Workers

Loco provides the following options for background jobs:

• Redis backed (powered by sidekiq-rs)
• Postgres backed (own implementation)
• Tokio-async based (same-process, evented thread based background jobs)

You enqueue and perform jobs without knowledge of the actual background queue implementation, similar to Rails' ActiveJob, so you can switch with a simple change of configuration and no code change.

Async vs Queue

When you generated a new app, you might have selected the default async configuration for workers. This means workers spin off jobs in Tokio's async pool, which gives you proper background processes in the same running server.

You might want to configure jobs to run in a separate process backed by a queue, in order to distribute the load across servers.

First, switch to BackgroundQueue:

# Worker Configuration
workers:
  # specifies the worker mode. Options:
  #   - BackgroundQueue - Workers operate asynchronously in the background, processing queued.
  #   - ForegroundBlocking - Workers operate in the foreground and block until tasks are completed.
  #   - BackgroundAsync - Workers operate asynchronously in the background, processing tasks with async capabilities.
  mode: BackgroundQueue

Then, configure a Redis based queue backend:

queue:
  kind: Redis
  # Redis connection URI
  uri: ""
  dangerously_flush: false

Or a Postgres based queue backend:

queue:
  kind: Postgres
  # Redis connection URI
  uri: ""
  dangerously_flush: false

Running the worker process

You can run in two ways, depending on which setting you chose for background workers:

Usage: demo_app start [OPTIONS]

Options:
  -w, --worker                     start worker
  -s, --server-and-worker          start same-process server and worker

Choose --worker when you configured a real Redis queue and you want a process for doing just background jobs. You can use a single process per server. In this case, you can run your main Web or API server using just cargo loco start.

$ cargo loco start --worker # starts a standalone worker job executing process
$ cargo loco start # starts a standalone API service or Web server, no workers

Choose -s when you configured async background workers, and jobs will execute as part of the current running server process.

For example, running --server-and-worker:

$ cargo loco start --server-and-worker # both API service and workers will execute

Creating background jobs in code

To use a worker, we mainly think about adding a job to the queue, so you use the worker and perform later:

    // .. in your controller ..
    DownloadWorker::perform_later(
        &ctx,
        DownloadWorkerArgs {
            user_guid: "foo".to_string(),
        },
    )
    .await

Unlike Rails and Ruby, with Rust you can enjoy strongly typed job arguments which gets serialized and pushed into the queue.

Using shared state from a worker

See How to have global state, but generally you use a single shared state by using something like lazy_static and then simply refer to it from the worker.

If this state can be serializable, strongly prefer to pass it through the WorkerArgs.

Creating a new worker

Adding a worker meaning coding the background job logic to take the arguments and perform a job. We also need to let loco know about it and register it into the global job processor.

Add a worker to workers/:

#[async_trait]
impl BackgroundWorker<DownloadWorkerArgs> for DownloadWorker {
    fn build(ctx: &AppContext) -> Self {
        Self { ctx: ctx.clone() }
    }
    async fn perform(&self, args: DownloadWorkerArgs) -> Result<()> {
        println!("================================================");
        println!("Sending payment report to user {}", args.user_guid);

        // TODO: Some actual work goes here...

        println!("================================================");
        Ok(())
    }
}

And register it in app.rs:

#[async_trait]
impl Hooks for App {
//..
    async fn connect_workers(ctx: &AppContext, queue: &Queue) -> Result<()> {
        queue.register(DownloadWorker::build(ctx)).await?;
        Ok(())
    }
// ..
}

Generate a Worker

To automatically add a worker using loco generate, execute the following command:

cargo loco generate worker report_worker

The worker generator creates a worker file associated with your app and generates a test template file, enabling you to verify your worker.

Configuring Workers

In your config/<environment>.yaml you can specify the worker mode. BackgroundAsync and BackgroundQueue will process jobs in a non-blocking manner, while ForegroundBlocking will process jobs in a blocking manner.

The main difference between BackgroundAsync and BackgroundQueue is that the latter will use Redis to store the jobs, while the former does not require Redis and will use async within the same process.

# Worker Configuration
workers:
  # specifies the worker mode. Options:
  #   - BackgroundQueue - Workers operate asynchronously in the background, processing queued.
  #   - ForegroundBlocking - Workers operate in the foreground and block until tasks are completed.
  #   - BackgroundAsync - Workers operate asynchronously in the background, processing tasks with async capabilities.
  mode: BackgroundQueue

Testing a Worker

You can easily test your worker background jobs using Loco. Ensure that your worker is set to the ForegroundBlocking mode, which blocks the job, ensuring it runs synchronously. When testing the worker, the test will wait until your worker is completed, allowing you to verify if the worker accomplished its intended tasks.

It's recommended to implement tests in the tests/workers directory to consolidate all your worker tests in one place.

Additionally, you can leverage the worker generator, which automatically creates tests, saving you time on configuring tests in the library.

Here's an example of how the test should be structured:

#[tokio::test]
#[serial]
async fn test_run_report_worker_worker() {
    // Set up the test environment
    let boot = testing::boot_test::<App, Migrator>().await.unwrap();

    // Execute the worker in 'ForegroundBlocking' mode, preventing it from running asynchronously
    assert!(
        ReportWorkerWorker::perform_later(&boot.app_context, ReportWorkerWorkerArgs {})
            .await
            .is_ok()
    );

    // Include additional assert validations after the execution of the worker
}