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| |
| <h1 id="context">使用Context传递控制消息</h1> |
| <p>Context 用于在微服务之间和微服务内部传递上下文数据。 Context 是 key/value 对，只能够使用 String 类型。 |
| Context 会序列化为 json 格式并通过 HTTP Header 传递，因此也不支持 ASCII 之外的字符，其他字符需要开发者先自行编码再传递。 |
| Context 在一次请求中，会在请求链上传递，不需要重新设置。<a href="../build-provider/access-log-configuration.html">access log</a>的 trace id 等 |
| 功能都基于这个特性实现的。</p> |
| <p>Context 保存的内容分为 context 和 localContext。 localContext 在调用过程中，会在进程内部传递， 而 context 的内容会传递到调用过程中 |
| 远端服务。 这种传递是单向的。比如在一个 Provider 内部， 调用接口， 那么 localContext 的内容会复制给调用接口运行过程中的 localContext， |
| 如果这个接口在调用过程中修改了 localContext， 接口返回后， Provider 后续的逻辑处理看不到对于 context 的修改。 </p> |
| <p>在 Handler 或者 Filter 中调用其他微服务， context 信息默认不会复制，需要开发者显示的将 context 信息传递过去。 使用 RestTemplate 或者 |
| RPC 方式传递 context 的例子，请参考本文后面的案例。 </p> |
| <h2 id="context_1">使用 Context 的场景</h2> |
| <ul> |
| <li>在认证场景，Edge Service 认证通过以后，需要将会话 ID、用户名称等信息传递给微服务，实现鉴权等逻辑。</li> |
| <li>灰度发布场景，需要结合自定义的 tag 实现引流，tag 信息需要传递给微服务。</li> |
| <li>开发 Handler 处理链的时候，一个 Handler 需要将计算结果传递给下一个 Handler。</li> |
| </ul> |
| <h2 id="_1">使用参考</h2> |
| <ul> |
| <li> |
| <p>在 Hanlder 中获取和设置Context |
| Handler 包含了 Invocation 对象，可以直接调用 invocation.addContext 和 invocation.getContext 。</p> |
| </li> |
| <li> |
| <p>在服务接口中获取Context</p> |
| <p>通过接口注入</p> |
| <pre><code>``` |
| public Response cseResponse(InvocationContext c1) |
| ``` |
| </code></pre> |
| <p>或者</p> |
| <pre><code>``` |
| ContextUtils.getInvocationContext() |
| ``` |
| </code></pre> |
| </li> |
| <li> |
| <p>在Edge Service中设置Context</p> |
| <p>通过重载EdgeInvocation</p> |
| <pre><code>``` |
| EdgeInvocation edgeInvocation = new EdgeInvocation() { |
| protected void createInvocation() { |
| super.createInvocation(); |
| this.invocation.addContext("hello", "world"); |
| } |
| }; |
| ``` |
| </code></pre> |
| </li> |
| </ul> |
| <h2 id="context-discoverytree">案例： 使用 Context 和 DiscoveryTree 实现轮询调用一个微服务的所有实例</h2> |
| <p>通常微服务的调用，是将请求发送到一个实例，这个实例是根据负载均衡策略决定的，业务开发不可控制。为了实现轮询调用一个微服务的所有实例， |
| 首先需要获取一个微服务的所有实例列表，然后逐个调用。 LoadBalance 模块支持通过 Context 传递 Endpoint 信息， 如果 Endpoint 已经 |
| 设置， 可以跳过负载均衡判断， 使用用户自己指定的 Endpoint。 </p> |
| <ul> |
| <li> |
| <p>使用 DiscoveryTree 获取微服务实例列表</p> |
| <pre><code>```java |
| public class TestDateTimeSchema { |
| private DiscoveryTree discoveryTree = new DiscoveryTree(); |
| |
| public TestDateTimeSchema() { |
| discoveryTree.addFilter(new CustomEndpointDiscoveryFilter()); |
| discoveryTree.sort(); |
| } |
| |
| private void testDateTimeSchemaMulticast() throws Exception { |
| DiscoveryContext context = new DiscoveryContext(); |
| VersionedCache serversVersionedCache = discoveryTree.discovery(context, "springmvctest", "springmvc", "0+"); |
| List<String> enpoints = serversVersionedCache.data(); // 获取到实例列表，可以给下面的处理流程使用 |
| } |
| } |
| |
| public class CustomEndpointDiscoveryFilter extends AbstractEndpointDiscoveryFilter { |
| @Override |
| protected String findTransportName(DiscoveryContext context, DiscoveryTreeNode parent) { |
| //only need rest endpoints |
| return "rest"; |
| } |
| |
| @Override |
| protected Object createEndpoint(String transportName, String endpoint, MicroserviceInstance instance) { |
| return endpoint; |
| } |
| |
| @Override |
| public int getOrder() { |
| return 0; |
| } |
| } |
| ``` |
| </code></pre> |
| <p>上面的代码通过 DiscoveryTree 发现实例列表， 并且实现了 CustomEndpointDiscoveryFilter ， 将发现的实例信息转换为 cache 的返回 |
| 类型， 即 String。</p> |
| </li> |
| <li> |
| <p>通过 InvocationContext 传递 Endpoint 信息给 Load Balance, 每次调用访问用户指定的 Endpoint。 |
| 访问 InvocationContext 分几种场景， 参考文章上面提到的情况。 在 Consumer 调用的场景下， 可能不在一个 Provider 的处理上下文中， |
| 这个时候系统中还没有 InvocationContext 实例， 这个时候可以新创建一个实例， 新创建的实例信息会复制到系统内部。 </p> |
| <p>使用 RPC Consumer 传递 InvocationContext 的例子：</p> |
| <pre><code>```java |
| interface DateTimeSchemaWithContextInf { |
| Date getDate(InvocationContext context, Date date); |
| } |
| |
| @RpcReference(microserviceName = "springmvc", schemaId = "DateTimeSchema") |
| private DateTimeSchemaWithContextInf dateTimeSchemaWithContextInf; |
| |
| // code slip |
| for (String endpoint : enpoints) { |
| InvocationContext invocationContext = new InvocationContext(); |
| invocationContext.addLocalContext(LoadbalanceHandler.SERVICECOMB_SERVER_ENDPOINT, parseEndpoint(endpoint)); |
| Date date = new Date(); |
| TestMgr.check(date.getTime(), dateTimeSchemaWithContextInf.getDate(invocationContext, date).getTime()); |
| } |
| |
| // code slip |
| private Endpoint parseEndpoint(String endpointUri) throws Exception { |
| URI formatUri = new URI(endpointUri); |
| Transport transport = SCBEngine.getInstance().getTransportManager().findTransport(formatUri.getScheme()); |
| return new Endpoint(transport, endpointUri); |
| } |
| ``` |
| </code></pre> |
| <p>使用 RestTemplate 传递 InvocationContext 的例子：</p> |
| <pre><code>``` |
| for (String endpoint : enpoints) { |
| CseHttpEntity<?> entity = new CseHttpEntity<>(null); |
| InvocationContext invocationContext = new InvocationContext(); |
| invocationContext.addLocalContext(LoadbalanceHandler.SERVICECOMB_SERVER_ENDPOINT, parseEndpoint(endpoint)); |
| entity.setContext(invocationContext); |
| |
| Date date = new Date(); |
| String dateValue = RestObjectMapperFactory.getRestObjectMapper().convertToString(date); |
| TestMgr.check(date.getTime(), |
| restTemplate |
| .exchange("cse://springmvc/dateTime/getDate?date={1}", HttpMethod.GET, |
| entity, Date.class, dateValue).getBody().getTime()); |
| ｝ |
| ``` |
| </code></pre> |
| <p><strong><em>注意：</em></strong> 2.0.2 版本开始， LoadbalanceHandler.SERVICECOMB_SERVER_ENDPOINT 传递的类型是 Endpoint, 早期版本可以直接传递 String 类型， |
| LoadBalance 模块会将 String 类型转换为 Endpoint。 在有大量 Endpoint 的情况， 提前使用 Endpoint 类型能够减少类型转换，节省处理时间。</p> |
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