Spring Cloud Hystrix 学习（三）请求合并

什么是请求合并？我们先来看两张图：

上方的两张图中，第二张可以看出服务端只执行了一次响应，这就是请求合并。客户端新增的请求合并模块，内部存在一个等待的时间窗口，将一定时间段内满足条件的请求进行合并，以此降低服务端的请求响应压力。

可以看出，请求合并是在客户端中实现的，接下来我们通过代码来实践一下。

首先给出服务端的代码，这里打印了入参ids，后续我们将通过这个入参打印的情况来对请求合并的情况进行观察。

@RequestMapping(value = "/mergeTest", method = RequestMethod.GET)
public List<Test> mergeTest(String ids) {

    System.out.println("ids:{" + ids + "}");
    String[] strs = ids.split(",");

    List<Test> lstResult = new ArrayList<Test>();
    for (int i = 0; i < strs.length; i++) {
        lstResult.add(new Test(i, Integer.parseInt(strs[i])));
    }
    return lstResult;
}

这里补充说明一点，对于测试用的Test类必须存在空构造函数，否则在客户端收到应答后反序列化时可能会抛出如下异常：

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `hystrix.dto.Test` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
 at [Source: (PushbackInputStream); line: 1, column: 3] (through reference chain: java.lang.Object[][0])
	at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1452) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1028) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1297) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.deser.std.ObjectArrayDeserializer.deserialize(ObjectArrayDeserializer.java:195) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.deser.std.ObjectArrayDeserializer.deserialize(ObjectArrayDeserializer.java:21) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4014) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3085) ~[jackson-databind-2.9.10.6.jar:2.9.10.6]
	at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:271) ~[spring-web-5.1.19.RELEASE.jar:5.1.19.RELEASE]
	at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:240) ~[spring-web-5.1.19.RELEASE.jar:5.1.19.RELEASE]
	at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:102) ~[spring-web-5.1.19.RELEASE.jar:5.1.19.RELEASE]
	at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:733) ~[spring-web-5.1.19.RELEASE.jar:5.1.19.RELEASE]
	at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:666) ~[spring-web-5.1.19.RELEASE.jar:5.1.19.RELEASE]
	at org.springframework.web.client.RestTemplate.getForObject(RestTemplate.java:307) ~[spring-web-5.1.19.RELEASE.jar:5.1.19.RELEASE]
	at hystrix.service.TestService.mergeTest(TestService.java:53) ~[classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_181]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_181]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_181]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_181]

接下来再看下客户端请求合并逻辑的实现，这里我们采用注解的方式来完成：

@Service
public class TestService {

    @Autowired
    private RestTemplate restTemplate;

    @Autowired
    private DiscoveryClient discoveryClient;

    @HystrixCollapser(batchMethod = "mergeTest", collapserProperties = {
            @HystrixProperty(name = "timerDelayInMilliseconds", value = "3000")
    })
    public Future<Test> singleTest(Integer id) {
        return null;
    }

    @HystrixCommand
    public List<Test> mergeTest(List<Integer> ids) {

        ServiceInstance instance = discoveryClient.getInstances("spring-cloud-service-provider").get(0);
        String url = "http://" + instance.getHost() + ":" + instance.getPort() + "/mergeTest";
        Test[] tests = restTemplate.getForObject(url + "?ids={1}", Test[].class, StringUtils.join(ids, ","));
        return Arrays.asList(tests);
    }
}

singleTest通过@HystrixCollapser注解实现请求合并，并指定了合并后调用的方法为mergeTest()，同时设置了时间窗口的大小为3000毫秒。

最后就是测试代码：

@GetMapping("/hystrix/test")
public String helloHystrix() throws ExecutionException, InterruptedException {

    HystrixRequestContext context = HystrixRequestContext.initializeContext();
    Future<Test> f1 = service.singleTest(101);
    Future<Test> f2 = service.singleTest(102);
    Future<Test> f3 = service.singleTest(103);
    Test t1 = f1.get();
    Test t2 = f2.get();
    Test t3 = f3.get();
    Thread.sleep(3000);
    Future<Test> f4 = service.singleTest(104);
    Test t4 = f4.get();
    context.close();

    return t1.getIndex() + ", " + t2.getIndex() + ", " + t3.getIndex() + ", " + t4.getIndex();
}

通过postman进行请求后，服务端打印信息如下：

可以看到前三条请求合并为一条汇总请求调用到服务端，由于第四条请求是在sleep(3000)之后进行发起，并未和前三条请求进入同一个时间窗口，因此单独调用到了服务端。

参考资料：

https://segmentfault.com/a/1190000011468804

https://www.cnblogs.com/yb-ken/p/15068392.html