如何在多线程环境中实现有限的API调用？

Question

I have a lot of requests to send to a server (Amazon Product Advertising API), however this server doesn't allow me to send more than 1 request each second. I am looking for a free solution, based on C# abilities.

I tried to code it like this:

public static WebResponse Request(Dictionary<string, string> parameters) {
  var response = LaunchRequest(parameters);
  Sleep(1000);
  return response;
}

The problem is that multiple threads are entering in the method "Request" at the same time. If there are 4 threads, then there will be 4 requests per second.

How can my threads wait each other ?

EDIT: I tried with a lock.

So I wrote this code :

public class RequestHandler {
  private static readonly RequestHandler instance = new RequestHandler();

  private Object thisLock = new Object();

  private RequestHandler() { }

  public static RequestHandler Instance {
    get {
      return instance;
    }
  }

  public WebResponse Request(Dictionary<string, string> parameters) {
    lock (thisLock) {
      Log();
      var response = LaunchRequest(parameters);
      Sleep(1000);
      return response;
    }
  }
}

So I call my method like this :

// ...
RequestHandler requestHandler = RequestHandler.Instance;
WebResponse response = requestHandler.Request(requestHelper, parameters);
// ...

It seems to work most of the time, but sometimes, my "LaunchRequest" method is fired almost at the same time.

EDIT 2: Here are the log results, the arrows are showing the calls where there is less than 1 second:

Request: 09:52:50.230  - Thread 22
Request: 09:52:48.830  - Thread 5
Request: 09:52:47.468  - Thread 10 <---
Request: 09:52:47.331  - Thread 13 <---
Request: 09:52:45.971  - Thread 12
Request: 09:52:44.767  - Thread 11
Request: 09:52:43.230  - Thread 5
Request: 09:52:30.546  - Thread 21 <--- 
Request: 09:52:30.357  - Thread 20 <---
Request: 09:52:29.232  - Thread 13
Request: 09:52:27.908  - Thread 11
Request: 09:52:26.471  - Thread 5
Request: 09:52:25.138  - Thread 11
Request: 09:52:23.835  - Thread 12

Answer 1

Create an SQS queue. In your controller, put the data to the queue. Create a .net service which would poll the queue and process messages one-by-one, ensuring that only one message is sent at any given time.

UPD Not sure why you'd tag the post as AWS if you don't like AWS services, but still.

Instead of SQS you can use any of the varying queues available, eg MSMQ, RabbitMQ etc. The cost would be in the need to install and maintain those yourself.

Absolute worst case scenario, use a ConcurrentQueue type from C#, and have a background thread running that will pop from that queue. That will ensure that only one message is processed at a time. However, this solution is not resilient as the app pool recycling would wipe your data.

Answer 2

Store the last call details in session or a local variable. Then you can call sleep if there is a call.

public static WebResponse Request(Dictionary<string, string> parameters) 
{
    lock (thisLock)
    {
        string lastCalled = Session["LastCalledTime"] as string;
        if (!string.IsNullOrEmpty(lastCalled) && DateTime.Parse(lastCalled) >= DateTime.Now.AddSeconds(-1))
        {
            Sleep(1000);
        }
        var response = LaunchRequest(parameters);
        Session["LastCalledTime"] = DateTime.Now.ToString("O");
    }

    return response;
 }

Added lock to make it threadsafe as highlighted by trailmax .