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Sharded Cluster Components

Config Servers

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  • Replica Set Config Servers
  • Read and Write Operations on Config Servers
  • Config Server Availability
  • Sharded Cluster Metadata
  • Sharded Cluster Security
  • Config Shards

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.

Starting in MongoDB 8.0, you can configure a config server to store application data in addition to the usual sharded cluster metadata. The config server is then known as a config shard. You'll learn more later on this page in the Config Shards section.

Replica Set Config Servers

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

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".

Metadata Views Must be Up-to-Date

For an operation to succeed, the view of the metadata on the specific shard member must be up-to-date. The shard and the router issuing the request must have the same version of the chunks metadata.

If the metadata is not up-to-date, the operation fails with the StaleConfig error and the metadata refresh process is triggered. Refreshing the metadata can introduce additional operational latency.

On a secondary, a metadata refresh can take a long time if there is significant replication lag. For secondary reads, set maxStalenessSeconds to minimize the impact of replication lag.

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.

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.

Config Shards

New in version 8.0.

Starting in MongoDB 8.0, you can:

  • Configure a config server to store your application data in addition to the usual sharded cluster metadata. A config server that stores application data is called a config shard.

  • Transition a config server between being a config shard and a dedicated config server.

A cluster requires a config server, but it can be a config shard instead of a dedicated config server. Using a config shard reduces the number of nodes required and can simplify your deployment.

If your application has demanding availability and resiliency requirements, consider deploying a dedicated config server. A dedicated config server provides isolation, dedicated resources, and consistent performance for critical cluster operations.

You cannot use the same config shard for multiple sharded clusters.

Commands

To configure a dedicated config server to run as a config shard, run the transitionFromDedicatedConfigServer command.

To configure a config shard to run as a dedicated config server, run the transitionToDedicatedConfigServer command.

Users

A user created on a config shard has the same behavior as a user created on a dedicated config server.

Config Shard Identity Document

To identify the config server as a config shard, inspect the document in the admin.system.version collection. In this example, shardName is set to 'config':

{
   _id: 'shardIdentity',
   shardName: 'config',
   clusterId: ObjectId("<objectID>"),
   configsvrConnectionString: '<config server replica set connection string>',
}

The following example retrieves a shard identity document from admin.system.version in the admin database:

use admin
db.system.version.find()

Output extract:

{
   _id: 'shardIdentity',
   shardName: 'config',
   clusterId: ObjectId("6441bdd6779584849dcac095"),
   configsvrConnectionString: 'configRepl/localhost:27007'
}

Downgrade Feature Compatibility Version

If your cluster has a config shard and you need to downgrade the feature compatibility version to earlier than 8.0, connect to mongos and perform this procedure:

1

Configure a config shard to run as a dedicated config server.

To begin configuring a config shard to run as a dedicated config server, run transitionToDedicatedConfigServer:

db.adminCommand( {
   transitionToDedicatedConfigServer: 1
} )
2

List all of the databases in the cluster.

To list all the databases in the cluster, run listDatabases:

db.adminCommand( { listDatabases: 1, nameOnly: true } )

Exclude admin and config databases.

For each database:

1
List all of the collections in the database.

To list all of the collections in the database, run listCollections.

db.adminCommand(
   {
     listCollections: 1,
     nameOnly: true,
     filter: { type: { $ne: "view" } }
   }
)

Exclude collections starting with system.

For each remaining collection:

1
Move the collection to a new shard.

To move the collection to a new shard, run moveCollection:

db.adminCommand(
   {
     moveCollection: "<database>.<collection>",
     toShard: "<new shard>",
   }
)
3

Wait for balancer to move sharded collection data off config server.

To verify the balancer has moved sharded collection data off the config server, run transitionToDedicatedConfigServer again:

db.adminCommand( {
   transitionToDedicatedConfigServer: 1
} )

The response after a successful data move contains state: "completed". If the response contains state: "pendingDataCleanup", wait a moment then continue calling transitionToDedicatedConfigServer until the command response contains state: "completed". For full response details, see removeShard.

4

Set the feature compatibility version.

To set the feature compatibility version, run setFeatureCompatibilityVersion:

db.adminCommand( { setFeatureCompatibilityVersion: "7.0" } )

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