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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.
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.
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
buildIndexessetting 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¶
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
MongoDB reads data from the config server in the following cases:
- A new
mongosstarts for the first time, or an existing
- After change in the cluster metadata, such as after a chunk migration.
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.
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
instances until after the config servers are accessible again. If you
mongos instances before the config servers are
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.