Problem Statement
Describe the components of a MongoDB sharded cluster and how they work together.
Explanation
A MongoDB sharded cluster consists of three main components: shards, config servers, and mongos routers. Each component plays a specific role in distributing data and routing queries.
Shards are the actual data stores in a sharded cluster. Each shard is typically deployed as a replica set to provide high availability and data redundancy. Shards hold subsets of the data based on the shard key ranges assigned to them. For example, one shard might hold documents with user IDs from 1 to 10000, while another holds 10001 to 20000.
Config servers store metadata about the cluster configuration. This includes which shards exist, what chunks of data each shard contains, and the ranges of shard key values in each chunk. Config servers must be deployed as a replica set for high availability because they are critical to cluster operations. If config servers are unavailable, the cluster cannot route queries or perform administrative operations, though existing connections continue to work.
Mongos routers are the query routing layer that applications connect to. Applications never connect directly to shards. Instead, they connect to mongos instances, which appear as normal MongoDB servers. When mongos receives a query, it consults the config servers to determine which shards contain relevant data. For targeted queries that include the shard key, mongos routes to specific shards. For scatter-gather queries without the shard key, mongos broadcasts to all shards and merges results.
The balancer is a background process that monitors chunk distribution across shards. When it detects imbalance, it migrates chunks from heavily loaded shards to less loaded ones. This ensures even distribution as data grows.
Together, these components enable transparent horizontal scaling. Applications interact with mongos routers as if querying a single database, while data is distributed across multiple shards for scalability and performance.