Advanced pattern: Concurrent operations with async actor¶
Sometimes, we’d like to have IO operations to other actors/tasks/components (e.g., DB) periodically within an actor (long polling). Imagine a process queue actor that needs to fetch data from other actors or DBs.
This is problematic because actors are running within a single thread. One of the solutions is to use a background thread within an actor, but you can also achieve this by using Ray’s async actors APIs.
Let’s see why it is difficult by looking at an example.
class LongPollingActor: def __init__(self, data_store_actor): self.data_store_actor = data_store_actor def run(self): while True: data = ray.get(self.data_store_actor.fetch.remote()) self._process(data) def other_task(self): return True def _process(self, data): # Do process here... pass
There are 2 issues here.
Since a long polling actor has a run method that runs forever with while True, it cannot run any other actor task (because the thread is occupied by the while loop). That says
l = long_polling_actor.remote() # Actor runs a while loop l.run.remote() # This won't be processed forever because the actor thread is occupied by the run method. ray.get(l.other_task.remote())
Since we need to call ray.get within a loop, the loop is blocked until ray.get returns (it is because
ray.getis a blocking API).
We can make this better if we use Ray’s async APIs. Here is a documentation about ray’s async APIs and async actors.
First, let’s create an async actor.
class LongPollingActorAsync: def __init__(self, data_store_actor): self.data_store_actor = data_store_actor async def run(self): while True: # Coroutine will switch context when "await" is called. data = await data_store_actor.fetch.remote() self._process(data) def _process(self): pass async def other_task(self): return True
Now, it will work if you run the same code we used before.
l = LongPollingActorAsync.remote() l.run.remote() ray.get(l.other_task.remote())
Now, let’s learn why this works. When an actor contains async methods, the actor will be converted to async actors. This means all the ray’s tasks will run as a coroutine. That says, when it meets the
await keyword, the actor will switch to a different coroutine, which is a coroutine that runs
You can implement interesting actors using this pattern. Note that it is also possible to switch context easily if you use await
asyncio.sleep(0) without any delay.