Implementing inter-process locks with child processes in Node.js

10 Oct 2023

When working with Node.js applications, it is common to encounter scenarios where multiple child processes need to access a shared resource concurrently. In such cases, it becomes essential to implement a mechanism to control access to this resource and prevent race conditions. One way to achieve this is by using inter-process locks.

Inter-process locks ensure that only one process can access the shared resource at a time, thereby preventing conflicts. In this article, we will explore how to implement inter-process locks with child processes in Node.js using the child_process module and the Cluster module.

Using the child_process module

The child_process module in Node.js allows us to create and manage child processes. We can utilize this module to implement inter-process locks as follows:

Create a lock file: Start by creating a lock file to indicate whether the resource is currently locked or not. A simple approach is to use a file on the file system.

const fs = require('fs');
const lockFilePath = './resource.lock';

function createLockFile() {
  fs.writeFileSync(lockFilePath, '');
}

function removeLockFile() {
  fs.unlinkSync(lockFilePath);
}

function lockFileExists() {
  return fs.existsSync(lockFilePath);
}

Acquiring the lock: Before accessing the resource, a child process should acquire the lock. If the lock file exists, the process waits until the lock is released.
```
function acquireLock() {
  while (lockFileExists()) {
    // Wait until the lock is released
  }
  createLockFile();
}
```
Releasing the lock: Once a child process has finished using the resource, it should release the lock by removing the lock file.
```
function releaseLock() {
  removeLockFile();
}
```

Using the lock:

function accessResource() {
  acquireLock();

  // Access the shared resource here

  releaseLock();
}

Using the Cluster module

The Cluster module in Node.js allows us to create a pool of child processes to handle incoming requests. We can leverage this module to implement inter-process locks by coordinating the lock acquisition and release among the child processes.

Here’s an example implementation using the Cluster module:

const cluster = require('cluster');
const numCPUs = require('os').cpus().length;

// Master process
if (cluster.isMaster) {
  // Function to acquire the lock
  function acquireLock(worker) {
    worker.send({ type: 'acquireLock' });
  }

  // Function to release the lock
  function releaseLock(worker) {
    worker.send({ type: 'releaseLock' });
  }

  // Create child processes
  for (let i = 0; i < numCPUs; i++) {
    const worker = cluster.fork();

    // Handle messages from child processes
    worker.on('message', (message) => {
      if (message.type === 'lockAcquired') {
        // Handle lock acquired event
      } else if (message.type === 'lockReleased') {
        // Handle lock released event
      }
    });
  }
}
// Child process
else {
  // Function to handle lock messages
  process.on('message', (message) => {
    if (message.type === 'acquireLock') {
      // Acquire the lock
      // Send lock acquired message back to the master process
      process.send({ type: 'lockAcquired' });
    } else if (message.type === 'releaseLock') {
      // Release the lock
      // Send lock released message back to the master process
      process.send({ type: 'lockReleased' });
    }
  });
}

The master process creates child processes using cluster.fork(), and each child process listens for messages from the master process using process.on('message'). With this setup, you can now acquire and release the lock by sending messages between the master and child processes.

Conclusion

Implementing inter-process locks is crucial when multiple child processes need to access a shared resource concurrently in a Node.js application. By utilizing the child_process or Cluster module, you can effectively prevent race conditions and ensure the resource is accessed in a controlled manner.

By adopting inter-process locks, you can enhance the performance and reliability of your Node.js applications, particularly in scenarios where shared resources are involved.

#nodejs #locking