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+# metadata
+## 简介
+metadata包提供了一个 API，允许插件将元数据添加到上下文中。每个元数据都存储在格式为`<plugin>/<name>` 的标签下。每个元数据作为 Func 返回。调用 Func 时返回元数据。如果某个 Func 执行时间很长，就需要自行提供某种形式的缓存。在处理一个查询时的元数据应该保持不变。
+## 用例
+Basic example:
+```go
+//
+// Implement the Provider interface for a plugin p:
+//
+func (p P) Metadata(ctx context.Context, state request.Request) context.Context {
+	metadata.SetValueFunc(ctx, "test/something", func() string { 
+		   return "myvalue" 
+	   })
+	return ctx
+    }
+```
+Basic example with caching:
+```go
+func (p P) Metadata(ctx context.Context, state request.Request) context.Context {
+	cached := ""
+	f := func() string {
+	    if cached != "" {
+		    return cached
+	    }
+	    cached = expensiveFunc()
+	    return cached
+	}
+	metadata.SetValueFunc(ctx, "test/something", f)
+	return ctx
+    }
+
+```
+
+
+If you need access to this metadata from another plugin:
+```go
+
+    // ...
+    valueFunc := metadata.ValueFunc(ctx, "test/something")
+    value := valueFunc()
+    // use 'value'
+
+```
+\ No newline at end of file