fifolock
Very simple FIFO task runner (usable as a lock or serial queue), plus a callback-wrapping helper.
Installation
npm install fifolock
Example
var q = require('fifolock')();
q.acquire(function (release) {
setTimeout(function () {
console.log("This task started first.");
release();
}, 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.");
done();
});
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); }
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);
else cb(null);
}
recursiveAttempt(0);
}); }
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.
API
var q = fifolock();
— the module exports a factory function, call it to get a queue instanceq.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.)
License
© 2014 Nathan Vander Wilt.
Funding for this work was provided in part by Technical Machine, Inc.
Reuse under your choice of: