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MongoDB Manual

Sharded Cluster Components

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  • Production Configuration
  • Development Configuration

A MongoDB sharded cluster consists of the following components:

  • shard: Each shard contains a subset of the sharded data. Each shard must be deployed as a replica set.

  • mongos: The mongos acts as a query router, providing an interface between client applications and the sharded cluster. mongos can support hedged reads to minimize latencies.

  • config servers: Config servers store metadata and configuration settings for the cluster. Config servers must be deployed as a replica set (CSRS).

In a production cluster, ensure that data is redundant and that your systems are highly available. Consider the following for a production sharded cluster deployment:

Where possible, consider deploying one member of each replica set in a site suitable for being a disaster recovery location.


Distributing replica set members across two data centers provides benefit over a single data center. In a two data center distribution,

  • If one of the data centers goes down, the data is still available for reads unlike a single data center distribution.

  • If the data center with a minority of the members goes down, the replica set can still serve write operations as well as read operations.

  • However, if the data center with the majority of the members goes down, the replica set becomes read-only.

If possible, distribute members across at least three data centers. For config server replica sets (CSRS), the best practice is to distribute across three (or more depending on the number of members) centers. If the cost of the third data center is prohibitive, one distribution possibility is to evenly distribute the data bearing members across the two data centers and store the remaining member in the cloud if your company policy allows.

Sharding requires at least two shards to distribute sharded data. Single shard sharded clusters may be useful if you plan on enabling sharding in the near future, but do not need to at the time of deployment.

mongos routers support high availability and scalability when deploying multiple mongos instances. If a proxy or load balancer is between the application and the mongos routers, you must configure it for client affinity. Client affinity allows every connection from a single client to reach the same mongos. For shard-level high availability, either:

  • Add mongos instances on the same hardware where mongos instances are already running.

  • Embed mongos routers at the application level.

mongos routers communicate frequently with your config servers. As you increase the number of routers, performance may degrade. If performance degrades, reduce the number of routers. Your deployment should not have more than 30 mongos routers.

The following diagram shows a common sharded cluster architecture used in production:

Diagram that shows a production-level sharded cluster
containing multiple shards and mongos routers.

For testing and development, you can deploy a sharded cluster with a minimum number of components. These non-production clusters have the following components:

The following diagram shows a sharded cluster architecture used for development only:

Diagram that shows a development sharded cluster
containing a single shard and mongos router.


Use the test cluster architecture for testing and development only.


See also: