable.

The mongos instances cache the metadata from the config servers. As such, if all config server members become unavailable, you can still use the cluster if you do not restart the mongos instances until after the config servers are accessible again. If you restart the mongos instances before the config servers are available, the mongos will be unable to route reads and writes.

Clusters become inoperable without the cluster metadata. To ensure that the config servers remain available and intact, backups of config servers are critical. The data on the config server is small compared to the data stored in a cluster, and the config server has a relatively low activity load.

See A Config Server Replica Set Member Become Unavailable for more information.

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