Optimizely Agent 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 Optimizely 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 the http.Handler interface.
The interceptor struct can also include a set of fields that can be configured via the config.yaml file
- HTTPLog Interceptor Plugin - Adds HTTP request logging based on go-chi/httplog.

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"