Firebase: Using .push() to maintain a queue and ensuring only one client is able to obtain the head of the queue at a time.

firebase_queue_pop.js

var Firebase = require("./firebase-node.js");

function Queue(ref) {
  this._ref = ref;
}

Queue.prototype.pop = function(cb) {
  this._ref.startAt().limit(1).once("child_added", this._pop.bind(this, cb));
}

Queue.prototype._pop = function(cb, snapshot) {
  var val = null;
  var self = this;

  // If the current head is empty just return.
  if (!snapshot.val()) {
    cb(null);
    return;
  }

  // The value returned by the transaction callback will be the new value.
  // If we return undefined, then the transaction will be cancelled.
  snapshot.ref().transaction(function(data) {
    // If the transaction succeded, val will still be null. Stash the value
    // and then delete it.
    if (!val) {
      val = data;
      return null;
    }
    // If the transaction failed (because some other process grabbed
    // the head of the queue first), just cancel this transaction and try again.
    // The next call to pop is asynchronous as recommended by node.js.
    process.nextTick(function() {
      self.pop(cb);
    });
    return;
  }, function(success, snapshot) {
    // The transaction succeded, just return the stashed value to process.
    if (success) {
      // node.js recommends making all callbacks asynchronous.
      // This prevents any blocking operations from holding up our queue and
      // also removes the possibility of recursion exhausting the scope chain.
      process.nextTick(function() {
        cb(val);
      });
    }
  });
}

Queue.prototype.push = function(val) {
  return this._ref.push(val);
}

var queue = new Queue(new Firebase("https://anant.firebaseio.com/queue/"));
queue._ref.once("value", function(val) {
  if (!val) {
    for (var i = 1; i <= 100; i++) {
      queue.push(i);
    }
  }

  console.log("Press return to fetch one item from head of queue.");
  var stdin = process.openStdin();
  stdin.on("data", function() {
    console.log("Fetching...");

    queue.pop(function(val) {
      if (val == null) {
        console.log("Failed!");
      } else {
        console.log("Success: " + val);
      }
    });
  });
});