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Ost

Redis based queues and workers.

Ost Cafe, by Arancia Project

Description

Ost makes it easy to enqueue object ids and process them with workers.

Say you want to process video uploads. In your application you will have something like this:

Ost[:videos_to_process].push(@video.id)

Then, you will have a worker that will look like this:

require "ost"

Ost[:videos_to_process].each do |id|
  # Do something with it!
end

Usage

Ost uses a lightweight Redis client called Redic. To connect to a Redis database, you will need to set an instance of Redic, with a URL of the form redis://:<passwd>@<host>:<port>/<db>.

You can customize the connection by calling Ost.redis=:

require "ost"

Ost.redis = Redic.new("redis://127.0.0.1:6379")

Then you only need to refer to a queue for it to pop into existence:

require "ost"

Ost.redis = Redic.new("redis://127.0.0.1:6379")

Ost[:rss_feeds] << @feed.id

Ost defaults to a Redic connection to redis://127.0.0.1:6379. The example above could be rewritten as:

require "ost"

Ost[:rss_feeds] << @feed.id

A worker is a Ruby file with this basic code:

require "ost"

Ost[:rss_feeds].each do |id|
  # ...
end

It will pop items from the queue as soon as they become available. It uses BRPOPLPUSH with a timeout that can be specified with the OST_TIMEOUT environment variable.

Note that in these examples we are pushing numbers to the queue. As we have unlimited queues, each queue should be specialized and the workers must be smart enough to know what to do with the numbers they pop.

Available methods

Ost[:example].push item, Ost[:some_queue] << item: add item to the :example queue.

Ost[:example].pop { |item| ... }, Ost[:example].each { |item| ... }: consume item from the :example queue. If the block doesn't complete successfully, the item will be left at a backup queue.

Ost.stop: halt processing for all queues.

Ost[:example].stop: halt processing for the example queue.

Failures

Ost stores in-process items in backup queues. That allows the developer to deal with exceptions in a way that results adequate for his application.

There is one backup queue for each worker, with the following convention for naming the key in Redis: given a worker using the :events queue, running in the hostname domU-12-31-39-04-49-C7 with the process id 28431, the key for the backup queue will be ost:events:domU-12-31-39-04-49-C7:28431.

Here's the explanation for each part:

  • ost: namespace for all Ost related keys.
  • events: name of the queue.
  • domU-12-31-39-04-49-C7: hostname of the worker.
  • 28431: process id of the worker.

Priorities

There's no concept of priorities, as each queue is specialized and you can create as many as you want. For example, nothing prevents the creation of the :example_high_priority or the :example_low_priority queues.

Differences with Delayed::Job and Resque

Both Delayed::Job and Resque provide queues and workers (the latter using Redis). They provide dumb workers that process jobs, which are specialized for each task. The specialization takes place in the application side, and the job is serialized and pushed into a queue.

Ost, by contrast, just pushes numbers into specialized queues, and uses workers that are subscribed to specific queues and know what to do with the items they get. The total sum of logic is about the same, but there's less communication and less data transfer with Ost.

Installation

$ gem install ost
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