Did you ever wish you could take a data structure you are familiar with and distribute it over grid? For example, why not take java.util.concurrent.BlockingDeque and add something to it on one node and poll it from another node? Or why not have a distributed primary key generator which would guarantee uniqueness on all nodes? Or how about a distributed java.util.concurrent.atomic.AtomicLong which can be updated and read from any node on the grid? GridGain gives you such capability. What GridGain did is actually take most of the data structures from java.util.concurrent framework and made sure they could be used in distributed fashion.

In this blog I want to show how flexible GridGain distributed queues are. On top implementing java.util.Collection interface and supporting different modes of operation, like collocated vs. non-collocated, or bounded vs. ubounded modes, you can actually control how elements are ordered within queues. GridGain supports FIFO, LIFO, and Priority based queues out of the box.

FIFO queues (first-in-first-out) are the most traditional queues where elements are added from the tail and polled form the queue head. LIFO queues (last-in-first-out) resemble more of stack features instead of queues. In LIFO queues elements are added and polled from the tail.

But the most interesting queue type is Priority queue where user can control the order of the elements. Priority queues order elements within the queue based on priority attribute specified by the user. Priority attribute of a queue element is annotated via @GridCacheQueuePriority annotation. Here is an example of how priority queue can be created and used.

public void priorityQueueExample() {
    Random rand = new Random();

    Grid grid = G.grid();

    // Initialize new unbounded collocated priority queue.
    GridCacheQueue<PriorityItem> queue = 
        grid.cache().queue("myqueue", PRIORITY);

    // Store 20 elements in queue with random priority.
    for (int i = 0; i < 20; i++) {
        int priority = rand.nextInt(20);

        queue.put(new PriorityItem(priority, "somedata-" + i));
    }

    PriorityItem item = null;

    int lastPriority = 0;

    do {
        item = queue.poll();

        // Ensure the elements are correctly ordered based on priority.
        assert lastPriority <= item.priority();

        lastPriority = item.priority();
    }
    while (item != null);
}

...

// Class defining sample queue element with its priority specified via
// @GridCacheQueuePriority annotation attached to priority field.
private static class PriorityItem implements Serializable {
    // Priority of queue item.
    @GridCacheQueuePriority
    private final int priority;

    private final String data;

    private SampleItem(int priority, String data) {
        this.priority = priority;
        this.data = data;
    }

    public int priority() {
        return priority;
    }
}

Read more about GridGain queues here.