mutual-exclusion (mutex)

Mutual Exclusion (mutex) object for JavaScript.

• Motivation
• API
  • Configuration
• Usage examples
  • Image server example
• Related libraries

Motivation

Promised based mutex allows to sequentially perform the same asynchronous operation. A typical use case example is checking if a resource exists and reading/ creating resource as an atomic asynchronous operation.

Suppose that you have a HTTP service that upon request downloads and serves an image. A naive implementation might look something like this:

router.use('/images/:uid', async (incomingMessage, serverResponse) => {
  const uid = incomingMessage.params.uid;

  const temporaryDownloadPath = path.resolve(
    downloadDirectoryPath,
    uid,
  );

  const temporaryFileExists = await fs.pathExists(temporaryDownloadPath);

  if (!temporaryFileExists) {
    try {
      await pipeline(
        // Some external storage engine.
        storage.createReadStream('images/' + uid),
        fs.createWriteStream(temporaryDownloadPath),
      );
    } catch (error) {
      await fs.remove(temporaryDownloadPath);

      throw error;
    }
  }

  fs
    .createReadStream(temporaryDownloadPath)
    .pipe(serverResponse);
});

In the above example, if two requests are made at near the same time, then both of them will identify that temporaryFileExists is false and at least one of them will fail. Mutex solves this problem by limiting concurrent execution of several asynchronous operations (see Image server example).

API

import {
  createMutex,
  HoldTimeoutError,
  WaitTimeoutError,,
} from 'mutual-exclusion';
import type {
  LockConfigurationInputType,
  MutexConfigurationInputType,
  MutexType,
} from 'mutual-exclusion';

/**
 * Thrown in case of `holdTimeout`.
 */
HoldTimeoutError;

/**
 * Thrown in case of `waitTimeout`.
 */
WaitTimeoutError;

const mutex: MutexType = createMutex(mutexConfigurationInput: MutexConfigurationInputType);

(async () => {
  // Lock is acquired.
  await mutex.lock(async () => {
    // Perform whatever operation that requires locking.

    mutex.isLocked();
    // true
  });

  // Lock is released.
  mutex.isLocked();
  // false
})()

Configuration

MutexConfigurationInputType (at the mutex-level) and LockConfigurationInputType (at the individual lock-level) can be used to configure the scope and timeouts.

/**
 * @property holdTimeout The maximum amount of time lock can be held (default: 30000).
 * @property key Used to scope mutex (default: a random generated value).
 * @property waitTimeout The maximum amount of time lock can be waited for (default: 5000).
 */
type MutexConfigurationInputType = {|
  +holdTimeout?: number,
  +key?: string,
  +waitTimeout?: number,
|};

type LockConfigurationInputType = {|
  +holdTimeout?: number,
  +key?: string,
  +waitTimeout?: number,
|};

Usage examples

Image server example

Motivation section of the documentation demonstrates a flawed implementation of an image proxy server. That same service can utilise Mutex to solve the illustrated problem:

router.use('/images/:uid', async (incomingMessage, serverResponse) => {
  const uid = incomingMessage.params.uid;

  const temporaryDownloadPath = path.resolve(
    downloadDirectoryPath,
    uid,
  );

  await mutex.lock(async () => {
    const temporaryFileExists = await fs.pathExists(temporaryDownloadPath);

    if (!temporaryFileExists) {
      try {
        await pipeline(
          // Some external storage engine.
          storage.createReadStream('images/' + uid),
          fs.createWriteStream(temporaryDownloadPath),
        );
      } catch (error) {
        await fs.remove(temporaryDownloadPath);

        throw error;
      }
    }
  }, {
    // Fail if image cannot be downloaded within 30 seconds.
    holdTimeout: 30000,

    // Restrict concurrency only for every unique image request.
    key: uid,

    // Fail subsequent requests if they are not attempted within 5 seconds.
    // This would happen if there is a high-concurrency or if the original request is taking a long time.
    waitTimeout: 5000,
  });

  fs
    .createReadStream(temporaryDownloadPath)
    .pipe(serverResponse);
});

Related libraries

• await-mutex – Similar implementation to mutual-exclusion, but without separation by key and timeouts.