AsyncSemaphore is a utility type that helps limit the number of concurrent usages of a given resource,
e.g. limit the number of concurrent requests to a database.
This is especially useful for actor systems, where a cold-start of a system might bring thousands of actors back to life. If you were to allow all these actors to individually request a database, the database could easily become overloaded.
Using a semaphore for this is one way to deal with this problem.
Example code using Redis
var maxConcurrency = 10; var semaphore = new AsyncSemaphore(maxConcurrency); var multiplexer = ConnectionMultiplexer.Connect("localhost:6379"); var db = multiplexer.GetDatabase(); // ... // code that concurrently reads from redis // this might be inside a component called from within an actor // ..or any other concurrent data flow var res = await semaphore.WaitAsync( async () => //we are guaranteed to have max `maxConcurrency` concurrent calls here await db.StringGetAsync("some key"));
AsyncSemaphoreonly guards concurrency inside a single process. If your system uses scale-out, the semaphore will only guard calls within the process on each instance.
Related: For other approaches to deal with similar problems, see Collective reads and writes