How many queues can I create?

[ericdude4]

Sorry, this is not an issue. I'm just wondering if someone from the Exq core team would be able to advise on how many queues can be created. I imagine this might come down to a Redis limitation? What is the upper limit for the number of queues I can create?

So far, my application is managing ~5,600 queues gracefully. How long can this scale for?

[ananthakumaran]

Exq polls redis for each and every queue independently using RPOPLPUSH. Let's say your poll_timeout is 50ms, it will poll redis 20 times per second, 20 * 5600 = 112000 ops. Redis usually can handle this, but there is a limit. I think up to 1 million you can easily scale, after that, it might get tricky. I am also talking about the job polling, it does a lot of other things. I would suggest you start looking at Redis ops per second metrics first, you can do a redis-cli monitor (not safe for production, run it locally) to get a rough idea about the commands executed by exq.

I think you are trying to design a queue per user? If the count could go up to 100k, I would suggest not to go this route. Also, Exq implementation itself might not be optimized. Most deployments I have seen are < 100 queues.

[akira]

Agreed with @ananthakumaran.

There's also some performance tests you can try here and adapt (we did some optimization a while back but not related to this case): https://github.com/akira/exq/blob/master/test/performance_test.exs
In the case you don't need stats, you can also disable those which would reduce the qps on Redis.

[ericdude4]

@akira @ananthakumaran Thank you for your insights on this. I appologize that I didn't respond sooner, but I have been thinking about this problem, especially as the application continues to grow and place more load on Exq. I am noticing more "gremlins" with regards to jobs being processed predictably. Sometimes, I notice that a queue which is subscribed to, simply doesn't execute the jobs within.

I have the following thought for a potential workaround to buy me some more time. Basically, out of the thousands of queues which exist in the application, only around 10 - 20 ever have any jobs queued up at a given time. I'm thinking that I can "prune" the queues which don't have any jobs every 5 minutes. Then, when I need to queue up a job for that client later on, the application will create the new queue and subscribe to it dynamically.

I expected I could make the "prune" function as follows:

def prune() do
  {:ok, queues} = Exq.Api.queue_size(Exq.Api)

  Enum.each(queues, fn {queue, jobs} ->
    if jobs == 0 do
      Exq.Api.remove_queue(Exq.Api, queue)
    end
  end)
end

The problem I am facing here though, is that redis-cli monitor still shows RPOPLPUSH commands being run for the queue, even after the queue has been removed. I also tried this with Exq.unsubscribe Exq.Api, "queue-name" but found the same result.

It seems like the queues remain in the Exq cache even after they are removed, causing the application to continue executing the RPOPLPUSH somehow.

[ericdude4]

Okay, quick update. After switching the above code to use Exq.unsubscribe Exq.Api, "queue-name", it seems to be working much better. However, I'm curious if you have any insight as to why this might be a bad idea? seems like I can keep my qps much lower on average if I run this worker every few minutes.

[ananthakumaran]

Exq.Api.remove_queue(Exq.Api, queue)

This is not necessary and might delete actual jobs. unsubscribe is all you need, though it needs to be run on all worker nodes

if I run this worker every few minutes.

From what I understand, you are delaying the job execution (after enqueue) by a few minutes for queues with infrequent jobs. If this is ok, then unsubscribe might work.

[ericdude4]

Hey @ananthakumaran, thank you for your thoughtful response. I enqueue jobs for users immediately based on incoming webhooks, with the following logic:

def enqueue_job(user) do
  # Check for an existing subscription for this user.
  {:ok, existing_subscriptions} = Exq.subscriptions(Exq)

  subscription_already_exists? = Enum.find(existing_subscriptions, &(&1 == user.name))

  unless subscription_already_exists? do
    # Subscribe to the queue if a subscription is not already in place.
    Exq.subscribe(Exq, user.name)
  end

  # Enqueue the job immediately, with the subscription is in place.
  Exq.enqueue(Exq, user.name, Foo.Worker, [])
end

def prune() do
  # Get list of queues along with their queue size
  {:ok, queues} = Exq.Api.queue_size(Exq.Api)

  {:ok, subscriptions} = Exq.subscriptions(Exq)

  Enum.each(queues, fn {queue, jobs} ->
    if queue in subscriptions and jobs == 0 do
      # If there are no jobs in the subscribed queue, unsubscribe from that queue
      Exq.unsubscribe(Exq, queue)
    end
  end)
end

The prune() worker gets run every 5 minutes, removing all queues which don't have any jobs. Enqueuing a job always checks for the presence of a queue, creating it if it doesn't exist.

I ran some tests and this really improved things a lot, since 99% of the queues which were created dynamically have 0 pending jobs at any given moment. This pruning approach keeps the ~10,000 subscriptions which were in place previously (making 20 RPOPLPUSH requests per second for each subscription) down below 100 - 200 subscriptions on average.

With this in mind, do you see anything which I might have failed to consider? Once again, I really appreciate your thoughts.

Eric

[ananthakumaran]

If you know when jobs are getting enqueued, then this approach would work, though I haven't given much thought about how it would play with multiple worker nodes.

Exq also has a Dequeue behaviour which can be overridden. This was added to support rate limiter (see https://github.com/ananthakumaran/exq_limit), you might be able to re-purpose it for your use case.

[nicnilov]

Please forgive my ignorance, but what is the reason Exq uses polling as opposed to Pub/Sub?

[ananthakumaran]

Pub/Sub is not persistent. There is a blocking variant of rpoplpush called brpoplpush, but it doesn't support multiple lists redis/redis#1785. Exq also needs to be compatible with Sidekiq, which restricts what data structures are allowed

If I were to write a Job processing library today, I would use redis streams, but that's not possible with Exq due to Sidekiq compatibility.

[nicnilov]

That makes sense, thanks!