How to retry a Polly rate limit when the rate limit has exceeded?

Question

I've got the following policy setup:

// Slack api for postMessage is generally 1 per channel per second.
// Some workspace specific limits may apply.
// We just limit everything to 1 second.
var slackApiRateLimitPerChannelPerSecond = 1;

var rateLimit = Policy.RateLimitAsync(slackApiRateLimitPerChannelPerSecond, TimeSpan.FromSeconds(slackApiRateLimitPerChannelPerSecond),
    (retryAfter, _) => retryAfter.Add(TimeSpan.FromSeconds(slackApiRateLimitPerChannelPerSecond)));

This should:

• Rate limit requests till 1 req/s
• Retry when rate limited

I can't wrap my head around wrapping this into a second policy that would retry...

I could retry this like so:

try
{
   _policy.Execute(...)
}
catch(RateLimitedException ex)
{
   // Policy.Retry with ex.RetryAfter
}

But that does not seem right.

I'd like to retry this a couple (3?) times so the method is abit more resilient - how would i do that?

Answer 1

You can omit the factory and wrap the rate-limiting policy into another one:

var ts = TimeSpan.FromSeconds(1);
var rateLimit = Policy.RateLimit(1, ts);
var policyWrap = Policy.Handle<RateLimitRejectedException>()
    .WaitAndRetry(3, _ => ts) // note that you might want to use more advanced back off policy here 
    .Wrap(rateLimit);
policyWrap.Execute(...);

If you want to respect the returned RetryAfter then try-catch approach is way to go, based on the documentation example:

public async Task SearchAsync(string query, HttpContext httpContext)
{
    var rateLimit = Policy.RateLimitAsync(20, TimeSpan.FromSeconds(1), 10);

    try
    {
        var result = await rateLimit.ExecuteAsync(() => TextSearchAsync(query));

        var json = JsonConvert.SerializeObject(result);

        httpContext.Response.ContentType = "application/json";
        await httpContext.Response.WriteAsync(json);
    }
    catch (RateLimitRejectedException ex)
    {
        string retryAfter = DateTimeOffset.UtcNow
            .Add(ex.RetryAfter)
            .ToUnixTimeSeconds()
            .ToString(CultureInfo.InvariantCulture);

        httpContext.Response.StatusCode = 429;
        httpContext.Response.Headers["Retry-After"] = retryAfter;
    }
}

UPD

There is WaitAndRetry overload with sleepDurationProvider which also passes the exception, so it can be used for the Wrap approach:

var policyWrap = Policy.Handle<RateLimitRejectedException>()
    .WaitAndRetry(5, 
        sleepDurationProvider: (_, ex, _) => (ex as RateLimitRejectedException)?.RetryAfter.Add(TimeSpan.From....) ?? TimeSpan.From...,
        onRetry:(ex, _, i, _) => { Console.WriteLine($"retry: {i}"); }) 
    .Wrap(rateLimit);