node-synchronized

Ensure that some code always executes exclusively, in the order it is called

Examples

This is particularly useful if you have some asynchronous code that should not be executing concurrently, e.g. where the next call relies on the result of the previous call. (Also see last example below.)

  var synchd = require('synchronized');

  MyObject.prototype.fetchedDocument = function(id, cb) {
    var self = this;

    synchd(self._documentCache, function(done){
      // Return cached response if available
      if (self._documentCache[id]) {
        return done(null, self._documentCache[id]);
      }

      // Otherwise fetch
      Model.findById(id, function(err, document){
        // ...

        self._documentCache[id] = document;
        done(null, document);
      });
    }, cb);
  }

A factory for when you're passing multiple asynchronous functions to a flow-control library method, such as async.parallel(), and you have a dependency or other reason for wanting a subset of the executed functions not be called concurrently.

  async.parallel({
    company: synchd.fn(this.companyId, function(done){
      person.fetchedCompany(done);
    }),

    // ... Other functions

    companyManager: synchd.fn(this.companyId, function(done){
      person.fetchedCompany(function(err, company){
        if (err) etc.

        done(null, company.manager);
      });
    })
  }, function(err, results){
    if (err) etc.

    // ...
  });

For the case of having a local cache to some expensive asynchronous call.

  var myObj = { id: 'abc', localCache: null, localCachedAt: null };

  myObj.lookup = synchd.cachedFn(function scopeLookup() {
    return this;
  }, function cacheLookup(cb, cont) {
    // Return cached response if available
    if (this.localCachedAt) return cb(null, this.localCache);

    cont();
  }, function remoteLookup(cb) {
    var self = this;

    // Otherwise fetch
    Model.findById(self.id, function(err, document){
      // ....

      self.localCachedAt = new Date();
      self.localCache = document;
      done(null, document);
    });
  })

  myObj.lookup(function(err, document){ console.log(myObj.localCachedAt) });
  myObj.lookup(function(err, document){ console.log(myObj.localCachedAt) });

Prints:

  Tue Sep 16 2014 15:17:35 GMT+0100 (BST)
  Tue Sep 16 2014 15:17:35 GMT+0100 (BST)

Usage

synchd(scopeObj, fn, done)

• scopeObj - An object or string as scope, or function returning either of these
• fn(cb) - The function to call one at a time per scope, first argument is the callback to call on completion, e.g. function(done) { process.nextTick(done) }
• done - (Optional) Called when execution of the provided function has completed, called with same arguments as provided callback

synchd.fn(scopeObj, fn)

• scopeObj - (Optional) As above, default: null (global)
• fn(..., cb) - cb as above. Other arguments passed from newFn.

Returns:

• newFn(..., cb) - First arguments passed to fn, last argument callback

synchd.cachedFn(scopeObj, cacheLookupFn, fn)

• scopeObj - (Optional) As above, default: null (global)
• cacheLookupFn(..., cb, cont) Must call either cb or cont, where cb is completion callback (as above) and call fn. Other arguments passed from newFn.
• fn(..., cb) - cb as above. Other arguments passed from newFn.

Returns:

• newFn(..., cb) - First arguments passed to fn, last argument callback

Note: if newFn is called in context of an object, this will be the context fn and cacheLookupFn.

Installation

Inspiration

License