user4341632 user4341632 - 3 months ago 25
C# Question

What are the downsides to Request throttling using delay (C# .Net 4 Web Server)

We are running a Http Api and want to be able to set a limit to the number of requests a user can do per time unit. When this limit has been reached, we don't want the users to receive errors, such as Http 429. Instead we want to increase the response times. This has the result that the users can continue to work, but slower, and can then choose to upgrade or not upgrade its paying plan. This solution can quite easily be implemented using Thread.sleep (or something similar) for x number of seconds, on all requests of a user that has passed its limit.

We think that in worst case there might be a problem with the number of possible connections for a single server, since as long as we keep delaying the response, we keep a connection open, and therefore limiting the number of possible other connections.

All requests to the Api is running asynchronously. The Server itself is built to be scalable and is running behind a load balancer. We can start up additional servers if necessary.

When searching for this type of throttling, we find very few examples of this way of limiting the users, and the examples we found seemed not concerned at all about connections running out. So we wonder is this not a problem?

Are there any downsides to this that we are missing, or is this a feasible solution? How many connections can we have open simultaneously without starting to get problems? Can our vision be solved in another way, that is without giving errors to the user?

Answer

Thread.Sleep() is pretty much the worst possible thing you can do on a web server. It doesn't matter that you are running things asynchronously because that only applies to I/O bound operations and then frees the thread to do more work.

By using a Sleep() command, you will effectively be taking that thread out of commission for the time it sleeps.

ASP.Net App Pools have a limited number of threads available to them, and therefore in the worst case scenario, you will max out the total number of connections to your server at 40-50 (whatever the default is), if all of them are sleeping at once.

Secondly

This opens up a major attack vector in terms of DOS. If I am an attacker, I could easily take out your entire server by spinning up 100 or 1000 connections, all using the same API key. Using this approach, the server will dutifully start putting all the threads to sleep and then it's game over.

UPDATE

So you could use Task.Delay() in order to insert an arbitrary amount of latency in the response. Under the hood it uses a Timer which is much lighter weight than using a thread.

await Task.Delay(numberOfMilliseconds);

However...

This only takes care of one side of the equation. You still have an open connection to your server for the duration of the delay. Because this is a limited resource it still leaves you vulnerable to a DOS attack that wouldn't have normally existed.

This may be an acceptable risk for you, but you should at least be aware of the possibility.