如何在Apache httpclient中使用指数回退策略？

Question

docs指定了一个ExponentialBackOffSchedulingStrategy类，但它似乎在4.5版中不存在。

此外，我没有找到任何解释如何使用它的东西。

以前有人用过这样的东西吗？

Answer 1

如果使用Maven包安装Apache HTTP Client 4.5.x，则如下所示(Grails Maven依赖关系语法)：

compile "org.apache.httpcomponents:httpclient:4.5.1"
compile "org.apache.httpcomponents:httpcore:4.4.3"

您需要添加另一个jar来获取这些类，如下所示：

compile 'org.apache.httpcomponents:httpclient-cache:4.5.1'

请参阅https://hc.apache.org/httpcomponents-client-4.5.x/download.html

然后你可以像这样把它连接起来：

 import org.apache.http.impl.client.*;
 import org.apache.http.impl.client.cache.*;
 import org.apache.http.client.methods.*;

 CloseableHttpClient createClient() {
     CachingHttpClientBuilder hcb = new CachingHttpClientBuilder();
     CacheConfig cc = CacheConfig.DEFAULT;
     ExponentialBackOffSchedulingStrategy ebo = new ExponentialBackOffSchedulingStrategy(cc);
     hcb.setSchedulingStrategy(ebo);
     CloseableHttpClient hc = hcb.build();
     return hc;
 }

 // You'll need to replace the URL below with something that returns a 5xx error
 CloseableHttpClient client = createClient();
 HttpUriRequest request = new HttpGet("http://www.example.com/throwsServerError");
 for (int i=0; i<4; i++) {
     CloseableHttpResponse response =  client.execute(request);
     println new Date().toString() + " " + response.getStatusLine().getStatusCode();
 }

Answer 2

ExponentialBackOffSchedulingStrategy在缓存条目上工作，这就是为什么它只能与CachingClient一起使用。我不认为它可以用来在失败的请求上实现指数回退重试机制。

我将实现一个简单的指数退避策略，如下所示：

class MyExponentialBackoffRetryHandler extends HttpRequestRetryHandler {
  public MyExponentialBackoffRetryHandler(int maxNumRetries, int backOffRate, int initialExpiry, int maxExpiry) {
  } 
 boolean retryRequest(IOException exception, int executionCount, HttpContext context) {
   if (executionCount == maxNumRetries) {
     return false;
   }else {
     long nextBackOffDelay = initialExpiry * Math.pow(backOffRate, executionCount - 
1)
     long delay = Math.min(maxExpiry, nextBackOffDelay)
     Thread.sleep(delay);
     return true;
   } 

}
}

此外，为了触发重试，您需要添加一个抛出IOException的响应拦截器，例如：

class MyResponseInterceptor extends HttpResponseInterceptor {
  void process(HttpResponse response, HttpContext context) throws HttpException, IOException {
    if (response.getStatusLine.getStatusCode == 503) {
        throw new IOException("Please retry")
    }
  }
}

最后，您可以构建HTTP客户端实例，如下所示：

HttpClientBuilder
    .create()
    .setRetryHandler(new MyExponentialBackoffRetryHandler(...))
    .addInterceptorFirst(new MyResponseInterceptor())
    .build()

您可以通过向https://httpstat.us/503发送请求来轻松地测试客户端