there are multiple ways to create a real-time chat, where user A can communicate with user B and user C. On this occasion, we’ll create a server chat where multiple users can enter a user name and send/receive messages in real-time.

For this article you will need to have previous experience with:

channels
goroutines

today we’ll learn how to create a TCP chat in Golang using channels and goroutines

index:

accept connections (accept incoming connections)
client
give it structure
chat
client
listen for connections
start new client and book new room
setting up user
receive messages
send messages
test #1
exit queue channel
exit guide
update handleConnection
test #2
wrap-up

let’s get started!

first, we need a way to accept incoming connections.

accept connections

package huevosrevueltos.com/simplechat

func buildServer() {
	server, err := net.Listen("tcp", ":8080") // localhost:8080

	if err != nil {
		log.Fatalf("could not start chat: %v", err)
	}

	for {
		conn, err := server.Accept()
		if err != nil {
			log.Fatal("connection err: %v", err)
			continue
		}
		go handleConnection(conn) // start new goroutine per connection
	}
}

with this function we start listening for connections in port 8080. In an infinite loop, we keep accepting new connections until the program stops.

client

func handleConnection(conn net.Conn) {
	scanner := bufio.NewScanner(conn)

	for scanner.Scan(){
		if scanner.Err() != nil{
			break
		}
		fmt.Println(scanner.Text())
	}

	defer conn.Close()

}

this simple connection handler will print a message every time a client writes to its console

give it structure

the next step it’s to give more structure to the chat and start sending and start processing the messages.

chat

the chat struct will be in charge of receiving, sending, and booking rooms for clients

type chat struct {
	clients      []*client
	messageQueue chan []byte
}

message queue is a channel that waits for incoming client messages, and then propagates them

client

client holds more logic, so let’s go by steps

type client struct {
	name  string
	conn  net.Conn
	in    chan []byte
	out   chan []byte
	token chan struct{}
}

name: is the username of the client, after the connection is established the server asks the user to put a user name to
conn: is the connection we start with the server
in: incoming messages channel
out: outgoing messages channel
token: room token that it’s held until the connection is finished

listen for connections

func (c *chat) buildServer() {
	server, err := net.Listen("tcp", ":8080") // localhost:8080

	if err != nil {
		log.Fatalf("could not start chat: %v", err)
	}
	go c.serve()
	for {
		conn, err := server.Accept()
		if err != nil {
			log.Fatalf("connection err: %v", err)
			continue
		}
		go c.handleConnection(conn) // start new goroutine per connection
	}
}

the structure here didn’t change that much, go c.serve creates a new goroutine that will handle the message queue and send the message to all the connected clients.

start the new client and book a new room

next, we need to create a new client and the channels where she can send/receive messages

func (c *chat) handleConnection(conn net.Conn) {
	in, out := make(chan []byte), make(chan []byte)
	client := newClient(conn, in, out)
	c.clients = append(c.clients, client)
	client.start()
	go c.bookRoom(client)
	<-client.token
	defer conn.Close()
}

the steps that this function follows are

start the user
add the client to the current chat
create a new “room” to listen for messages from the client

setting up user


func newClient(conn net.Conn, in chan []byte, out chan []byte) *client {
	return &client{in: in, out: out, conn: conn}
}

here we ask the user for a username and start goroutines for incoming and outgoing messages

func (cl *client) start() {
	cl.setUpUsername()
	go cl.receiveMessages()
	go cl.sendMessages()
}

receive messages

here we create a new scanner and go until the connection ends or an error happens

func (cl *client) receiveMessages() {
	scanner := bufio.NewScanner(cl.conn)
	for scanner.Scan() {
		if scanner.Err() != nil {
			log.Fatalf("receive messages %v: ", scanner.Err())
			break
		}
		cl.in <- scanner.Bytes()
	}
	cl.token <- struct{}{} // end connection
}

the content of the scanners is redirected to the in-client channel and then it’s added to the chat message queue

send messages

to send messages to a specific client, we listen to the out channel and write to the connection

func (cl *client) sendMessages() {
	for msg := range cl.out {

		cl.conn.Write(msg)
	}
}

test #1

let’s give it a go and run the server and a couple of clients server

go run main. go

this will start listening to connections on port 8080

for the clients we’ll use nc

client A

nc localhost 8080
Enter your username: megumin
welcome megumin

client B

nc localhost 8080
Enter your username: gopher
welcome gopher

if we write a message on either client we should see the message in both places

client A

nc localhost 8080
Enter your username: megumin
welcome megumin
gopher says: hello megumin

client B

 nc localhost 8080
Enter your username: gopher
welcome gopher
hello megumin
gopher says: hello megumin

Cool, we have a working chat in place, the next and final step for this time it’s to let the other users know when someone leaves the chat

exit queue channel

this will help us close the channels, and the connection and let the other users in the room who left the chat

type chat struct {
	clients      map[*client]bool //<- notice the update to clients
	messageQueue chan []byte
	exitQueue    chan *client
}

the exit queue is a channel that holds clients

exit guide

func (c *chat) exitGuide(client *client) {
	delete(c.clients, client)
	close(client.in)
	close(client.out)

	c.messageQueue <- []byte(fmt.Sprintf("%s left the chat\n", client.name))
	defer client.conn.Close()
}

update handleConnection

func (c *chat) handleConnection(conn net.Conn) {
	in, out := make(chan []byte), make(chan []byte)
	client := newClient(conn, in, out)
	c.clients[client] = true
	client.start()
	go c.bookRoom(client)
	<-client.token // wait until the client ends writting
	c.exitQueue <- client // send client to exit queue
}

test #2

last but not least the handle connection waits until the client ends writing to the connection and writes to the exit queue passing the client pointer to the channel

client A

nc localhost 8080
Enter your username: megumin
welcome megumin
trump says: hello megumin
hi trum
megumin says: hi trum
bye
megumin says: bye
^C

client B

nc localhost 8080
Enter your username: trump
welcome trump
hello megumin
trump says: hello megumin
megumin says: hi trum
megumin says: bye
megumin left the chat

TLDR;

the overall parts of the chat are

handleConnection // creates new goroutines for incomming connection
messagesManager // sends and receives messages from clients
server // single serve function that waits for channel changes, newuser, disconnect, sendmessages
bookRoom: to format and send messages
setUpUsername: at the beginning of the connection we ask for a user name
exitGuide: closes the in / out channels of the client, deletes the client to the chat and closes the connection

check the full code here package here code here

Simple golang TCP chat

accept connections

client

give it structure

chat

client

listen for connections

start the new client and book a new room

setting up user

receive messages

send messages

test #1

exit queue channel

exit guide

update handleConnection

test #2

TLDR;