Why use a Message Queue?
When building an application, there usually comes a time when it makes sense to add a Message Queue. You can get pretty far with a RDMS or a It happened here at Iron.
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It’s extremely difficult to predict, at the start of a project, what the future needs of the project will be. By introducing a layer in between processes, message queues create an implicit, data-based interface that both processes implement. This allows you to extend and modify these processes independently, by simply ensuring they adhere to the same interface requirements.
Sometimes processes fail when processing data. Unless that data is persisted, it’s lost forever. Queues mitigate this by persisting data until it has been fully processed. The put-get-delete paradigm, which many message queues use, requires a process to explicitly indicate that it has finished processing a message before the message is removed from the queue, ensuring your data is kept safe until you’re done with it.
Because message queues decouple your processes, it’s easy to scale up the rate with which messages are added to the queue or processed; simply add another process. No code needs to be changed, no configurations need to be tweaked. Scaling is as simple as adding more power.
Elasticity & Spikability
When your application hits the front page of Hacker News, you’re going to see unusual levels of traffic. Your application needs to be able to keep functioning with this increased load, but the traffic is an anomaly, not the standard; it’s wasteful to have enough resources on standby to handle these spikes. Message queues will allow beleaguered components to struggle through the increased load, instead of getting overloaded with requests and failing completely. Check out our Spikability blog post for more information about this.
When part of your architecture fails, it doesn’t need to take the entire system down with it. Message queues decouple processes, so if a process that is processing messages from the queue fails, messages can still be added to the queue to be processed when the system recovers. This ability to accept requests that will be retried or processed at a later date is often the difference between an inconvenienced customer and a frustrated customer.
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The redundancy provided by message queues guarantees that a message will be processed eventually, so long as a process is reading the queue. On top of that, IronMQ provides an only-delivered-once guarantee. No matter how many processes are pulling data from the queue, each message will only be processed a single time. This is made possible because retrieving a message "reserves" that message, temporarily removing it from the queue. Unless the client specifically states that it's finished with the message, the message will be placed back on the queue to be processed after a configurable amount of time.
In a lot of situations, the order with which data is processed is important. Message queues are inherently ordered, and capable of providing guarantees that data will be processed in a specific order. IronMQ guarantees that messages will be processed using FIFO (first in, first out), so the order in which messages are placed on a queue is the order in which they'll be retrieved from it.
In any non-trivial system, there are going to be components that require different processing times. For example, it takes less time to upload an image than it does to apply a filter to it. Message queues help these tasks operate at peak efficiency by offering a buffer layer--the process writing to the queue can write as fast as it’s able to, instead of being constrained by the readiness of the process reading from the queue. This buffer helps control and optimize the speed with which data flows through your system.
Understanding Data Flow
In a distributed system, getting an overall sense of how long user actions take to complete and why is a huge problem. Message queues, through the rate with which they are processed, help to easily identify under-performing processes or areas where the data flow is not optimal.
A lot of times, you don’t want to or need to process a message immediately. Message queues enable asynchronous processing, which allows you to put a message on the queue without processing it immediately. Queue up as many messages as you like, then process them at your leisure.
UPDATE: We posted a related article recently on the uses of a worker system titled Top 10 Uses of IronWorker (although the uses can apply to any worker system/task queue). They often work in concert with message queues and are used for background processing, schedule jobs, event processing, as a mobile compute cloud, and for many other uses.
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