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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&lt;String&gt; 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&lt;?&gt; entity = new CseHttpEntity&lt;&gt;(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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