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[WIP]: One Way to write your own Task Queue

One way to write your own Task Queue

I was reading a Hacker News post a while back about why you should implement a Task Queue - I’m not going to go there as I think Dan Palmer’s article and the original hacker news thread

Since it’s a and rewrite - I imagine one could fork one of the Codecrafters Challenges and put together

One could read the source code on your own. Despite this , I decide to write his was for my own learning and I hope that it’d make reading faster for you too

An Overview of the application as a whole

Soft timeouts -> The soft timeouts allow the task to catch an exception to clean up before it is killed. Hard timeouts -> Similar to soft timeouts but hard timeouts aren’t catch-able and forcefully terminates the task. SIG_DFL -> This is a default signal which can be used in place over a generic signal SIG_CHLD -> Child signs

Breakdown of a task

This is enabled by pubsub on the broker which allows signals to be propagated

Breakdown of code within worker

queues_by_name -> Passed to tasks so we can quickly identify the queues that tasks are associated to

What’s a ZRANGEPOP

To save you the hassle of looking it up here’s the corresponding documentation for ZREMRANGEBYSCORE and ZRANGEBYSCORE. Keys are 1-indexed

What happens within the scheduler

How are tasks scheduled?

Let’s take a look at the run function.

While starting off we have no upcoming tasks.

  1. Advances the schedules of all cronjobs to the next interval
  2. Find the next closest task which will be executed
  3. Add all tasks with same minute precision to the queue and sleep until then

On second pass, we push all tasks to the corresponding queue

Runs at minute precision Here’s how the scheduler would look like.

Waitpid, does what you’d expect to do

State being passed around -> Queues by name,

What does the core architecture look like

Minute level task scheduler

How the ETA parameter helps with scheduling

Each queue has a corresponding eta queue which can be used as an override at each cycle the parent does a check for eta tasks and dumps them at the front of the queue

Redis LPush:

Insert all the specified values at the head of the list stored at key

Redis ZAdd:

Adds all the specified members with the specified scores to the sorted set stored at key. It is possible to specify multiple score / member pairs. https://redis.io/commands/zadd/

Redis BLPop:

BLPOP is a blocking list pop primitive. It is the blocking version of LPOP because it blocks the connection when there are no elements to pop from any of the given lists.

Additional Resources

https://github.com/tomarrell/miniqueue https://github.com/beanstalkd/beanstalkd https://github.com/rq/rq

Resources: https://www.slideshare.net/bryanhelmig/task-queues-comorichweb-12962619