49 lines
3.2 KiB
Markdown
49 lines
3.2 KiB
Markdown
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# Asynchronism
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[Source: Intro to architecting systems for scale](http://lethain.com/introduction-to-architecting-systems-for-scale/#platform_layer)
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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
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- 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.
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## Task queues
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Tasks queues receive tasks and their related data, runs them, then delivers their results. They can support scheduling and can be used to run computationally-intensive jobs in the background.
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Celery has support for scheduling and primarily has python support.
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## Back pressure
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If queues start to grow significantly, the queue size can become larger than memory, resulting in cache misses, disk reads, and even slower performance. [Back pressure](http://mechanical-sympathy.blogspot.com/2012/05/apply-back-pressure-when-overloaded.html) can help by limiting the queue size, thereby maintaining a high throughput rate and good response times for jobs already in the queue. Once the queue fills up, clients get a server busy or HTTP 503 status code to try again later. Clients can retry the request at a later time, perhaps with [exponential backoff](https://en.wikipedia.org/wiki/Exponential_backoff) .
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## Disadvantage(s) : asynchronism
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- Use cases such as inexpensive calculations and realtime workflows might be better suited for synchronous operations, as introducing queues can add delays and complexity.
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## Source(s) and further reading
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- [It's all a numbers game](https://www.youtube.com/watch?v=1KRYH75wgy4)
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- [Applying back pressure when overloaded](http://mechanical-sympathy.blogspot.com/2012/05/apply-back-pressure-when-overloaded.html)
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- [Little's law](https://en.wikipedia.org/wiki/Little%27s_law)
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- [What is the difference between a message queue and a task queue?](https://www.quora.com/What-is-the-difference-between-a-message-queue-and-a-task-queue-Why-would-a-task-queue-require-a-message-broker-like-RabbitMQ-Redis-Celery-or-IronMQ-to-function) |