Rate Limit

Controls the rate of the operations by specifying a maximum amount of executions within a given time window.

Configuration

// Create a new bot policy
var policy = new BotPolicy();

// Or with a return value
var policy = new BotPolicy<HttpResponseMessage>();

// Configure the policy to use rate limit on operations
policy.Configure(policyConfig => policyConfig
    .RateLimit(rateLimitConfig => rateLimitConfig
        .SlidingWindow(              // Set the rate limiter strategy
            5,                       // Set the amount of maximum allowed operations within the given time interval.
            TimeSpan.FromSeconds(10) // Set the time interval within only the given maximum amount of operations are allowed.
        )));

Available configuration options

• .SlidingWindow(int, TimeSpan) - Sets the underlying strategy to a sliding window rate limiter.
  • int: The maximum allowed operation count within the given time interval.
  • TimeSpan: The time interval within only the given maximum amount of operations are allowed.
• .FixedWindow(int, TimeSpan) - Sets the underlying strategy to a fixed time window rate limiter.
  • int: The maximum allowed operation count within the given time interval.
  • TimeSpan: The time interval within only the given maximum amount of operations are allowed.
• .UseStrategy(IRateLimiterStrategy) - Sets the underlying strategy to a custom rate limiter.

Custom Rate Limit strategies

The functionality of the Rate Limiter bot can be customized with a IRateLimiterStrategy implementation. It can gives you more control over the internal behavior by determining when should the rate of the executions limited.

public class CustomStrategy : IRateLimiterStrategy
{
    public bool ShouldLimit(out TimeSpan retryAfter)
    {
        // You can decide here that you allow the current operation or not.
    }
}

Then you can use your custom strategy by configuring the Botpolicy like this:

policy.Configure(policyConfig => policyConfig
    .RateLimit(rateLimitConfig => rateLimitConfig
        .UseStrategy(new CustomStrategy())));

Built-in Rate Limit strategies

• Fixed window: It counts the number of operations within a fix time window and resets the counters at constant points in time.
  Let's check an example where a configuration allows 2 requests within a given time window:

  ^ <- this sign means that an operation is executed on the timeline
  
  |=============|=============|=============|
  ^     ^       ^ 
                | <- the next point in time an operation allowed to execute
  |-------------| <- the fix sized window                     
  

  However that should be kept in mind that this strategy allows "bursts" to be formed:

  |=============|=============|=============|
  ^            ^ ^ ^  
               |---| <- 3 quick calls in a much less time window that we wanted to allow
  

  You should use the Sliding Window strategy if you want to avoid these bursts.

• Sliding window (used by default): Checks the relative time window related to the current request.
  The same example (2 requests allowed) with this strategy would look like this:

  |=============|=============|=============|
  ^     ^             ^ 
                      | <- the next point in time an operation allowed to execute 
        |-------------| <- the sliding window 
                       
  

When the configured rate limit is exceeded, the framework throws a custom RateLimitExceededException exception. From its RetryAfter property you can read the remaining time - as a TimeSpan - left until the limiter allows the next execution.