How to delay Laravel jobs and listeners within database transactions
Tags: laravel, php, programming
Published: 2023-03-30 07:07:10

As a Laravel developer, you may have encountered issues with jobs and listeners fired within database transactions. These issues can lead to `ModelNotFoundException`, inconsistencies in your data, and other problems that can affect the reliability of your application. In this blog post, we will explore why database transactions are important, common issues that can arise when using them, and how to properly handle jobs and listeners within transactions.

## Why use database transactions?

Database transactions allow you to group multiple database operations into a single atomic unit. If any part of the transaction fails, all changes are rolled back, ensuring that your database remains in a consistent state. In Laravel, you can use transactions to execute multiple database queries within a single transaction by using the `DB::transaction` method. Here's an example:

```php
use Illuminate\Support\DB;

DB::transaction(function () {
    // Perform database queries here
});
```

Alternatively, to have more control you can also use a non-callback method of controlling database transactions:

```php
use Illuminate\Support\Facades\DB;

DB::beginTransaction();

try {
    // Perform database operations here
    DB::commit();
} catch (\Exception $e) {
    DB::rollback();
    // Handle the exception here
}
```

But for the sake of this article, we will stick to the first option - using callbacks.

## Common issues with database transactions

While database transactions are crucial for maintaining data consistency in Laravel, there are some common issues that developers may encounter when using them, particularly with jobs and listeners. In this section, we will discuss these issues in more detail.

### ModelNotFoundException in queued jobs

Jobs firing with models that have never been saved because of rollbacks, a.k.a. `ModelNotFoundException` in jobs.

Consider a scenario where you use a job to create a new user and add some records to the database. However, if the transaction rolls back due to an error, the job may be dispatched with incomplete or non-existent data. This can cause the `ModelNotFoundException` exception to be thrown, as the job will attempt to access a model that has never been saved. This can be particularly problematic if the job is responsible for sending important notifications or performing other critical actions.

### Event listeners performing actions that cannot be rolled back

Suppose you have a listener that performs an external API call to sync data with a third-party service. If the transaction is rolled back, this action cannot be undone, leading to data inconsistencies between your application and the external service. This can cause significant problems, especially if the external service is business-critical.

## The solutions

Laravel has great and easy solutions for both problems to ensure that the jobs, or listeners, are not executed until after the database transaction is committed.

Let's have a look!

### Dispatching a Laravel job after a transaction is committed

To ensure that jobs are only dispatched after the transaction has been committed, you can use the `afterCommit` method. This method will only dispatch the job after the transaction has been committed successfully. Here's an example:

```php
DB::transaction(function () use ($data) {
    // Perform database queries here

    dispatch(new MyJob($data))->afterCommit();
    
    // alternatively, if the job uses the Dispatchable trait:
    // MyJob::dispatch($data)->afterCommit();

    // Perform other operations that could potentially fail
    // and roll back the transaction.
});
```

Alternatively, for more control, you can also use a `DB::afterCommit()` method to provide a callback that will run after the transaction is committed:

```php
DB::transaction(function () use ($data) {
    // Perform database queries here

    DB::afterCommit(function () {
        dispatch(new MyJob($data));
    });

    // Perform other operations that could potentially fail
    // and roll back the transaction.
});
```

### Delaying a Laravel event listener to run after a transaction is committed

To ensure that listeners are only executed after the transaction has been committed, you can set the `$afterCommit` property on your listener. This method will delay the execution of the listener until the transaction has been committed successfully. Here's an example:

```php
class SendNotificationListener
{
    public $afterCommit = true;

    public function handle(MyEvent $event)
    {
        // Send notification email here
    }
}
```

It does not matter whether the listener is synchronous or asynchronous (queued, implementing `ShouldQueue` interface) - Laravel will only execute this event **after** the database transaction is committed.

Very convenient!

---

Handling jobs and listeners within database transactions requires careful consideration to ensure that your application's data remains consistent. By using the `afterCommit` method to dispatch jobs later, and the `$afterCommit = true` property on listeners to delay them until after the transaction has been committed, you can avoid common issues such as `ModelNotFoundException` and external API calls that cannot be rolled back. Using these methods you can ensure that your Laravel application operates reliably and consistently.

Hope you learned something new today!