Retry & Backoff

[ph1ps]

No docs nor a pitch nor unit tests, yet. But I am looking for early feedback on the interface of the proposed algorithm.
Usage would look something like this:

// Example of when backoff is calculated by a client
let rng = SystemRandomNumberGenerator() // or a seeded rng for unit tests
var backoff = Backoff
  .exponential(factor: 2, initial: .seconds(5))
  .fullJitter(using: rng)
  .minimum(.seconds(2))
  .maximum(.seconds(100))

let response = try await retry {
  try await URLSession.shared.data(from: url)
} strategy: { error in
  return .backoff(backoff.nextDuration())
}

// Example of when backoff is returned by a server
let response = try await retry {
  let (data, response) = try await URLSession.shared.data(from: url)
  if
    let response = response as? HTTPURLResponse,
    response.statusCode == 429,
    let retryAfter = response.value(forHTTPHeaderField: "Retry-After")
  {
   throw TooManyRequestsError(retryAfter: Double(retryAfter)!)
  }
  return (data, response)
} strategy: { error in
  if let error = error as? TooManyRequestsError {
    return .backoff(.seconds(error.retryAfter))
  } else {
    return .stop
  }
}

(Of course nothing in this algorithm is limited to networking, in fact I have used this multiple times with Bluetooth implementations, but network code is more widely used, so I used this in my examples...)

Retry on an AsyncSequence could be a future direction, although I have not yet found a use case for this.

Thank you.

[phausler]

Generally this is an interesting idea of a set of algorithms to include, I think there are some ergonomics improvements that can be made and generally some API cleanup but it is an interesting set of additions.

[ph1ps]

@phausler Very valid and good points raised. Thank you for that. Exactly what I was hoping for.

[ph1ps]

@phausler Fully embracing stateful backoffs was a good call. I adapted the MR according to your feedback. Would you mind taking a look again? Thanks.

The only thing I am still unhappy with is that with this approach we have to copy the RNG. Can't come up with a way around this unfortunately.