Plugins

Plugins are a way of extending the functionality of Micro

Overview

Plugins enable Micro to be extending and intercepted to provide additional functionality and features. This may include logging, metrics, tracing, authentication, etc. The Plugin model requires registering a struct that matches a plugin interface. It's then registered and setup when Micro starts.

Design

Here's the interface design

// Plugin is the interface for plugins to micro. It differs from go-micro in that it's for
// the micro API, Web, Sidecar, CLI. It's a method of building middleware for the HTTP side.
type Plugin interface {
	// Global Flags
	Flags() []cli.Flag
	// Sub-commands
	Commands() []cli.Command
	// Handle is the middleware handler for HTTP requests. We pass in
	// the existing handler so it can be wrapped to create a call chain.
	Handler() Handler
	// Init called when command line args are parsed.
	// The initialized cli.Context is passed in.
	Init(*cli.Context) error
	// Name of the plugin
	String() string
}

// Manager is the plugin manager which stores plugins and allows them to be retrieved.
// This is used by all the components of micro.
type Manager interface {
        Plugins() map[string]Plugin
        Register(name string, plugin Plugin) error
}

// Handler is the plugin middleware handler which wraps an existing http.Handler passed in.
// Its the responsibility of the Handler to call the next http.Handler in the chain.
type Handler func(http.Handler) http.Handler

How to use it

Here's a simple example of a plugin that adds a flag and then prints the value

The plugin

Create a plugin.go file in the top level dir

package main

import (
	"log"
	"github.com/urfave/cli/v2"
	"github.com/micro-community/micro/plugin"
)

func init() {
	plugin.Register(plugin.NewPlugin(
		plugin.WithName("example"),
		plugin.WithFlag(&cli.StringFlag{
			Name:   "example_flag",
			Usage:  "This is an example plugin flag",
			EnvVars: []string{"EXAMPLE_FLAG"},
			Value: "avalue",
		}),
		plugin.WithInit(func(ctx *cli.Context) error {
			log.Println("Got value for example_flag", ctx.String("example_flag"))
			return nil
		}),
	))
}

Building the code

Simply build micro with the plugin

go build -o micro ./main.go ./plugin.go