Locker

Locker provides exclusive, stackable locking, to control concurrent access to shared resource(s).

var lock = Locker();

lock.when(async function myOperation(){
   // safely operate against shared resource(s),
   // such as global state variables, files, etc
});

Or:

var lock = Locker();

await lock.get();

// safely operate against shared resource(s),
// such as global state variables, files, etc
myOperation();

lock.release();

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Library Tests (Demo)

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Overview

The main purpose of Locker is to aid in managing asynchrony by gating an operation until an exclusive lock can be obtained. If an operation already holds that lock, subsequent requests to obtain the exclusive lock will stack on top of each other, each waiting in turn.

Essentially, this provides a strictly-sequential asynchronous queue, either explicitly -- with a lock.when().when()... chain -- or implicitly -- by gating subsequent, separate lock.get() calls.

Deployment / Import

npm install @byojs/locker

The @byojs/locker npm package includes a dist/ directory with all files you need to deploy Locker (and its dependencies) into your application/project.

Note: If you obtain this library via git instead of npm, you'll need to build dist/ manually before deployment.

Using a bundler

If you are using a bundler (Astro, Vite, Webpack, etc) for your web application, you should not need to manually copy any files from dist/.

Just import like so:

import Locker from "@byojs/locker";

The bundler tool should pick up and find whatever files (and dependencies) are needed.

Without using a bundler

If you are not using a bundler (Astro, Vite, Webpack, etc) for your web application, and just deploying the contents of dist/ as-is without changes (e.g., to /path/to/js-assets/locker/), you'll need an Import Map in your app's HTML:

<script type="importmap">
{
    "imports": {
        "locker": "/path/to/js-assets/locker.mjs"
    }
}
</script>

Now, you'll be able to import the library in your app in a friendly/readable way:

import Locker from "locker";

Note: If you omit the above locker import-map entry, you can still import Locker by specifying the proper full path to the locker.mjs file.

Locker API

The API provided by Locker is a single factory function, typically named Locker. This function takes no arguments, and produces independent exclusive-lock instances.

import Locker from "..";

var lockA = Locker();
var lockB = Locker();

Exclusive Lock instance API

Exclusive lock instances have 3 methods.

The when(..) method is the preferred and safest way to use Locker. It executes the passed-in function once an exclusive lock is obtained; the function can be normal/synchronous, or an async function. And once the function completes (normally, or abnormally with an exception), the lock is automatically released.

import Locker from "..";

var lock = Locker();

lock.when(async function doStuff(){
    // now ready to safely access shared resource(s)
});

Note: when() normalizes the invocation of the passed-in function to always be asynchronous; even if the lock is not currently held, a promise microtask tick passes before invocation.

To support externally cancelling a pending request for the lock, pass an options object (e.g., { signal: .. }) as the second argument to when(..), with an AbortSignal instance (from an AbortController instance).

For example, use a timeout signal to cancel the lock if it takes too long to obtain:

import Locker from "..";

var lock = Locker();

lock.when(doStuff,{ signal: AbortSignal.timeout(5000), });

Note: signal aborting has no effect on the doStuff() operation if it has already started; it only cancels obtaining the lock (which prevents doStuff() from starting).

Set a pass property on the options object (second argument), with an array of arguments to pass to the function when it's invoked:

import Locker from "..";

var lock = Locker();

lock.when(doStuff,{ pass: [ 1, 2, 3 ] });

when() returns the lock instance, so you can chain multiple when() calls together, conceptually expressing an asynchronous queue:

import Locker from "..";

var lock = Locker();

lock.when(doTaskA).when(doTaskB).when(doTaskC);

Note: when() does not return a promise, so by design there's no way to wait for the completion of the operation (or a chain of operations). If other parts of the application need to coordinate (wait for) a lock-bound task, those parts should be invoked inside the lock-bound task.

Lower-level mechanisms

As asserted earlier, the preferred and safest usage of Locker is with when(). However, sometimes it's more convenient to use two lower-level methods: get() and release():

• get(..): obtains an exclusive lock for this instance.

  If the lock is available, returns undefined (so as to not block an await unnecessarily). But if lock is already held, returns a promise that will resolve once the exclusive lock is obtained:

  import Locker from "..";
  
  var lock = Locker();
  
  await lock.get();
  
  // now ready to safely access shared resource(s)
  
  lock.release();

  Note: lock.release() should be called as soon as the lock obtained by lock.get() is no longer needed. See below for important clarifications.

  Pass true as an argument (or the options object { forceResolvedPromise: true }) to always return a promise; if lock is inactive, that promise will already be resolved. This safely enables promise chaining like lock.get(true).then(..), if desired:

  import Locker from "..";
  
  var lock = Locker();
  
  lock.get(true)
      // safe to call .then()
      .then(/* .. */)
      .finally(() => lock.release());

  To support externally cancelling a pending request for the lock, pass an options object (e.g., { signal: .. }) as the argument to get(..), with an AbortSignal instance (from an AbortController instance).

  For example, use a timeout signal to cancel the lock if it takes too long to obtain:

  import Locker from "..";
  
  var lock = Locker();
  
  try {
      await lock.get({ signal: AbortSignal.timeout(5000), });
  
      // ..
  
      lock.release();
  }
  catch (err) {
      // lock may have timed out
  }

  Note: If obtaining the lock is canceled by the signal, an exception will be thrown -- hence, the try..catch. In this case, lock.release() should not be called; doing so might prematurely release a subsequent hold on the lock (elsewhere in the app).

• release(): releases a current hold on the lock (if any); takes no input and has no return value. Has no effect if the lock is not currently active.

  import Locker from "..";
  
  var lock = Locker();
  
  await lock.get();
  
  // now ready to safely access shared resource(s)
  
  lock.release();

  Calls to lock.get() and lock.release() must always be paired together, and pairs of calls must never be nested. Otherwise, the application will experience deadlocks or other unexpected behavior.

  Further, take care not to call lock.release() unless its paired lock.get() call succeeded (no cancellation exception). Otherwise, this may prematurely release a subsequent hold on the lock (elsewhere in the app), causing bugs.

Re-building dist/*

If you need to rebuild the dist/* files for any reason, run:

# only needed one time
npm install

npm run build:all

Tests

Visit https://byojs.dev/locker/ and click the "run tests" button.

Run Locally (browser)

To instead run the tests locally in a browser, first make sure you've already run the build, then:

npm run test:start

This will start a static file webserver (no server logic), serving the interactive test page from http://localhost:8080/; visit this page in your browser and click the "run tests" button.

By default, the test/browser.js file imports the code from the src/* directly. However, to test against the dist/* files (as included in the npm package), you can modify test/browser.js, updating the /src in its import statements to /dist (see the import-map in test/index.html for more details).

Run Locally (CLI)

To run the tests on the CLI, first make sure you've already run the build, then:

npm test

By default, the test/cli.js file imports the code from the src/* directly. However, to test against the dist/* files (as included in the npm package), swap out which of the two import statements at the top of test/cli.js is commented out, to use the ./dist/locker.mjs import specifier.

License

All code and documentation are (c) 2024 locker and released under the MIT License. A copy of the MIT License is also included.