added: service discovery example with mdns (#34)

examples/mdns/Makefile

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+PROJECT_BINARY=mdns
+PROJECT_BINARY_OUTPUT=bin
+
+.PHONY: all
+
+all: help
+
+## Build:
+tidy: ## Tidy project
+	@go mod tidy
+
+clean: ## Cleans temporary folder
+	@rm -rf ${PROJECT_BINARY_OUTPUT}
+	@rm -rf ${PROJECT_RELEASER_OUTPUT}
+
+build: clean tidy build-arm build-amd ## Build all
+	@echo "DONE"
+
+build-arm: ## Build for arm64
+	@GO111MODULE=on CGO_ENABLED=0 go build -ldflags="-w -s" -o ${PROJECT_BINARY_OUTPUT}/arm/${PROJECT_BINARY} main.go
+
+build-amd: ## Build for amd64
+	@GO111MODULE=on CGO_ENABLED=0 GOOS=darwin GOARCH=amd64 go build -ldflags="-w -s" -o ${PROJECT_BINARY_OUTPUT}/amd/${PROJECT_BINARY} main.go
+
+test: build ## Run unit tests
+	@go clean -testcache
+	@go test ./... 
+
+pre-commit: test ## Checks everything is allright
+	@echo "Commit Status: OK"
+
+## Help:
+help: ## Show this help.
+	@echo ''
+	@echo 'Usage:'
+	@echo '  make <target>'
+	@echo ''
+	@echo 'Targets:'
+	@awk 'BEGIN {FS = ":.*?## "} { \
+		if (/^[a-zA-Z_-]+:.*?##.*$$/) {printf "    %-20s%s\n", $$1, $$2} \
+		else if (/^## .*$$/) {printf "  %s\n", substr($$1,4)} \
+		}' $(MAKEFILE_LIST)

examples/mdns/README.md

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+# Remote Engine Discovery via mDNS
+This example shows us how service discovery can be done via mDNS.
+
+# About
+mDNS or Multicast DNS can be used to discover services on the local network without the use of an authoritative DNS server.
+This enables peer-to-peer discovery. It is important to note that many networks restrict the use of multicasting, which prevents mDNS from functioning.
+Notably, multicast cannot be used in any sort of cloud, or shared infrastructure environments.
+However it works well in most office, home, or private infrastructure environments.
+
+# Quickstart
+
+> The **[examples](https://github.com/anthdm/hollywood/tree/master/examples/mdns)** folder is the best place to explore Hollywood's Service Discovery
+
+```
+make build
+```
+
+# Flow
+ 0. When start engine with remote configuration, mDNS starting to announce itself and searches for other nodes
+ 1. If node founds new engine, discovery actor publishes a `DiscoveryEvent` via `actor.EventStream`
+ 2. (For demo purposes) An `chat` actor receives the discovery event and try to send a message to validate the flow.
+
+# Execution 
+
+```
+./bin/arm/mdns -port 4001
+    <omitted standart logs>
+    INFO[0000] [REMOTE] server started                       listenAddr="127.0.0.1:4001"
+    TRAC[0000] [EVENTSTREAM] subscribe                       id=1432518515 subs=1
+    TRAC[0000] [PROCESS] started                             pid="127.0.0.1:4001/chat"
+    TRAC[0000] [PROCESS] started                             pid="127.0.0.1:4001/mdns"
+    INFO[0001] [DISCOVERY] remote discovered                 ID=engine_1682994946742073000 addrs="127.0.0.1:4002"
+    TRAC[0001] [STREAM WRITER] connected                     remote="127.0.0.1:4002"
+    TRAC[0001] [STREAM ROUTER] new stream route              pid="127.0.0.1:4001/stream/127.0.0.1:4002"
+    INFO[0001] new message                                   fields.msg=hello
+```
+```
+./bin/arm/mdns -port 4002
+    <omitted standart logs>
+    INFO[0000] [REMOTE] server started                       listenAddr="127.0.0.1:4002"
+    TRAC[0000] [EVENTSTREAM] subscribe                       id=1432518515 subs=1
+    TRAC[0000] [PROCESS] started                             pid="127.0.0.1:4002/chat"
+    TRAC[0000] [PROCESS] started                             pid="127.0.0.1:4002/mdns"
+    INFO[0000] [DISCOVERY] remote discovered                 ID=engine_1682994395132833000 addrs="127.0.0.1:4001"
+    TRAC[0000] [INBOX] started                               pid="127.0.0.1:4002/stream/127.0.0.1:4001"
+    TRAC[0000] [STREAM WRITER] connected                     remote="127.0.0.1:4001"
+    TRAC[0000] [STREAM ROUTER] new stream route              pid="127.0.0.1:4002/stream/127.0.0.1:4001"
+    INFO[0000] new message                                   fields.msg=hello
+```
+
+# References
+- [mDNS](https://github.com/grandcat/zeroconf.git) library for golang
+
+# License
+
+Hollywood is licensed under the MIT licence.

examples/mdns/chat/actor.go

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+package chat
+
+import (
+	"github.com/anthdm/hollywood/actor"
+	"github.com/anthdm/hollywood/examples/mdns/chat/types"
+	"github.com/anthdm/hollywood/examples/mdns/discovery"
+	"github.com/anthdm/hollywood/log"
+)
+
+type server struct {
+	eventStream  *actor.EventStream
+	subscription *actor.EventSub
+	ctx          *actor.Context
+}
+
+func New(e *actor.EventStream) actor.Producer {
+	return func() actor.Receiver {
+		ret := &server{
+			eventStream: e,
+		}
+		return ret
+	}
+}
+
+func (s *server) Receive(ctx *actor.Context) {
+	switch msg := ctx.Message().(type) {
+	case actor.Initialized:
+		s.ctx = ctx
+		s.subscription = s.eventStream.Subscribe(s.onMessage)
+	case actor.Started:
+		_ = msg
+	case actor.Stopped:
+		s.shutdown()
+	case *types.Message:
+		s.handleMessage(ctx, msg)
+	}
+}
+func (s *server) onMessage(event any) {
+	switch evt := event.(type) {
+	case *discovery.DiscoveryEvent:
+		pid := actor.NewPID(evt.Addr[0], "chat")
+		s.ctx.Engine().Send(pid, &types.Message{
+			Username: evt.ID,
+			Msg:      "hello",
+		})
+	}
+}
+
+func (s *server) shutdown() {
+	s.eventStream.Unsubscribe(s.subscription)
+}
+
+// handle the incoming message by broadcasting it to all connected clients.
+func (s *server) handleMessage(ctx *actor.Context, msg *types.Message) {
+	log.Infow("new message", log.M{"msg": msg.Msg})
+}