MongoDB sharded clusters provide a mechanism for horizontal scaling, distributing large datasets, and high throughput operations across multiple machines. This document outlines the key components of a MongoDB sharded cluster, their roles, the general workflow, and scenarios where a sharded cluster is the optimal choice.

Key Components

Application Server

The application server represents the client that interfaces with the MongoDB cluster. It sends requests for data retrieval or modification, which are then handled by the cluster.

Router (mongos)

The mongos acts as the query router in the MongoDB sharded cluster. It receives all operations from the application servers and routes them to the appropriate shards. mongos uses metadata, stored in the config servers, to determine the location of the data within the cluster.

Config Servers

Config servers are a set of three replicated servers that store the metadata of the MongoDB sharded cluster. This metadata includes information about the cluster’s composition, the configuration of shards, and the mapping of data across the shards. The config servers enable mongos to route operations to the correct shard.

Shards (Primary Shard)

Each shard in a MongoDB sharded cluster holds a subset of the sharded data. A primary shard is responsible for handling all write operations for its subset of data. Each primary shard typically represents a replica set to ensure redundancy and high availability.

Workflow

  1. Request Handling: The application server sends a request to the MongoDB cluster.
  2. Routing: The mongos router receives the request and consults the config servers to determine where the relevant data is located across the shards.
  3. Data Retrieval/Modification: mongos routes the request to the appropriate primary shard(s). If the operation is a read, the shard retrieves the data; if it’s a write, the data is modified.
  4. Response Aggregation: For operations that affect multiple shards, mongos compiles results from each relevant shard and aggregates them into a cohesive response.
  5. Response: The aggregated response is sent back to the application server, completing the operation.

Suitable Application Scenarios

A MongoDB sharded cluster is particularly well-suited for:

  • Large Data Volumes: When data size exceeds the capacity of a single machine, sharding enables MongoDB to distribute data across multiple servers.
  • High Throughput Requirements: Applications that require the ability to perform many operations per second can benefit from sharding, which allows concurrent operations across multiple shards.
  • Horizontal Scaling: For applications that need to scale out rather than up, adding more shards to a cluster can increase capacity without the need for higher-specification hardware.
  • Geographically Distributed Data: Sharded clusters can manage data distributed across different geographical locations, optimizing latency and regulatory compliance.

Conclusion

MongoDB sharded cluster

The MongoDB sharded cluster is a powerful architecture for managing very large datasets and workloads that exceed the hardware limitations of a single server. Its ability to distribute data and load across multiple servers makes it an ideal choice for data-intensive applications that require scalability, high availability, and robust performance.