聊聊feign的Retryer

序

本文主要研究一下feign的Retryer

Retryer

feign-core-10.2.3-sources.jar!/feign/Retryer.java

public interface Retryer extends Cloneable {

  /**
   * if retry is permitted, return (possibly after sleeping). Otherwise propagate the exception.
   */
  void continueOrPropagate(RetryableException e);

  Retryer clone();

  class Default implements Retryer {

    private final int maxAttempts;
    private final long period;
    private final long maxPeriod;
    int attempt;
    long sleptForMillis;

    public Default() {
      this(100, SECONDS.toMillis(1), 5);
    }

    public Default(long period, long maxPeriod, int maxAttempts) {
      this.period = period;
      this.maxPeriod = maxPeriod;
      this.maxAttempts = maxAttempts;
      this.attempt = 1;
    }

    // visible for testing;
    protected long currentTimeMillis() {
      return System.currentTimeMillis();
    }

    public void continueOrPropagate(RetryableException e) {
      if (attempt++ >= maxAttempts) {
        throw e;
      }

      long interval;
      if (e.retryAfter() != null) {
        interval = e.retryAfter().getTime() - currentTimeMillis();
        if (interval > maxPeriod) {
          interval = maxPeriod;
        }
        if (interval < 0) {
          return;
        }
      } else {
        interval = nextMaxInterval();
      }
      try {
        Thread.sleep(interval);
      } catch (InterruptedException ignored) {
        Thread.currentThread().interrupt();
        throw e;
      }
      sleptForMillis += interval;
    }

    /**
     * Calculates the time interval to a retry attempt. <br>
     * The interval increases exponentially with each attempt, at a rate of nextInterval *= 1.5
     * (where 1.5 is the backoff factor), to the maximum interval.
     *
     * @return time in nanoseconds from now until the next attempt.
     */
    long nextMaxInterval() {
      long interval = (long) (period * Math.pow(1.5, attempt - 1));
      return interval > maxPeriod ? maxPeriod : interval;
    }

    @Override
    public Retryer clone() {
      return new Default(period, maxPeriod, maxAttempts);
    }
  }

  /**
   * Implementation that never retries request. It propagates the RetryableException.
   */
  Retryer NEVER_RETRY = new Retryer() {

    @Override
    public void continueOrPropagate(RetryableException e) {
      throw e;
    }

    @Override
    public Retryer clone() {
      return this;
    }
  };
}

• Retryer继承了Cloneable接口，它定义了continueOrPropagate、clone方法；它内置了一个名为Default以及名为NEVER_RETRY的实现
• Default有period、maxPeriod、maxAttempts参数可以设置，默认构造器使用的period为100，maxPeriod为1000，maxAttempts为5；continueOrPropagate方法首先判断attempt是否达到阈值，达到则抛出异常，否则进一步计算interval，然后进行sleep
• NEVER_RETRY的continueOrPropagate直接抛出异常，而clone方法直接返回当前实例

SynchronousMethodHandler

feign-core-10.2.3-sources.jar!/feign/SynchronousMethodHandler.java

final class SynchronousMethodHandler implements MethodHandler {
    //......

  public Object invoke(Object[] argv) throws Throwable {
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    Retryer retryer = this.retryer.clone();
    while (true) {
      try {
        return executeAndDecode(template);
      } catch (RetryableException e) {
        try {
          retryer.continueOrPropagate(e);
        } catch (RetryableException th) {
          Throwable cause = th.getCause();
          if (propagationPolicy == UNWRAP && cause != null) {
            throw cause;
          } else {
            throw th;
          }
        }
        if (logLevel != Logger.Level.NONE) {
          logger.logRetry(metadata.configKey(), logLevel);
        }
        continue;
      }
    }
  }


    //......
}

• SynchronousMethodHandler的invoke的方法首先使用retryer.clone()创建一个retryer，然后在捕获到RetryableException的时候，会执行retryer.continueOrPropagate(e)

RetryableException

feign-core-10.2.3-sources.jar!/feign/RetryableException.java

public class RetryableException extends FeignException {

  private static final long serialVersionUID = 1L;

  private final Long retryAfter;
  private final HttpMethod httpMethod;

  /**
   * @param retryAfter usually corresponds to the {@link feign.Util#RETRY_AFTER} header.
   */
  public RetryableException(int status, String message, HttpMethod httpMethod, Throwable cause,
      Date retryAfter) {
    super(status, message, cause);
    this.httpMethod = httpMethod;
    this.retryAfter = retryAfter != null ? retryAfter.getTime() : null;
  }

  /**
   * @param retryAfter usually corresponds to the {@link feign.Util#RETRY_AFTER} header.
   */
  public RetryableException(int status, String message, HttpMethod httpMethod, Date retryAfter) {
    super(status, message);
    this.httpMethod = httpMethod;
    this.retryAfter = retryAfter != null ? retryAfter.getTime() : null;
  }

  /**
   * Sometimes corresponds to the {@link feign.Util#RETRY_AFTER} header present in {@code 503}
   * status. Other times parsed from an application-specific response. Null if unknown.
   */
  public Date retryAfter() {
    return retryAfter != null ? new Date(retryAfter) : null;
  }

  public HttpMethod method() {
    return this.httpMethod;
  }
}

• RetryableException继承了FeignException，它的构造器会接收retryAfter，该参数可以为null

FeignException

feign-core-10.2.3-sources.jar!/feign/FeignException.java

public class FeignException extends RuntimeException {
    //......

  static FeignException errorReading(Request request, Response response, IOException cause) {
    return new FeignException(
        response.status(),
        format("%s reading %s %s", cause.getMessage(), request.httpMethod(), request.url()),
        cause,
        request.requestBody().asBytes());
  }

    //......
}

• FeignException定义了errorReading静态方法，它创建的是FeignException

ErrorDecoder

feign-core-10.2.3-sources.jar!/feign/codec/ErrorDecoder.java

public interface ErrorDecoder {
    //......

  public static class Default implements ErrorDecoder {

    private final RetryAfterDecoder retryAfterDecoder = new RetryAfterDecoder();

    @Override
    public Exception decode(String methodKey, Response response) {
      FeignException exception = errorStatus(methodKey, response);
      Date retryAfter = retryAfterDecoder.apply(firstOrNull(response.headers(), RETRY_AFTER));
      if (retryAfter != null) {
        return new RetryableException(
            response.status(),
            exception.getMessage(),
            response.request().httpMethod(),
            exception,
            retryAfter);
      }
      return exception;
    }

    private <T> T firstOrNull(Map<String, Collection<T>> map, String key) {
      if (map.containsKey(key) && !map.get(key).isEmpty()) {
        return map.get(key).iterator().next();
      }
      return null;
    }
  }

  static class RetryAfterDecoder {

    static final DateFormat RFC822_FORMAT =
        new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss 'GMT'", US);
    private final DateFormat rfc822Format;

    RetryAfterDecoder() {
      this(RFC822_FORMAT);
    }

    RetryAfterDecoder(DateFormat rfc822Format) {
      this.rfc822Format = checkNotNull(rfc822Format, "rfc822Format");
    }

    protected long currentTimeMillis() {
      return System.currentTimeMillis();
    }

    /**
     * returns a date that corresponds to the first time a request can be retried.
     *
     * @param retryAfter String in
     *        <a href="https://tools.ietf.org/html/rfc2616#section-14.37" >Retry-After format</a>
     */
    public Date apply(String retryAfter) {
      if (retryAfter == null) {
        return null;
      }
      if (retryAfter.matches("^[0-9]+$")) {
        long deltaMillis = SECONDS.toMillis(Long.parseLong(retryAfter));
        return new Date(currentTimeMillis() + deltaMillis);
      }
      synchronized (rfc822Format) {
        try {
          return rfc822Format.parse(retryAfter);
        } catch (ParseException ignored) {
          return null;
        }
      }
    }
  }

    //......
}

• ErrorDecoder提供了Default的默认实现，其decode方法会使用RetryAfterDecoder来计算retryAfter值，在该值不为null时会返回RetryableException；RetryAfterDecoder的apply方法会根据retryAfter来计算retryAfter日期，该retryAfter参数是从response的名为Retry-After的header读取而来

小结

• Retryer继承了Cloneable接口，它定义了continueOrPropagate、clone方法；它内置了一个名为Default以及名为NEVER_RETRY的实现
• Default有period、maxPeriod、maxAttempts参数可以设置，默认构造器使用的period为100，maxPeriod为1000，maxAttempts为5；continueOrPropagate方法首先判断attempt是否达到阈值，达到则抛出异常，否则进一步计算interval，然后进行sleep
• NEVER_RETRY的continueOrPropagate直接抛出异常，而clone方法直接返回当前实例

doc

• Retryer