Using SQS FIFO Queues

Tutorials

September 20th, 2021

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AWS's Simple Queue Service (SQS) is a great, high-scale, and extremely inexpensive way to get reliable queues.

One minor quibble with SQS is that you can't gurantee what order jobs will be processed in.

If there are 10 jobs in a queue, you don't know which job a queue worker will be given.

For many of us, that's just fine. If you need some jobs to be done in order, you can use Laravels Job Chaining.

However, that doesn't work in all cases.

For example, in Chipper CI, we need to process builds in the order we receive them. We have to wait on incoming webhooks that can come at any time, and so we can't simply add to a Job Chain. Instead rely on SQS FIFO queues.

FIFO Queues

FIFO stands for First In, First Out. That means that the oldest job will always be the first to be given to a queue worker.

The job "behind" the oldest job will not be given to a queue worker until the job before it is complete.

This is great when you need jobs to be processed in order, but this limits scaling queue workers.

If this was all there was to it, you'd never be able to use more than one queue worker nor process more than one job at a time (without creating and somehow managing multiple FIFO queues).

Message Group IDs

Luckily FIFO queues account for this. It turns out that messages order is only guaranteed within a Group ID.

Each job (message) in FIFO queues must have a Group ID. Any jobs that have the same Group ID will be processed in order they are received.

If you assigned a random Group ID to each job, then no order would be maintained as there would be no shared Group ID.

In Chipper CI, each Team can have multiple Projects. Each Project is a git repository that gets built when someone pushes an update to their git repository.

To keep builds in order, we use FIFO queues, creating a Group ID using the Team ID and Project ID. It looks like this:

1$build = App\Build::createFromWebhook($payload);
2 
3$groupId = sprintf('build-%s-%s', $build->project->team_id, $build->project_id);
4 
5dispatch(
6 (new ProcessBuild($build))
7 ->onMessageGroup($groupId)
8);

SQS FIFO Laravel Package

To help with this, we can use the shiftonelabs/laravel-sqs-fifo-queue Composer package.

This exends the SQS queue message type and adds some functionality.

Most importantly, it gives us the onMessageGroup() method, which allows us to add the message group ID when adding a job (message) to the SQS FIFO queue.

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Chris Fidao

Teaching coding and servers at CloudCasts and Servers for Hackers. Co-founder of Chipper CI.