最後更新: 2019-10-10

socket.io

real-time bidirectional event-based communication, it relies on Engine.IO

Socket.IO is not a WebSocket implementation.

(它是用 Long Polling 的, WebSocket as a transport when possible)

i.e.

wss://URL/socket.io/?version=95&lastupdate=Epoch&EIO=3&transport=websocket&sid=XXXXXXXXXXXXX

Connections are established even in the presence of

• proxies and
• load balancers

* sticky-session requirement

封裝了多種協議(Comet: iframe, long polling ...), 蔽了所有底層細節

設計

 - Binary streaming (image, audio, video)

 - Multiplexing support

Socket.IO allows you to create several Namespaces,

which will act as separate communication channels but will share the same underlying connection.

 - Room support

Within each Namespace, you can define arbitrary channels, called Rooms,

that sockets can join and leave. (用圖: to send notifications to a group of users)

 - Auto-reconnection support

 - Disconnection detection(heartbeat mechanism)

Comet

彗星

把發出的 Request 像彗星的尾巴一樣拉的很長, 讓伺服器保持連線的狀態, 持續讓 Server 端 Response 資料回來

Long Polling

 * It works on every platform, browser or device

 * non-blocking IO

Client 向 Server 發出請求, Server 接收到請求後,

server 並不一定立即發送回應給 client, 而是看數據是否更新,

如果數據已經更新了的話, 那就立即斷掉 client,

接著 Client 觸發新一個的 Request. (新的數據更新)

但如果數據沒有更新, 那就把這個請求保持住(不回應).

如果 server 的數據長時間沒有更新, 一段時間後，請求便會超時,

client 收到超時信息後, 再發送一個新的請求給 server 建立連線.