An example for using retry policy with dart http package

http_retry_duration_list_generator.dart

import 'dart:async';
import 'dart:math';

import 'package:http/http.dart' as http;
/// A utility class for generating a list of waiting times (delays) for HTTP request retries.
/// This class uses an exponential backoff strategy with jitter for calculating delay times.
class HttpRetryDurationListGenerator {

  /// Generates a list of waiting times (durations) to be used between retry attempts.
  ///
  /// This method calculates delay times based on exponential backoff strategy, which gradually increases
  /// the delay between retry attempts, helping to reduce the load on the server and increase the
  /// probability of a successful response in case of transient server issues.
  ///
  /// Parameters:
  ///   - `maxRetryCount`: The maximum number of retries for which to generate delay times.
  ///   - `baseDelay`: The initial delay time before the first retry attempt.
  ///   - `maxDelay`: The maximum allowable delay time. No calculated delay will exceed this value.
  ///   - `backoffFactor`: The factor by which the delay time is multiplied for each subsequent retry.
  ///
  /// Returns a list of `Duration` objects representing the delay times for each retry attempt.
  static List<Duration> generateWaitingTimes({
    required int maxRetryCount,
    required Duration baseDelay,
    required Duration maxDelay,
    required double backoffFactor,
  }) {
    assert(maxDelay >= baseDelay, 'maxDelay should be greater than or equal to baseDelay');

    final List<Duration> waitingTimes = [];
    Duration currentDelay = baseDelay;

    for (var i = 0; i < maxRetryCount; i++) {
      // Add the current delay to the list of waiting times.
      waitingTimes.add(currentDelay);
      currentDelay = _calculateNextDelay(
        backoffFactor: backoffFactor,
        currentDelay: currentDelay,
        maxDelay: maxDelay,
      );
    }
    return waitingTimes;
  }

  /// A private helper method to calculate the delay for the next retry attempt.
  ///
  /// This method applies the exponential backoff algorithm, which multiplies the current delay
  /// by the backoff factor, and adds a randomized jitter. The jitter helps to avoid the scenario
  /// where many clients retry simultaneously, causing spikes in load on the server.
  ///
  /// Parameters:
  ///   - `currentDelay`: The delay used in the last retry attempt.
  ///   - `backoffFactor`: The factor by which to multiply the current delay.
  ///   - `maxDelay`: The maximum allowable delay.
  ///
  /// Returns the calculated delay for the next retry attempt, ensuring it doesn't exceed `maxDelay`.
  static Duration _calculateNextDelay({
    required Duration currentDelay,
    required double backoffFactor,
    required Duration maxDelay,
  }) {
    // Calculates the jitter, which adds randomness to the delay times.
    // The jitter is a random duration up to one second, which helps in distributing
    // retry attempts over time when multiple clients are involved.
    final jitter = Duration(milliseconds: Random().nextInt(1000));

    // Calculate the next delay time by multiplying the current delay by the backoff factor
    // and adding the jitter. The resulting delay is compared with `maxDelay` to ensure it
    // doesn't exceed the maximum allowed delay.
    final nextDelay = (currentDelay * backoffFactor) + jitter;

    // Ensure the next delay doesn't exceed the maximum delay.
    return nextDelay < maxDelay ? nextDelay : maxDelay;
  }
}

retry_client_example.dart

RetryClient.withDelays(
        http.Client(),
        WoltHttpRetryDurationListGenerator.generateWaitingTimes(
          maxRetryCount: 3,
          baseDelay: const Duration(seconds: 1),
          maxDelay: const Duration(seconds: 15),
          backoffFactor: 2,
        ),
        when: (response) => response.statusCode >= 500 && response.statusCode == 429,
        onRetry: (request, response, retryCount) {
          Randall.d(
            'Retrying request ${request.url}\nresponse code: ${response?.statusCode}\nretry count: $retryCount',
          );
        },
      )