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Azure Service Fabric - Actors and Reminders execution order

The main topic of this post is the order execution of requests that are coming to a Reliable Actor of Azure Service Fabric.

Actor and Single-Thread
Of the most important characteristic of an Actor is that is single-thread. This means that if you have 3 different requests that are coming to the Actor, the actor will execute them one at a time.
As we can see in the above example, while the actor execute the first request, it will not start executing the second request. All the requests will be added to an 'execution queue'. In this way, there will be only one thread executing in the same time on an actor.
This is why, all the functionality that is exposed by an actor is asynchronous and should all the time have a cancellation token. We might have cases when we want to cancel a request because of a timeout or other causes.

Reminders
On top of this we have Timers and Reminders that are used in combination with an Actor when we want to execute a specific action after a period of time. For example we have an actor that checks every 10 minutes if new content is available for him or we can have another actor that checks every 12 hours if the commands that are stored by him are expired or not.

Execution order
Now, in combination with the execution flows, comes the big question. If we already have Request 2 and Request 3 in the 'execution queue', when our reminder will be executed?

We might have a clean-up or check action that needs to execute every 10 minutes. If Request 2 and 3 takes to much, we might end up executing the Reminder action to late.

It is important to know that the priority of an action triggered by a Reminder is higher than a normal Request that arrives to an actor. Because of this, the Reminder action is executed immediately after the current request.

This means that even if we have Request 2 and Request 3 in the 'execution queue', the Reminder action will be executed once the Request 1 ends. Even if Request 2 and 3 arrived before the Reminder action, the Reminder will have a higher priority.

Conclusion
It is important to remember that:

  • Each request (action) is executed one at a time
  • If actor is executing another action, the second action will be putted in a queue until the first action is executed
  • The action that is triggered by a reminder will be putted in the 'execution queue' with a higher priority

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