Question

Is there a way to assure FIFO (first in, first out) behavior with Task Queues on GAE?

GAE Documentation says that FIFO is one of the factors that affect task execution order, but the same documentation says that “the system’s scheduling may ‘jump’ new tasks to the head of the queue” and I have confirmed this behavior with a test. The effect: my events are being processed out of order.

Docs says:

https://developers.google.com/appengine/docs/java/taskqueue/overview-push

The order in which tasks are executed depends on several factors:

The position of the task in the queue. App Engine attempts to process tasks based on FIFO > (first in, first out) order. In general, tasks are inserted into the end of a queue, and
executed from the head of the queue.

The backlog of tasks in the queue. The system attempts to deliver the lowest latency
possible for any given task via specially optimized notifications to the scheduler.
Thus, in the case that a queue has a large backlog of tasks, the
system’s scheduling may “jump” new tasks to the head of the queue.

The value of the task’s etaMillis property. This property specifies the
earliest time that a task can execute. App Engine always waits until
after the specified ETA to process push tasks.

The value of the task’s countdownMillis property. This property specifies the minimum
number of seconds to wait before executing a task. Countdown and eta
are mutually exclusive; if you specify one, do not specify the other.

What I need to do? In my use case, I’ll process 1-2 million events/day coming from vehicles. These events can be sent at any interval (1 sec, 1 minute or 1 hour). The order of the event processing has to be assured. I need process by timestamp order, which is generated on a embedded device inside the vehicle.

What I have now?

1. A Rest servlet that is called by the consumer and creates a Task (Event data is on payload).

2. After this, a worker servlet get this Task and:

   • Deserialize Event data;

   • Put Event on Datastore;

   • Update Vehicle On Datastore.

So, again, is there any way to assure just FIFO behavior? Or how can I improve this solution to get this?

Answer 1

Ok. This is how I’ve done it.

1) Rest servlet that is called from the consumer:

    If Event sequence doesn't match Vehicle sequence (from datastore)

        Creates a task on a "wait" queue to call me again

    else

       State validation

       Creates a task on the "regular" queue (Event data is on payload).


2) A worker servlet gets the task from the "regular" queue, and so on... (same pseudo code)

This way I can pause the “regular” queue in order to do a data maintenance without losing events.

Thank you for your answers. My solution is a mix of them.