Recipe: Distributed Queue

caution

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

Description

An implementation of the Distributed Queue ZK recipe. Items put into the queue are guaranteed to be ordered (by means of ZK's PERSISTENTSEQUENTIAL node). If a single consumer takes items out of the queue, they will be ordered FIFO. If ordering is important, use a LeaderSelector to nominate a single consumer.

Participating Classes

• QueueBuilder
• QueueConsumer
• QueueSerializer
• DistributedQueue

Creating a DistributedQueue

// Parameters:
// client - the curator client
// consumer - functor to receive messages
// 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 ...
DistributedQueue<MessageType queue = builder.build();

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);

Your consumer (QueueConsumer.consumeMessage()) will get called as messages arrive.

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

The Distributed queue writes messages using this format:

Offset	Size	Description
0	4	Format version. Currently 0x00010001
4	1	Opcode: 0x01 = message, 0x02 = End of data
5	4	Message byte length
9	n	Message: serialized message bytes
9 + n	...	Next set of opcode-size-bytes until end of data

Error Handling

The QueueConsumer class extends ConnectionStateListener. When the queue is started, it adds the listener to the Curator instance. Users of the DistributedQueue 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.