How to Send a Specific Websocket Message to a Client in Go (Gorilla)?

Sending a Specific Websocket Message to a Client in Go (Using Gorilla)

Websockets provide a connection-oriented protocol for low-latency communication between clients and servers. In Go, several frameworks simplify websocket handling, including Gorilla. However, understanding how to send messages to specific clients can be challenging.

Client and Server Setup

In Gorilla, the server establishes a hub to manage client connections. The hub includes a map of clients and channels for broadcasting messages. Each client has a websocket connection and a send channel.

Identifying a Specific Client

To send a message to a specific client, you need a way to uniquely identify it. Typically, this is done by creating a unique ID field in the client struct.

Sending a Specific Message

To send a message to a specific client, you can either modify the hub or write directly to the client's websocket connection. Modifying the hub requires creating a message type that includes the target client ID and data. You would then replace the broadcast channel with a message channel and modify the hub's for loop accordingly:

<code class="go">type message struct {
    ID idType
    data []byte
}

func (h *Hub) run() {
    for {
        select {
        case client := <-h.register:
            h.clients[client.ID] = client
        case client := <-h.unregister:
            if _, ok := h.clients[client.ID]; ok {
                delete(h.clients, client.ID)
                close(client.send)
            }
        case message := <-h.send:
            if client, ok := h.clients[message.ID]; ok {
                select {
                case client.send <- message.data:
                default:
                    close(client.send)
                    delete(h.connections, client)
                }
            }
        }
    }
}</code>

Sending a message to a specific client would then involve:

<code class="go">hub.send <- message{ID: targetID, data: data}</code>

Alternatively, you can write directly to the client's websocket connection using NextWriter or WriteMessage. This approach requires maintaining a connection to each client and ensuring single-threaded writing to the connection.

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