Plugin System (#330). Closes #147

README.md

@@ -34,6 +34,8 @@ yet ready for production applications.*
    1. [Subscriptions](#subscriptions)
 1. [Events API](#events-api)
 1. [Configuration API](#configuration-api)
+1. [System Events](#system-events)
+1. [Plugin System](#plugin-system)
 1. [Client Libraries](#client-libraries)
 1. [Versioning](#versioning)
 1. [Comparison](#comparison)
@@ -281,13 +283,6 @@ the data block is base64 encoded.
 
 `invoke` is a built-in type of event allowing to call functions synchronously.
 
-#### System Events
-
-The Event Gateway emits system events allowing to react on internal events. Currently, only one internal event is
-emitted. `gateway.info.functionError` happens when function invocation failed.
-
-If you are looking for more system events, please comment [the corresponding issue](https://github.com/serverless/event-gateway/issues/215).
-
 ### Emit a Custom Event
 
 Creating a subscription requires `path` property (by default it's "/"). `path` indicates path under which you can push.
@@ -551,6 +546,52 @@ Dummy endpoint (always returning `200 OK` status code) for checking if the event
 
 `GET <Configuration API URL>/v1/status`
 
+## System Events
+
+System Events are special type of events emitted by the Event Gateway instance. They are emitted on each stage of event
+processing flow starting from receiving event to function invocation end. Those events are:
+
+- `gateway.event.received` - the event is emitted when an event was received by Events API. Data fields:
+  - `event` - event payload
+  - `path` - Events API path
+  - `headers` - HTTP request headers
+- `gateway.function.invoking` - the event emitted before invoking a function. Data fields:
+  - `event` - event payload
+  - `functionId` - registered function ID
+- `gateway.function.invoked` - the event emitted after successful function invocation. Data fields:
+  - `event` - event payload
+  - `functionId` - registered function ID
+  - `result` - function response
+- `gateway.function.invocationFailed` - the event emitted after failed function invocation. Data fields:
+  - `event` - event payload
+  - `functionId` - registered function ID
+  - `error` - invocation error
+
+## Plugin System
+
+The Event Gateway is built with extensibility in mind. Built-in plugin system allows reacting on system events and
+manipulate how an event is processed through the Event Gateway.
+
+_Current implementation supports plugins written only in Golang. We plan to support other languages in the future._
+
+Plugin system is based on [go-plugin](https://github.com/hashicorp/go-plugin). A plugin needs to implement the following
+interface:
+
+```go
+type Reacter interface {
+	Subscriptions() []Subscription
+	React(event event.Event) error
+}
+```
+
+`Subscription` model indicates the event that plugin subscribes to and the subscription type. A subscription can be either
+sync or async. Sync (blocking) subscription means that in case of error returned from `React` method the event won't be
+further processed by the Event Gateway.
+
+`React` method is called for every system event that plugin subscribed to.
+
+For more details, see [the example plugin](plugin/example).
+
 ## Client Libraries
 
 - [FDK for Node.js](https://github.com/serverless/fdk)

api/events.go

@@ -6,34 +6,33 @@ import (
 	"time"
 
 	"github.com/rs/cors"
-	"github.com/serverless/event-gateway/internal/cache"
 	"github.com/serverless/event-gateway/internal/httpapi"
-	"github.com/serverless/event-gateway/internal/metrics"
-	"github.com/serverless/event-gateway/router"
 )
 
-// StartEventsAPI creates a new gateway endpoint and listens for requests.
-func StartEventsAPI(config httpapi.Config) httpapi.Server {
-	targetCache := cache.NewTarget("/serverless-event-gateway", config.KV, config.Log)
-	router := router.New(targetCache, metrics.DroppedPubSubEvents, config.Log)
-	router.StartWorkers()
+// EventsAPIConfig stores configuration for Events API. Events API expects cofigured router. That's why new
+// configuration object is needed.
+type EventsAPIConfig struct {
+	httpapi.Config
+	Router http.Handler
+}
 
+// StartEventsAPI creates a new gateway endpoint and listens for requests.
+func StartEventsAPI(config EventsAPIConfig) httpapi.Server {
 	handler := &http.Server{
 		Addr:         ":" + strconv.Itoa(int(config.Port)),
-		Handler:      cors.AllowAll().Handler(router),
+		Handler:      cors.AllowAll().Handler(config.Router),
 		ReadTimeout:  3 * time.Second,
 		WriteTimeout: 3 * time.Second,
 	}
 
 	server := httpapi.Server{
-		Config:      config,
+		Config:      config.Config,
 		HTTPHandler: handler,
 	}
 
 	config.ShutdownGuard.Add(1)
 	go func() {
 		server.Listen()
-		router.Drain()
 		config.ShutdownGuard.Done()
 	}()
 

cmd/event-gateway/main.go

@@ -15,18 +15,25 @@ import (
 	"go.uber.org/zap/zapcore"
 
 	"github.com/serverless/event-gateway/api"
+	"github.com/serverless/event-gateway/internal/cache"
 	"github.com/serverless/event-gateway/internal/embedded"
 	"github.com/serverless/event-gateway/internal/httpapi"
 	"github.com/serverless/event-gateway/internal/metrics"
 	"github.com/serverless/event-gateway/internal/sync"
+	"github.com/serverless/event-gateway/plugin"
+	"github.com/serverless/event-gateway/router"
 )
 
 var version = "dev"
 
 func init() {
 	etcd.Register()
+
+	prometheus.MustRegister(metrics.RequestDuration)
+	prometheus.MustRegister(metrics.DroppedPubSubEvents)
 }
 
+// nolint: gocyclo
 func main() {
 	showVersion := flag.Bool("version", false, "Show version.")
 	logLevel := zap.LevelFlag("log-level", zap.InfoLevel, `The level of logging to show after the event gateway has started. The available log levels are "debug", "info", "warn", and "err".`)
@@ -42,17 +49,16 @@ func main() {
 	eventsPort := flag.Uint("events-port", 4000, "Port to serve events API on.")
 	eventsTLSCrt := flag.String("events-tls-cert", "", "Path to events API TLS certificate file.")
 	eventsTLSKey := flag.String("events-tls-key", "", "Path to events API TLS key file.")
+	plugins := paths{}
+	flag.Var(&plugins, "plugin", "Path to a plugin to load.")
 	flag.Parse()
 
 	if *showVersion {
 		fmt.Printf("Event Gateway version: %s\n", version)
 		os.Exit(0)
 	}
 
-	prometheus.MustRegister(metrics.RequestDuration)
-	prometheus.MustRegister(metrics.DroppedPubSubEvents)
-
-	log, err := loggerConfiguration(*developmentMode, *logLevel, *logFormat).Build()
+	log, err := logger(*developmentMode, *logLevel, *logFormat).Build()
 	if err != nil {
 		panic(err)
 	}
@@ -64,10 +70,9 @@ func main() {
 		embedded.EmbedEtcd(*embedDataDir, *embedPeerAddr, *embedCliAddr, shutdownGuard)
 	}
 
-	dbHostStrings := strings.Split(*dbHosts, ",")
 	kv, err := libkv.NewStore(
 		store.ETCDV3,
-		dbHostStrings,
+		strings.Split(*dbHosts, ","),
 		&store.Config{
 			ConnectionTimeout: 10 * time.Second,
 		},
@@ -76,13 +81,26 @@ func main() {
 		log.Fatal("Cannot create KV client.", zap.Error(err))
 	}
 
-	eventServer := api.StartEventsAPI(httpapi.Config{
-		KV:            kv,
-		Log:           log,
-		TLSCrt:        eventsTLSCrt,
-		TLSKey:        eventsTLSKey,
-		Port:          *eventsPort,
-		ShutdownGuard: shutdownGuard,
+	pluginManager := plugin.NewManager(plugins, log)
+	err = pluginManager.Connect()
+	if err != nil {
+		log.Fatal("Loading plugins failed.", zap.Error(err))
+	}
+
+	targetCache := cache.NewTarget("/serverless-event-gateway", kv, log)
+	router := router.New(targetCache, pluginManager, metrics.DroppedPubSubEvents, log)
+	router.StartWorkers()
+
+	eventServer := api.StartEventsAPI(api.EventsAPIConfig{
+		Config: httpapi.Config{
+			KV:            kv,
+			Log:           log,
+			TLSCrt:        eventsTLSCrt,
+			TLSKey:        eventsTLSKey,
+			Port:          *eventsPort,
+			ShutdownGuard: shutdownGuard,
+		},
+		Router: router,
 	})
 
 	configServer := api.StartConfigAPI(httpapi.Config{
@@ -108,14 +126,19 @@ func main() {
 	}
 
 	shutdownGuard.Wait()
+	router.Drain()
+
+	if pluginManager != nil {
+		pluginManager.Kill()
+	}
 }
 
 const (
 	consoleEncoding = "console"
 	jsonEncoding    = "json"
 )
 
-func loggerConfiguration(dev bool, level zapcore.Level, format string) zap.Config {
+func logger(dev bool, level zapcore.Level, format string) zap.Config {
 	cfg := zap.Config{
 		Level:            zap.NewAtomicLevelAt(level),
 		Development:      false,
@@ -153,3 +176,14 @@ func loggerConfiguration(dev bool, level zapcore.Level, format string) zap.Confi
 
 	return cfg
 }
+
+type paths []string
+
+func (p *paths) String() string {
+	return strings.Join(*p, ",")
+}
+
+func (p *paths) Set(value string) error {
+	*p = append(*p, value)
+	return nil
+}

event/event.go

@@ -1,8 +1,12 @@
 package event
 
 import (
+	"encoding/json"
+	"strings"
 	"time"
 
+	"go.uber.org/zap/zapcore"
+
 	uuid "github.com/satori/go.uuid"
 )
 
@@ -34,3 +38,20 @@ const TypeInvoke = Type("invoke")
 
 // TypeHTTP is a special type of event for sync http subscriptions.
 const TypeHTTP = Type("http")
+
+// MarshalLogObject is a part of zapcore.ObjectMarshaler interface
+func (e Event) MarshalLogObject(enc zapcore.ObjectEncoder) error {
+	enc.AddString("type", string(e.Type))
+	enc.AddString("id", e.ID)
+	enc.AddUint64("receivedAt", e.ReceivedAt)
+	payload, _ := json.Marshal(e.Data)
+	enc.AddString("data", string(payload))
+	enc.AddString("dataType", e.DataType)
+
+	return nil
+}
+
+// IsSystem indicates if th event is a system event.
+func (e Event) IsSystem() bool {
+	return strings.HasPrefix(string(e.Type), "gateway.")
+}

event/system.go

@@ -0,0 +1,46 @@
+package event
+
+import (
+	"net/http"
+
+	"github.com/serverless/event-gateway/functions"
+)
+
+// SystemEventReceivedType is a system event emmited when the Event Gateway receives an event.
+const SystemEventReceivedType = Type("gateway.event.received")
+
+// SystemEventReceivedData struct.
+type SystemEventReceivedData struct {
+	Path    string      `json:"path"`
+	Event   Event       `json:"event"`
+	Headers http.Header `json:"header"`
+}
+
+// SystemFunctionInvokingType is a system event emmited before invoking a function.
+const SystemFunctionInvokingType = Type("gateway.function.invoking")
+
+// SystemFunctionInvokingData struct.
+type SystemFunctionInvokingData struct {
+	FunctionID functions.FunctionID `json:"functionId"`
+	Event      Event                `json:"event"`
+}
+
+// SystemFunctionInvokedType is a system event emmited after successful function invocation.
+const SystemFunctionInvokedType = Type("gateway.function.invoked")
+
+// SystemFunctionInvokedData struct.
+type SystemFunctionInvokedData struct {
+	FunctionID functions.FunctionID `json:"functionId"`
+	Event      Event                `json:"event"`
+	Result     []byte               `json:"result"`
+}
+
+// SystemFunctionInvocationFailedType is a system event emmited after successful function invocation.
+const SystemFunctionInvocationFailedType = Type("gateway.function.invocationFailed")
+
+// SystemFunctionInvocationFailedData struct.
+type SystemFunctionInvocationFailedData struct {
+	FunctionID functions.FunctionID `json:"functionId"`
+	Event      Event                `json:"event"`
+	Error      []byte               `json:"result"`
+}