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    2.0.3 • Public • Published

    Better Lock

    A (better) node.js lock library.

    Travis tests


    • Typescript-ready
    • Named and keyed locks
    • Queue and execution timeouts
    • Queue size limit
    • Lock on multiple keys
    • Extended stack traces
    • Promise and callback interface
    • JSDoc annotations
    • Good error messages
    • Unit tests, good code coverage
    • No dependencies


    npm install --save better-lock
    # or
    yarn add better-lock


    Full docs:

    Minimal example
    const lock = new BetterLock();
    try {
      const res = await lock.acquire(async () => {
        // Inside the lock. It will stay closed until the promise you return resolves or rejects.
        await doSomeAsyncTask();
        return 'my result';
      // Outside the lock. You will get whatever the promise chain has returned.
      console.log(res); // "my result"
    catch (err) {
      // Either your or BetterLock's error
    Advanced example
    const lock = new BetterLock({
      name: 'FileLock',                  // To be used in error reporting and logging
      log: winstonLogger.debug,          // Give it your logger with appropeiate level
      wait_timeout: 1000 * 30,           // Max 30 sec wait in queue
      execution_timeout: 1000 * 60 * 5,  // Time out after 5 minutes
      queue_size: 1,                     // At most one pending job
    async function processFile(filename) {
      try {
        const result = await lock.acquire(filename, async () => {
          const appended = await appendToFile(filename);
          return updateDb(appended);
        return {
          status: true,
      catch (err) {
        if (err instanceof BetterLock.QueueOverflowError) {
          // The job was discarded
          return {
            status: false
        if (err instanceof BetterLock.ExecutionTimeoutError) {
          winstonLogger.warn('Potential swallowed callback! Stack trace to the entry site:', err.stack);
        throw err;
    Locking on multiple keys
    const userLock = new BetterLock({
      name: 'User lock',
      executionTimeout: 1000 * 60 * 60, // Note you can also use camelCase
    function transferBetweenUsers(fromId, toId, amount) {
      userLock.acquire([fromId, toId], () => {
        return Promise.all([
        ]).then(([fromUser, toUser]) => {
          fromUser.amount -= amount;
          toUser.amount += amount;
          return Promise.all([
      }).then(() => {
        console.log('Transfer completed');
    Using callback interface
    const BetterLock = require('better-lock');
    const lock = new BetterLock();
    lock.acquire(done => {
      // Inside the lock
      doMyAsyncStuffHere((err) => {
        // Call done when done
    }, (err, result) => {
      // Outside the lock
      if (err) {
        // Either your or BetterLock's error

    You can see a bunch more usage examples in the spec file, here;


    • new BetterLock(options)
      Create a new instance of BetterLock. Options should match interface BetterLockOptions. See below for details.

    • BetterLock.acquire([key], executor, [callback], [jobOptions])
      The main method you'll want to call. For each key, given executor will be called only one at a time. If you don't provide callback, it will return a promise that will be resolved with whatever executor returns.

      • key: Arbitrary string under which to lock. It allows you to use the same lock instance for multiple parallel concerns. Eg. this might be a database record id or filename.
      • executor: Function that will be called within the lock. This function should have one of two forms.
        1. Without arguments, in which case it should return a promise. Lock will remain locked until the promise resolves.
        2. With single done argument. In this case, the executor should call done(err, res) once it is done. Arguments passed to done will be passed to the callback of the lock.
      • callback: Optional callback that will be called once executor exits. Results from executor (resolved/rejected value or arguments given to done) will be passed along. This can be used in addition to the returned promise.
      • jobOptions: An object that should match interface BetterLockJobOptions. A subset of main options that will serve as overrides for this particular job (for example, timeout settings).
    • BetterLock.canAcquire([key])
      Returns true if given key can be acquired.

    • BetterLock.abort([key])
      Abort all jobs for a given key (or from the default job queue, if no key is given). Job executors will not be called. Callbacks will be called with JobAbortedError. Currently executing job will not be interrupted.

    • BetterLock.abortAll()
      Abort all jobs for all keys. This is suitable to be called during shutdown of your app.


    All available options can be seen here.

    BetterLockOptions are provided when you construct a lock instance. A subset of options given in LockJobOptions can be provided when you call lock.acquire, as the last argument.


    lock.acquire(executor, callback, {
      wait_timeout: 1000

    Most commonly used options are:

    • wait_timeout
      How long can jobs wait in queue before timing out (ms). Null to disable timeout.

    • execution_timeout
      How long can a job be executing before timing out (ms). Null to disable timeout. If you do that, though, and you have a swallowed callback, the lock can remain locked permanently.

    • queue_size
      Max queue size for waiting jobs.

    Default options are a static member DEFAULT_OPTIONS on the BetterLock class. During runtime, you can change the defaults like this:

    import BetterLock from 'better-lock';
    BetterLock.DEFAULT_OPTIONS.wait_timeout = 1000;

    When to use

    This library is a good fit if:

    • You need a local lock in your node.js application.
    • You have some advanced needs, like multiple keys per lock, timeouts, aborting...
    • You like good error messages, with full stack traces.
    • You like good types, either through typescript or JSDoc comments

    The library is not a good fit if:

    • You need lock reentrancy.
      There is no good solution for this in node.js that I know of. And this library doesn't offer any.

    • You need a shared lock between different nodes
      This library is a single process only. If you need to coordinate multiple apps or services, you need a different library.

    Change log

    2.0.3 (2022/05/28)

    Introduce generated API docs. Remove beta tag from npm version.

    2.0.0 (2021/05/30)

    Major update. The entire library was rewritten in typescript, so you should now get typings in most editors. We also had to switch tests from mocha + chai to jest (easier ts integration).

    The API and features have remained largely the same, just a bit of a refresh.

    Non-breaking and internal changes:

    • We now export all error names as a type script type. A few other types as well.
    • Errors now have name parameter, which matches these names.

    Breaking changes:

    • You can no longer use camel case versions of external-facing objects. Eg. you can no longer pass waitTimeout instead of wait_timeout. In retrospect, this was a pretty flaky API to maintain.

    • Internal LockJob class is no longer exported.

    • Also, errors no longer expose internal LockJob instances (err.job). We now instead provide the most important fields from the job (id and keys).

    • Error names have been renamed to have BetterLock prefix. Eg. WaitTimeoutError -> BetterLockWaitTimeoutError. This will influence and err.message parameters. The idea here is, if you see BetterLock error in the wild, you will know what generated it.

    • We have renamed BetterLock.BetterLockError to BetterLock.BaseError and BetterLock.BetterLockInternalError to BetterLock.InternalError to better match the naming scheme.

    • Since Options are no longer a class but interface, we are no longer exporting them under BetterLock.Options. You can do import {BetterLockOptions} from 'better-lock'; to get the typescript type.

    1.0.1 (2019/01/28)

    Handle empty key list

    1.0.0 (2019/01/28)

    Major version bump.

    • Added multi-key locks and refactored a bunch of internals.
    • Removed OVERFLOW_STRATEGIES and related options, which is mostly the reason for the major version bump. The library should otherwise work the same.

    0.3.1 (2018/10/01)

    Updated CI to use the current node versions (0.8 & 0.10). Older node versions should continue to work, but are no longer tested. Also, README updates.

    0.3.0 (2018/10/01)

    Can abort jobs waiting in queue.

    0.2.1 (2018/09/27)

    • Better and customizable Promise detection.
    • Restored DEFAULT_OPTIONS.

    0.2.0 (2018/09/27)

    Code reformat, better pattern for loading options. No feature upgrades.

    0.1.1 (2018/06/04)

    You can now use a Number as job name


    Fork, then git clone. The project is already set up with a WebStorm project, if that's your cup of tee.

    To run tests, with coverage:

    npm run test

    If you want to contribute, create a branch off master, do your work and then make a pull request against master. Unit tests would be appreciated.




    npm i better-lock

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