Why use Queueing systems such as RabbitMQ

Question

I am not a senior programmer but I have been deploying applications for a while and devloped small complete systems.

I am starting to hear about queueing systems such as RabbitMQ. May be, I never developed any systems that had to use a queueing system. But, I am worried if I am not using it because I have no idea what to do with this. I have read RabbitMQ tutorial on their site but I am not sure why I would use this for. I am not sure if any of those cannot be achieved by conventional programming with no additional component and regular databases or similar.

Can someone please explain why I would use a queueing system with a small example. I mean not a hello world example, but a a practical scenario.

Thanks a lot for your time

• RM

Answer 1

One of the key uses of middleware like message queues is to be able to send data between non homogenous systems. The messages themselves can be many things. Strings are the easiest to be understood by different languages on different systems but are often less useful for transferring more meaningful data. As a result JSON and XML are very popular for the messages. These are just structured strings that can be converted into objects in the language of choice at the consumer end.

Additional useful features:

• In some MQ systems like RabbitMQ (not true in all MQ systems) is that the client handles the communication side of things very nicely.
• The messages can be asynchronous. If the consumer goes down, the messages will remain until the consumer is back online.
• The MQ system can be setup to varying degrees of message durability. They can be removed from the queue once read or remain until the are acknowledged. They can be persistent so even if the MQ systems goes down message will not be lost.

Here goes with some possibly contrived examples. A Java program on a local system wants to send a message to a system on the connected through the internet. The local system has a server connected to the internet. Everything is blocked coming from the internet except a connection to the MQ. The Java program can publish the message to the MQ with out needing access to the internet. The message sits on the queue until the external system picks it up. The Java program publishes a message, lets say XML, and the consumer could be a Perl program. As long as they have some way of understanding the XML with a predefined way of serialization and deserialization it will be fine.

Answer 2

MQ systems tend to work best in "fire-and-forget" scenarios. If an event happens and others need to be notified of it, but the source system has no need for feedback from the other systems, then MQ might be a good fit.

If you understand the pros and cons of MQ and still don't understand why it would be a good fit for a particular system, then it probably isn't. I've seen systems where MQ was used but not needed, and the result was not pretty.

Most of the scenarios I've seen where it's worked out well is integration between unrelated systems (usually out-of-the-box type system). Let's say you have one system that takes orders, and a different system that fills the orders and ships them. In that scenario, the order system can use a MQ to notify the fulfillment system of the order, but the order system has no interest in waiting until the fulfillment system receives the order. So it puts a message in a queue keep going.

Answer 3

This is a very simplified answer, but it gives the general ideas.

Let's think about this in terms of telephone vs. email. Pretend for a minute that email does not exist. To get work done, you must phone everyone. When you communicate with someone via telephone, you need to have them at their desk in order to reach them (assume they are in a factory and can't hear their cell phone ring) :-) If the person you wish to reach isn't at the desk, you are stuck waiting until they return your call (or far more likely, you call them back later). It's the same with you - you don't have any work to do until someone calls you up. If multiple people call at once, you don't know about it because you can only handle one person at a time.

However, if we have email, it is possible for you to "queue" your requests with someone else, to answer (but more likely ignore) at their convenience. If they do ignore your email, you can always re-send it. You don't have to wait for them to be at the desk, and they don't have to wait until you are off the phone. The workload evens out and things run much more smoothly. As an added bonus, you can forward messages that you don't want to deal with to your peons.

In systems engineering, we use the term "closely coupled" to define programs (or parts of programs) that work like the telephone scenario above. They depend very closely upon each other, often sharing implementations among various parts of the program. In these programs, data is processed in serial order, one at a time. These systems are typically easy to build, but there are a few important drawbacks to consider: (1) changing any part of the program likely will cause cascading changes throughout the code, and this introduces bugs; (2) the system is not very scalable, and typically must be scrapped and rebuilt as needs grow; (3) all parts of the system must be functioning simultaneously or the whole system will not work.

Basically, closely-coupled programs are good if the program is very simple or if there is some specialized reason to use a closely-coupled program.

In the real world, things are much more complex. Programs cannot be that simple, and it becomes a nightmare to develop enterprise applications in a closely-coupled manner. Therefore, we use the term "loosely-coupled" to define large systems that are composed of many smaller pieces. The pieces have very well-defined boundaries and functions, so that changing of the system may be accomplished more easily. It is the essence of object-oriented design. Message queues (like RabbitMQ) allow email-like communication to take place among various programs and parts of programs, thus making workflow much more like it would be with people. Adding extra capacity then becomes a simple matter of starting up and additional computer wherever you need it.

Obviously, this is a gross simplification, but I think it conveys the general idea. Building applications that use message queuing enables you to deploy massively scalable applications leveraging cloud service providers. Here is an article that talks about designing for the cloud: http://blogs.msdn.com/b/silverlining/archive/2011/08/23/designing-and-building-applications-for-the-cloud.aspx