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Recipe: Distributed Delay Queue

caution

We recommend that you do NOT use ZooKeeper for Queues. Please see Tech Note 4 for details.

Description​

An implementation of a Distributed Delay Queue. A Delay Queue is similar to a Priority Queue. When items are added to the queue, a delay value is given. The item will not be sent to a consumer until the time elapses.

Participating Classes​

  • QueueBuilder
  • QueueConsumer
  • QueueSerializer
  • DistributedDelayQueue

Creating a DistributedDelayQueue​

// Parameters:
// client - the curator client
// consumer - message consumer
// serializer - serializer to use for items
// queuePath - path to store queue
public static <T> QueueBuilder<T> builder(
    CuratorFramework client,
    QueueConsumer<T> consumer,
    QueueSerializer<T> serializer,
    java.lang.String queuePath
);
QueueBuilder<MessageType> builder = QueueBuilder.builder(client, consumer, serializer, path);
// ... more builder method calls as needed ...
DistributedDelayQueue<MessageType> queue = builder.buildDelayQueue();

General Usage​

The queue must be started via the start() method. Call close() when you are done with the queue.

To add messages to the queue:

queue.put(aMessage, delayUntilEpoch);

The consumer (QueueConsumer.consumeMessage()) will get called as messages arrive. delayUntilEpoch is a future epoch (milliseconds) when this item will be available to consumers.

Lock Safety​

In the general usage case, the message is removed from the queue prior to the consumer being called. A more atomic mode is provided that removes the item from the queue only after the consumer successfully returns. To enable this mode, call the lockPath() method of the Queue Builder. This uses a lock to make the message recoverable. A lock is held while the message is being processed - this prevents other processes from taking the message. The message will not be removed from the queue until the consumer functor returns. Thus, if there is a failure or the process dies, the message will get sent to another process. There is a small performance penalty for this behavior however.

Data Format​

Same as Distributed Queue.

Error Handling​

The QueueConsumer class extends ConnectionStateListener. When the queue is started, it adds the listener to the Curator instance. Users of the DistributedPriorityQueue must pay attention to any connection state changes.

If the SUSPENDED state is reported, the instance must assume that, until it receives a RECONNECTED state, the queue is no longer being updated. If the LOST state is reported, the queue is permanently down.