solidqueue: A solid and safe (stateful and crash-safe) queue with a simple interface

solidqueue can be used either on synchronous and asynchronous ways. It saves the queue state on a file and supports journaling.

Synchronous:

var
    SolidQueue = require('solidqueue'),
    queue = new SolidQueue({file:file.db",sync:true}),
	rv;

queue.push(123);
rv = queue.shift();
console.log("Got: ",rv.data);
rv.ack();

Available methods (synchronous mode):

• push(value): Pushes an item into the queue. Throws an error if something unexpected happens.
• shift(): Picks an item from the queue and returns an object like {data:XX,ack:fn()} where data is the previously pushed item and ack is the acknowledge function. Throws an error if something unexpected happens.

Available events (synchronous mode):

• ready: Fired when the queue is ready.
• error: Fired when some error happens. The event handler is called with two arguments, the phase on what the error happened and the error itself.

Asynchronous:

var
    SolidQueue = require('solidqueue'),
    queue = new SolidQueue({file:"file.db"});

queue.on('ready',function(){
    return queue.push({bla:"bli"},function(err){
        if ( err ) {
            console.log("Error pushing item to queue: ",err);
            return;
        }

        return queue.shift(function(err,item,ack){
            if ( err ) {
                console.log("Error shifting item from queue: ",err);
                return;
            }
            console.log("Got: ",item);
            ack(function(err,ok){
                if ( err ) {
                    console.log("Error acknowledging item ",item,": ",err);
                    return process.exit(-1);
                }
                return process.exit(0);
            });
        });
    });
});

Constructor options (asynchronous mode):

• file: Specifies the queue database file path.

• concurrentItems: The number of items that can be processed at the same time (the number of times that 'nextItem' event can be called before getting an acknowledge). Watch nextItem event bellow.

Available methods (asynchronous mode):

• push(value,callback): Pushes an item into the queue and calls the callback. The arguments of callback function are: (err).

• shift(callback): Picks an item from the queue and calls the callback. The arguments of the callback function are (err,value,ack) where item is the value previously pushed into the queue and ack is the acknowledge function that should be called after processing this queue item.

Available events (asynchronous mode):

• ready: Fired when the queue is ready.
• error: Fired when some error happens. The event handler is called with two arguments, the phase on what the error happened and the error itself.
• nextItem: Called when a new/next item is available for being processed, which is: after initialization if the queue is not empty, after an acknowledge if the queue is not empty and after a push() if the queue was empty.
• drain: Fired when the queue gets empty.
• free: Fired then the queue is empty and all the items got acknowledge.