I am planning to write an Asynchronous Trigger but I am not any documentation, if Async Trigger can run parallelly with other async process or it's added in queue based on availaible resources.
Salesforce Stack Exchange is a question and answer site for Salesforce administrators, implementation experts, developers and anybody in-between. It only takes a minute to sign up.Sign up to join this community
I think the only real indicator we have is this statement:
Unlike triggers on standard or custom objects, triggers on platform events don’t execute in the same Apex transaction as the one that published the event. The trigger runs asynchronously in its own process. As a result, there can be a delay between when an event is published and when the trigger processes the event.
But this doesn't tell us if these triggers share the same queue as Scheduleable/Queueable/future/Batchable calls, or if they're a separate queue. I'm pretty sure that asynchronous code would have a lower priority than these events, however, given that these events are still under synchronous limits.
Synchronous Governor Limits
When governor limits are different for synchronous and asynchronous Apex, the synchronous limits apply to platform event triggers. Asynchronous limits are for long-lived processes, such as Batch Apex and future methods. Synchronous limits are for short-lived processes that execute quickly. Although platform event triggers run asynchronously, they’re short-lived processes that execute in batches rather quickly.
This restriction on execution time means that they're expecting a lot of these events, and that they must be processed quickly. This would suggest a higher priority than the background tasks that have a lower urgency.
So, it's probably safe to say that, under heavy load, these triggers are likely to execute sooner than non-priority jobs from Batchable, Queuable, and future.