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reqeust context(请求上下文)

一般在一个web应用中，Hystrix会在一个filter里面，对每个请求都添加一个请求上下文。即Tomcat容器内，每一次请求，就是一次请求上下文。

在一次请求上下文中，如果有多个command,参数及调用的接口也是一样的，其实结果也可以认为是一样的

这样避免在一次请求上下文中多次执行一样的command，避免重复执行网络请求,从而提升整个请求的性能

• request cache的原理图
  

对于请求缓存（request caching），请求合并（request collapsing），请求日志（request log），等等技术，都必须自己管理HystrixReuqestContext的声明周期

在一个请求执行之前，都必须先初始化一个request context

HystrixRequestContext context = HystrixRequestContext.initializeContext();

然后在请求结束之后，需要关闭request context

context.shutdown();

一般来说，在java web的应用中，都是通过filter过滤器来实现的

public class HystrixRequestContextServletFilter implements Filter {    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)      throws IOException, ServletException {        HystrixRequestContext context = HystrixRequestContext.initializeContext();        try {            chain.doFilter(request, response);        } finally {            context.shutdown();        }    }}@Beanpublic FilterRegistrationBean indexFilterRegistration() {    FilterRegistrationBean registration = new FilterRegistrationBean(new IndexFilter());    registration.addUrlPatterns("/");    return registration;}

结合业务背景，我们做了一个批量查询商品数据的接口，在这个里面，我们其实通过HystrixObservableCommand一次性批量查询多个商品id的数据

但是这里有个问题，如果说nginx在本地缓存失效了，重新获取一批缓存，传递过来的productId都没有进行去重，1,1,2,2,5,6,7

那么可能说，商品id出现了重复，如果按照我们之前的业务逻辑，可能就会重复对productId=1的商品查询两次，productId=2的商品查询两次

我们对批量查询商品数据的接口，可以用request cache做一个优化，就是说一次请求，就是一次request context，对相同的商品查询只能执行一次，其余的都走request cache

public class CommandUsingRequestCache extends HystrixCommand
   
     {    private final int value;    protected CommandUsingRequestCache(int value) {        super(HystrixCommandGroupKey.Factory.asKey("ExampleGroup"));        this.value = value;    }    @Override    protected Boolean run() {        return value == 0 || value % 2 == 0;    }    @Override    protected String getCacheKey() {        return String.valueOf(value);    }}@Testpublic void testWithCacheHits() {    HystrixRequestContext context = HystrixRequestContext.initializeContext();    try {        CommandUsingRequestCache command2a = new CommandUsingRequestCache(2);        CommandUsingRequestCache command2b = new CommandUsingRequestCache(2);        assertTrue(command2a.execute());        // this is the first time we've executed this command with        // the value of "2" so it should not be from cache        assertFalse(command2a.isResponseFromCache());        assertTrue(command2b.execute());        // this is the second time we've executed this command with        // the same value so it should return from cache        assertTrue(command2b.isResponseFromCache());    } finally {        context.shutdown();    }    // start a new request context    context = HystrixRequestContext.initializeContext();    try {        CommandUsingRequestCache command3b = new CommandUsingRequestCache(2);        assertTrue(command3b.execute());        // this is a new request context so this         // should not come from cache        assertFalse(command3b.isResponseFromCache());    } finally {        context.shutdown();    }}
   

缓存的手动清理

public static class GetterCommand extends HystrixCommand
   
     {    private static final HystrixCommandKey GETTER_KEY = HystrixCommandKey.Factory.asKey("GetterCommand");    private final int id;    public GetterCommand(int id) {        super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("GetSetGet"))                .andCommandKey(GETTER_KEY));        this.id = id;    }    @Override    protected String run() {        return prefixStoredOnRemoteDataStore + id;    }    @Override    protected String getCacheKey() {        return String.valueOf(id);    }    /**     * Allow the cache to be flushed for this object.     *      * @param id     *            argument that would normally be passed to the command     */    public static void flushCache(int id) {        HystrixRequestCache.getInstance(GETTER_KEY,                HystrixConcurrencyStrategyDefault.getInstance()).clear(String.valueOf(id));    }}public static class SetterCommand extends HystrixCommand
    
      {    private final int id;    private final String prefix;    public SetterCommand(int id, String prefix) {        super(HystrixCommandGroupKey.Factory.asKey("GetSetGet"));        this.id = id;        this.prefix = prefix;    }    @Override    protected Void run() {        // persist the value against the datastore        prefixStoredOnRemoteDataStore = prefix;        // flush the cache        GetterCommand.flushCache(id);        // no return value        return null;    }}
    
   

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