Config Servers
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Important
Starting in 3.4, the use of the deprecated mirrored mongod
instances as config servers (SCCC) is no longer supported. Before you
can upgrade your sharded clusters to 3.4, you must convert your config
servers from SCCC to CSRS.
To convert your config servers from SCCC to CSRS, see the MongoDB 3.4 manual Upgrade Config Servers to Replica Set.
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.
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 adding a shard. Shards also read
chunk metadata from the config servers.
The config servers also store Authentication configuration information such as Role-Based Access Control or internal authentication settings for the cluster.
MongoDB also uses the config servers to manage distributed locks.
Each sharded cluster must have its own config servers. Do not use the same config servers for different sharded clusters.
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.
Replica Set Config Servers
Changed in version 3.4.
Starting in MongoDB 3.2, config servers for sharded clusters can be deployed as a replica set (CSRS) instead of three mirrored config servers (SCCC). 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.
In version 3.4, MongoDB removes support for SCCC config servers.
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
members[n].buildIndexessetting set to false).
Read and Write Operations on Config Servers
The admin database and the config database exist on the config servers.
Writes to Config Servers
The admin database contains the collections related to the
authentication and authorization as well as the other system.*
collections for internal use.
The config database contains the collections that contain the sharded cluster metadata. MongoDB writes data to the config database when the metadata changes, such as after a chunk migration or a chunk split.
Users should avoid writing directly to the config database in the course of normal operation or maintenance.
When writing to the config servers, MongoDB uses a write concern of "majority".
Reads from Config Servers
MongoDB reads from the admin database for authentication and
authorization data and other internal uses.
MongoDB reads from the config database when a mongos
starts or after a change in the metadata, such as after a chunk
migration. Shards also read chunk metadata from the config servers.
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.
In a sharded cluster, mongod and mongos instances
monitor the replica sets in the sharded cluster (e.g. shard replica
sets, config server replica set).
If all config servers become unavailable, the cluster can become inoperable. 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.
For 3.2 sharded clusters, if the number of consecutive unsuccessful
attempts to monitor the config server replica set exceeds
replMonitorMaxFailedChecks parameter value, the monitoring
mongos or mongod instance becomes unusable until
you restart the instance. See
v3.2 troubleshooting guide
for a workaround.
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 in
mongosh:
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/Membership 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.
Starting in MongoDB 5.3, SCRAM-SHA-1 cannot be used for intra-cluster authentication. Only SCRAM-SHA-256 is supported.
In previous MongoDB versions, SCRAM-SHA-1 and SCRAM-SHA-256 can both be used for intra-cluster authentication, even if SCRAM is not explicitly enabled.
See Deploy Sharded Cluster with Keyfile Authentication for a tutorial on deploying a secured sharded cluster.