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Agent plugins

This topic describes how to extend Optimizely Agent using plugins.

Agent Plugins

Optimizely Agent can be extended through the use of plugins. Plugins are distinct from the standard Agent packages that provide a namespaced environment for custom logic. Plugins must be compiled as part of the Agent distribution and are enabled through configuration.

Interceptor Plugins

Interceptors can be added to Agent to customize the request and/or response by implementing the Interceptor interface.
This interface defines a Handler() method that returns a standard net/http middleware handler based on http.Handler.
The interceptor struct can also include a set of fields that can be configured via config.yaml.

Example Interceptor definition

package example
import (
    "context"
    "net/http"

    "github.com/optimizely/agent/plugins/interceptors"
)

// Example implements the Interceptor plugin interface
type Example struct {
    // set of configuration fields
    RequestHeader  string
    ResponseHeader string
    ContextValue   string
}

func (i *Example) Handler() func(next http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            r.Header.Add("X-Example-Request", i.RequestHeader)

            // Example adding context to the request path
            ctx := context.WithValue(r.Context(), "example-context", i.ContextValue)

            // Continuing with the normal serving
            next.ServeHTTP(w, r.WithContext(ctx))

            // Modify the response in some way
            w.Header().Add("X-Example-Response", i.ResponseHeader)
        })
    }
}

// Register our interceptor as "example".
func init() {
    interceptors.Add("example", func() interceptors.Interceptor {
        return &Example{}
    })
}
```

To make the interceptor available to Agent, add the plugin as an anonymous import into [all.go](./interceptors/all/all.go).
```go
package all

// Add imports here to trigger the plugin `init()` function
import (
    _ "github.com/optimizely/agent/plugins/interceptors/example"
)
```

Enable the example interceptor by adding to `server.interceptors` within your `config.yaml`. Note that the yaml fields should match the struct definition of your plugin.
```yaml
server:
  interceptors:
    example:
      requestHeader: "example-request"
      responseHeader: "example-response"
      contextValue: "example-context"

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