Add Message queues section

README.md

@@ -1407,3 +1407,18 @@ Refresh-ahead can result in reduced latency vs read-through if the cache can acc
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 Asynchronous workflows help reduce request times for expensive operations that would otherwise be performed in-line.  They can also help by doing time-consuming work in advance, such as periodic aggregation of data.
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+### Message queues
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+Message queues receive, hold, and deliver messages.  If an operation is too slow to perform inline, you can use a message queue with the following workflow:
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+* An application publishes a job to the queue, then notifies the user of job status
+* A worker picks up the job from the queue, processes it, then signals the job is complete
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+The user is not blocked and the job is processed in the background.  During this time, the client might optionally do a small amount of processing to make it seem like the task has completed.  For example, if posting a tweet, the tweet could be instantly posted to your timeline, but it could take some time before your tweet is actually delivered to all of your followers.
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+**Redis** is useful as a simple message broker but messages can be lost.
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+**RabbitMQ** is popular but requires you to adapt to the 'AMQP' protocol and manage your own nodes.
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+**Amazon SQS**, is hosted but can have high latency and has the possibility of messages being delivered twice.