[FIXED] redis block until key exists

Issue

I'm new to Redis and was wondering if there is a way to be able to await geting a value by it's key until the key exists. Minimal code:

async def handler():
    data = await self._fetch(key)

async def _fetch(key):
    return self.redis_connection.get(key)

As you know, if such key doesnt exist, it return's None. But since in my project, seting key value pair to redis takes place in another application, I want the redis_connection get method to block untill key exists. Is such expectation even valid?

Solution

It is not possible to do what you are trying to do without implementing some sort of polling redis GET on your client. On that case your client would have to do something like:

async def _fetch(key):
    val = self.redis_connection.get(key)
    while val is None:
       # Sleep and retry here
       asyncio.sleep(1)  
       val = self.redis_connection.get(key)
    return val

However I would ask you to completelly reconsider the pattern you are using for this problem. It seems to me that what you need its to do something like Pub/Sub https://redis.io/topics/pubsub.

So the app that performs the SET becomes a publisher, and the app that does the GET and waits until the key is available becomes the subscriber.

I did a bit of research on this and it looks like you can do it with asyncio_redis:

• Subscriber https://github.com/jonathanslenders/asyncio-redis/blob/b20d4050ca96338a129b30370cfaa22cc7ce3886/examples/pubsub/receiver.py.

• Sender(Publisher): https://github.com/jonathanslenders/asyncio-redis/blob/b20d4050ca96338a129b30370cfaa22cc7ce3886/examples/pubsub/sender.py

Hope this helps.

Answered By - andrefsp