MongoDB.Messaging - MongoDB Messaging Library

April 10, 2016

Overview

The MongoDB Messaging library is a lightweight queue pub/sub processing library based on MongoDB data store.

Features

Download

The MongoDB.Messaging library is available on nuget.org via package name MongoDB.Messaging.

To install MongoDB.Messaging, run the following command in the Package Manager Console

PM> Install-Package MongoDB.Messaging

NuGet Version

Concepts

Queue

A queue is equivalent to a MongoDB collection. The name of the queue will match the MongoDB collection name.

Queue names must be alphanumeric, without spaces or symbols.

It is a good practice to suffix the queue name with Queue.

Message

A message is the high level object that is a generic definition of a messages. The message contains processing level information. The message object is automatically created and updated by the Fluent API and should not be updated directly by the publisher or subscriber.

Data

Data is the message payload to be processed with the message. Use data to pass information you need to process the message.

The data object must be serializable by MongoDB driver.

It is a good practice to have one queue per data object being passed to limit confusion and to maintain simplicity when subscribing to the queue.

Publish

Publishing a message adds the message with the corresponding data to a queue for processing.

Subscribe

In order to process a message on a queue, an application needs to subscribe to a queue. There can be many subscribers to a queue to scale the load across processes. A subscriber can also set the worker count to scale the number of processing threads for that subscriber.

The framework ensures that only one subscriber can process a messages.

Queue Configuration

The queue configuration is used to set default values on messages published to a queue.

An example of using the fluent api to configure the sleep queue.

MessageQueue.Default.Configure(c => c
    .Connection("MongoMessaging")
    .Queue(s => s
        .Name(SleepMessage.QueueName)
        .Priority(MessagePriority.Normal)
        .ResponseQueue("ReplyQueueName")
        .Retry(5)
    )
);

Properties

Connection is the app.config connection string name used to connect to MongoDB.

Name is the name of the queue to configure.
Retry is the number of times the message should be retried on error. Set to zero, default, to not retry.
Priority is the default priority to publish the message with.
ResponseQueue is the name of the queue where responses should be sent.

Publish Message

To publish a message to a queue, use the fluent api.

var message = await MessageQueue.Default.Publish(m => m
    .Queue(SleepMessage.QueueName)
    .Data(sleepMessage)
    .Correlation("321B4671-3B4C-4B97-8E81-D6A8CF22D4F0")
    .Description("User friendly description of the message")
    .Priority(MessagePriority.Normal)
    .Retry(1)
);

Properties

Required
Queue is the name of the queue to publish to.
Data is the object to pass in the message. Used to process the message by the subscriber.

Optional
Correlation is an identifier used to link messages together.
Description is a user friendly description of the message.

Overrides
Retry is the number of times the message should be retried on error.
Priority is the default priority to publish the message with.
ResponseQueue is the name of the queue where responses should be sent.

Notes

Subscribe to Message

To subscribe to a queue, use the fluent api. The subscribe handler must implement IMessageSubscriber.

MessageQueue.Default.Configure(c => c
    .Connection("MongoMessaging")
    .Subscribe(s => s
        .Queue(SleepMessage.QueueName)
        .Handler<SleepHandler>()
        .Workers(4)
    )
);

To speed up processing, you can monitor the oplog for changes to trigger processing. The connection must have access to local.oplog.rs

MessageQueue.Default.Configure(c => c
    .Connection("MongoMessaging")
    .Subscribe(s => s
        .Queue(SleepMessage.QueueName)
        .Handler<SleepHandler>()
        .Workers(4)
        .Trigger()
    )
);

Properties

Required
Queue is the name of the queue to publish to.
Handler is the class that implements IMessageSubscriber. This is what processes the message.

Optional
Workers is the number of worker processes.
ExpireError is how long to keep error messages.
ExpireWarning is how long to keep warning messages.
ExpireSuccessful is how long to keep successful messages.
PollTime is the amount of time between work polling. If using Triggger, set to longer time. Retry is a class that implements IMessageRetry. IMessageRetry controls if an error message should be retried.
Timeout is the amount of time before a processing message times out.
TimeoutAction is how to handle timed out messages. Options are Fail or Retry.
Trigger to enable monitoring of the oplog for changes to trigger processing.

Message Service

In order for the message subscribers to process messages off queue, the MessageService needs to be created and Start called. Note, the MessageService.Stop() method tries to gracefully stop by waiting for active processes to finish.

_messageService = new MessageService();

// on service or application start
_messageService.Start();

// on service stop.  
_messageService.Stop();

IMessageSubscriber Interface

The following is a sample implementation of IMessageSubscriber

public class SleepHandler : IMessageSubscriber
{
    public MessageResult Process(ProcessContext context)
    {
        // get message data
        var sleepMessage = context.Data<SleepMessage>();

        // Do processing here

        return MessageResult.Successful;
    }

    public void Dispose()
    {
        // free resources
    }
}

IMessageRetry Interface

The IMessageRetry interface allows for customization of the retry of failed messages.

The following is the default implementation of IMessageRetry

public class MessageRetry : IMessageRetry
{
    public virtual bool ShouldRetry(ProcessContext processContext, Exception exception)
    {
        // get current message 
        var message = processContext.Message;

        // true to retry message
        return message.ErrorCount < message.RetryCount;
    }

    public virtual DateTime NextAttempt(ProcessContext processContext)
    {
        var message = processContext.Message;

        // retry weight, 1 = 1 min, 2 = 30 min, 3 = 2 hrs, 4+ = 8 hrs
        if (message.ErrorCount > 3)
            return DateTime.Now.AddHours(8);

        if (message.ErrorCount == 3)
            return DateTime.Now.AddHours(2);

        if (message.ErrorCount == 2)
            return DateTime.Now.AddMinutes(30);

        // default
        return DateTime.Now.AddMinutes(1);
    }
}

Process Locks

The library has supports distributed locks. The following are the lock types supported.

DistributedLock Distributed Lock manager provides synchronized access to a resources over a network
ThrottleLock Throttle Lock Manager controls how frequent a process can run

This is an example of using the DistributedLock.

var lockName = "PrintMessage";

// get MongoDB collection to store lock
var collection = GetCollection();

// create lock with timeout, max time it will wait for lock, of 5 minutes
var locker = new DistributedLock(collection, TimeSpan.FromMinutes(5));

// acquire lock; if can't, it will retry to get lock up to timeout value
var result = locker.Acquire(lockName);
if (!result)
    return; // acquire lock timeout

try
{
    // do processing here

}
finally
{
    // release lock
    locker.Release(lockName);
}

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