June 17, 2024

Node.js is a server-side JavaScript runtime that uses an event-driven, non-blocking input-output (I/O) model. It’s widely recognized for building fast and scalable web apps. It also has a large community and a rich library of modules that simplify various tasks and processes.

Clustering enhances the performance of Node.js applications by enabling them to run on multiple processes. This technique allows them to use the full potential of a multi-core system.

This article takes a comprehensive look at clustering in Node.js and how it affects the performance of an application.

What is clustering?

By default, Node.js applications run on a single thread. This single-threaded nature means Node.js cannot use all the cores in a multi-core system — which most systems currently are.

Node.js can still handle multiple requests simultaneously by leveraging non-blocking I/O operations and asynchronous programming techniques.

However, heavy computational tasks can block the event loop and cause the application to become unresponsive. Consequently, Node.js comes with a native cluster module — irrespective of its single-threaded nature — to take advantage of the total processing power of a multi-core system.

Running multiple processes leverages the processing power of multiple central processing unit (CPU) cores to enable parallel processing, reduce response times, and increase throughput. This, in turn, improves the performance and scalability of Node.js applications.

How does clustering work?

The Node.js cluster module allows a Node.js application to create a cluster of concurrently running child processes, each handling a portion of the application’s workload.

When initializing the cluster module, the application creates the primary process, which then forks child processes into worker processes. The primary process acts as a load balancer, distributing the workload to the worker processes while each worker process listens for incoming requests.

The Node.js cluster module has two methods of distributing incoming connections.

  • The round-robin approach — The primary process listens on a port, accepts new connections and evenly distributes the workload to ensure no process is overloaded. This is the default approach on all operating systems except Windows.
  • The second approach — The primary process creates the listen socket and sends it to “interested” workers, which accept incoming connections directly.

Theoretically, the second approach — which is more complicated — should provide a better performance. But in practice, the distribution of the connections is very unbalanced. The Node.js documentation mentions that 70% of all connections end up in just two processes out of eight.

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