@chriscdn/promise-semaphore

Limit or throttle the simultaneous execution of asynchronous code in separate iterations of the event loop.

Installing

Using npm:

npm install @chriscdn/promise-semaphore

Using yarn:

yarn add @chriscdn/promise-semaphore

Updating v1 to v2

Version 2 adds TypeScript and better inline documentation. The API remains the same, and doesn't introduce any breaking changes.

API

Create an instance

import Semaphore from "@chriscdn/promise-semaphore";
const semaphore = new Semaphore([maxConcurrent]);

The maxConcurrent parameter is optional, and defaults to 1 (making it an exclusive lock or binary semaphore). Use an integer value greater than one to limit how many times the code block can be simultaneously executing from separate iterations of the event loop.

Acquire a lock

semaphore.acquire([key]);

This returns a Promise, which resolves once a lock has been acquired. The key parameter is optional and permits the same Semaphore instance to be used in different contexts. See the second example.

Release a lock

semaphore.release([key]);

The release call should be executed from a finally block (whether using promises or a try/catch block) to guarantee it gets called.

Check if a lock can be acquired

semaphore.canAcquire([key]);

This method is synchronous, and returns true if a lock can be immediately acquired, false otherwise.

request function

const results = await semaphore.request(fn [,key])

This function reduces boilerplate when using acquire and release. It returns a promise, which resolves once fn has completed. It is functionally equivalent to:

try {
  await semaphore.acquire([key]);
  const results = await fn();
} finally {
  semaphore.release([key]);
}

See the examples below.

requestIfAvailable function

const results = await semaphore.requestIfAvailable(fn [,key])

This is functionally equivalent to:

const results = semaphore.canAcquire([key] ?
  await semaphore.request(fn, [key]) :
  null

This is useful in situations when only one instance of a function block should run, while discarding other attempts to execute the block. E.g., a button is repeatedly clicked.

Example 1

import Semaphore from "@chriscdn/promise-semaphore";
const semaphore = new Semaphore();

// using promises
semaphore
  .acquire()
  .then(() => {
    // This block executes once a lock is acquired.  If already locked,
    // then wait and execute once all preceeding locks have been released.
    //
    // do your critical stuff here
  })
  .finally(() => {
    // release the lock permitting the next queued block to continue
    semaphore.release();
  });

// or, using async/await
try {
  await semaphore.acquire();

  // do your critical stuff here
} finally {
  semaphore.release();
}

// or, using the request function
semaphore.request(() => {
  // do your critical stuff here
});

Example 2

Say you have an asynchronous function to download a file and save it to disk:

async function downloadAndSave(url) {
  const filePath = urlToFilePath(url);

  if (await pathExists(filePath)) {
    // the file is on disk, so no action is required
  } else {
    await downloadAndSaveToFilepath(url, filePath);
  }

  return filePath;
}

This works until a process calls downloadAndSave() multiple times in short succession with the same url. This can cause multiple simultaneous downloads that attempt to write to the same file at the same time.

This can be resolved with a Semaphore instance using the key parameter:

import Semaphore from "@chriscdn/promise-semaphore";
const semaphore = new Semaphore();

async function downloadAndSave(url) {
  try {
    await semaphore.acquire(url);

    // This block continues once a lock on url is acquired.  This
    // permits multiple simulataneous downloads for different urls.

    const filePath = urlToFilePath(url);

    if (await pathExists(filePath)) {
      // the file is on disk, so no action is required
    } else {
      await downloadAndSaveToFilepath(url, filePath);
    }

    return filePath;
  } finally {
    semaphore.release(url);
  }
}

Alternatively, this can be accomplished with the request function:

async function downloadAndSave(url) {

  return semaphore.request(() => {
    const filePath = urlToFilePath(url)

    if (await pathExists(filePath)) {
      // the file is on disk, so no action is required
    } else {
      await downloadAndSaveToFilepath(url, filePath)
    }

    return filePath
  }, url)

}

License

MIT