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Config Servers

Config servers store the metadata for a sharded cluster. The metadata reflects state and organization for all data and components within the sharded cluster. The metadata includes the list of chunks on every shard and the ranges that define the chunks.

In MongoDB 3.2+, you can deploy config servers as a replica set (CSRS). See Replica Set Config Servers.

The mongos instances cache this data and use it to route read and write operations to the correct shards. mongos updates the cache when there are metadata changes for the cluster, such as Chunk Splits or adding a shard.

The config servers store Authentication configuration information such as Role-Based Access Control or internal authentication settings for the cluster.

A single sharded cluster must have exclusive use of its config servers. If you have multiple sharded clusters, each cluster must have its own CSRS.

Warning

Administrative operations conducted on config servers may have significant impact on sharded cluster performance and availability. Depending on the number of config servers impacted, the cluster may be read-only or offline for a period of time.

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 sharded clusters can be deployed as a replica set. Using a replica set for the config servers improves consistency across the config servers, since MongoDB can take advantage of the standard replica set read and write protocols for the config data. 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. To deploy config servers as a replica set, the config servers must run the WiredTiger storage engine.

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. If you are using mirrored config servers, each server’s system clock must be within 30 seconds of each other server for the distributed lock manager to work properly. With mirrored config servers, minimize clock skew by running the network time protocol (NTP) ntpd on your servers. MongoDB 3.2 deprecates the use of three mirrored mongod instances for config servers.

With replica set config servers, clock skew does not affect distributed lock management.

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 a replica set config server deployment, 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 cannot 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.

Sharded Cluster Metadata

Config servers store metadata in the Config Database.

Important

Always back up the config database before doing any maintenance on the config server.

To access the config database, issue the following command from the mongo shell:

use config

In general, you should never edit the content of the config database directly. The config database contains the following collections:

For more information on these collections and their role in sharded clusters, see Config Database. See Read and Write Operations on Config Servers for more information about reads and updates to the metadata.

Sharded Cluster Security

Use Internal Authentication to enforce intra-cluster security and prevent unauthorized cluster components from accessing the cluster. You must start each mongod in the cluster with the appropriate security settings in order to enforce internal authentication.

See Deploy Sharded Cluster with Keyfile Access Control for a tutorial on deploying a secured sharded cluster.

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