Not-quite-barebones serial queue / lock.
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Very simple FIFO task runner (usable as a lock or serial queue), plus a callback-wrapping helper.


npm install fifolock


var q = require('fifolock')();

q.acquire(function (release) {
    setTimeout(function () {
        console.log("This task started first.");
    }, 1e3);

attemptSomething(function (e, d) {
    if (e) console.warn("Attempt happened to fail.", e);
    else console.log("Attempt happened to succeed,", d);

attemptSeveralSomethings(42, function (e) {
    if (e) console.warn("Got the inevitable error.", e);
    else console.log("Wow, that was improbable…");

q.acquire(function (done) {
    console.log("This task was started last.");

// callers of q.TRANSACTION_WRAPPER wait to acquire the queue just like any direct users
function attemptSomething(cb, _nested) { cb = q.TRANSACTION_WRAPPER(cb, function () {
    if (Math.random() > 0.75) process.nextTick(function () {
        cb(new Error("Chaos monkey struck!"));
    }); else setTimeout(function () {
        cb(null, "here's the data.");
    }, 0);

}, _nested); }

// advanced trick: sharing a single acquisition with nested/nestable helper routines
function attemptSeveralSomethings(n, cb) { cb = q.TRANSACTION_WRAPPER(cb, function () {
    function recursiveAttempt(i) {
        if (i < n) attemptSomething(function (e) {
            if (e) cb(new Error("Only succeeded at "+i+" of "+n+" attempts"));
            else recursiveAttempt(i+1);
        }, true);       // note how, unlike normal callers, we use our internal _nested parameter
        else cb(null);
}); }

This code will output something like:

This task started first. Attempt happened to succeed, here's the data. Got the inevitable error. [Error: Only succeeded at 8 of 42 attempts] This task was started last.


  • var q = fifolock(); — the module exports a factory function, call it to get a queue instance
  • q.acquire(task) — enqueues task to start when all previously-queued tasks have finished (or in the next tick if q was empty). When it is started, task will receive one argument e.g. (finish) which is a function it must call to release the lock and allow any next task to proceed.
  • cb = q.TRANSACTION_WRAPPER(cb, fn, [skipAcquisition]) — this method helps you ensure "normal" async logic happens in a consistent fashion. Before executing fn it waits to aquire a lock, but immediately returns a wrapper around cb which will forward the call while making sure the lock is properly released. The skipAcquisition argument is optional, intended to keep code that must work both within its own transactions, and under the auspices of another caller's already-acquired lock. (See example above.)
  • var CUSTOM_WRAPPER = q.TRANSACTION_WRAPPER.bind({postAcquire:preFn, preRelease:postFn}); — the wrapper method also allows you to provide postAcquire and/or preRelease hooks to bracket a transaction with additional custom logic. The postAcquire hook preFn(proceed) should do any addition setup require and then must call proceed. Note that while the hook must proceed it could signal an error condition by passing custom arguments to proceed; these will be forwarded to the main function(s) which would then need to participate in any such failure-handling protocol. The preRelease hook postFn(finish, [args, ctx]) should do any cleanup necessary and then must release the queue by calling finish. Note that the preRelease hook gets an array copy of the arguments which will be passed to the wrapped callback in case it affects cleanup, but cannot control what the callback receives. (See "examples/custom_wrapper.js" for a sample custom wrapper.)


© 2014 Nathan Vander Wilt. Funding for this work was provided in part by Technical Machine, Inc.

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