0Pricing
WebSockets & Realtime Systems Programming · レッスン

接続のライフサイクルと状態

WebSocket接続が開始してから終了するまでのさまざまな状態と、その管理方法を学びます。

「接続のライフサイクルと状態」はCoddyKit上の無料WebSockets & Realtime Systems Programmingレッスンです。 これはレッスン3/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはWebSockets & Realtime Systems Programming学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 WebSockets & Realtime Systems Programmingコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Understanding Connection States

When you work with WebSockets, it's crucial to know the current state of your connection. This helps you manage data flow and handle disconnections gracefully.

Just like a phone call goes through phases (dialing, connected, hanging up), a WebSocket connection has a defined lifecycle with specific states.

The CONNECTING State (0)

The very first state of a WebSocket is CONNECTING. This happens immediately after you create a new WebSocket object.

  • Its numeric value is 0.
  • In this state, the client is trying to establish a connection with the server.
  • This includes the initial HTTP handshake process that upgrades the connection to WebSocket.
  • You cannot send messages yet.

Connecting with Node.js

When a WebSocket client attempts to connect, it enters the CONNECTING state (readyState = 0). This is where the initial handshake occurs.

To run this Node.js example:

  1. Make sure you have Node.js installed.
  2. Open your terminal and run npm init -y
  3. Install the ws library: npm install ws
  4. Save the code below as index.js.
  5. Run node index.js.

Note: You'll need a WebSocket server running on ws://localhost:8080 for a successful connection.

const WebSocket = require('ws');

const ws = new WebSocket('ws://localhost:8080');

console.log(`Initial state: ${ws.readyState}`); // Expected: 0 (CONNECTING)

ws.onopen = () => {
  console.log(`Connection opened! State: ${ws.readyState}`); // Expected: 1 (OPEN)
  ws.send('Hello from CoddyKit client!');
  setTimeout(() => ws.close(1000, 'Demo complete'), 2000);
};

ws.onmessage = (event) => {
  console.log(`Received: ${event.data}`);
};

ws.onclose = (event) => {
  console.log(`Connection closed. Code: ${event.code}, Reason: ${event.reason}`);
  console.log(`Final state: ${ws.readyState}`); // Expected: 3 (CLOSED)
};

ws.onerror = (error) => {
  console.error('WebSocket Error:', error.message);
};

The OPEN State (1)

Once the handshake is complete and the connection is successfully established, the WebSocket enters the OPEN state.

  • Its numeric value is 1.
  • In this state, the connection is ready for bidirectional communication.
  • You can send data to the server, and the server can send data to you.
  • This is the active communication phase.

Handling the OPEN Event

The onopen event is fired once the WebSocket connection transitions from CONNECTING to OPEN. This is your cue that you can start sending messages.

Run this Node.js example to see the onopen event in action:

const WebSocket = require('ws');
const ws = new WebSocket('ws://localhost:8080');

ws.onopen = () => {
  console.log('WebSocket connection is now OPEN!');
  console.log(`Current state: ${ws.readyState}`); // Expected: 1 (OPEN)
  ws.send('Ready to chat!');
  setTimeout(() => ws.close(), 1500); // Close after a short delay
};

ws.onmessage = (event) => {
  console.log('Received:', event.data);
};

ws.onclose = () => {
  console.log('Connection closed.');
};

ws.onerror = (error) => {
  console.error('Error:', error.message);
};

The CLOSING State (2)

When either the client or the server decides to terminate the connection, it enters the CLOSING state.

  • Its numeric value is 2.
  • This state occurs after ws.close() is called, but before the connection is fully shut down.
  • During this phase, a closing handshake is performed to ensure a clean termination.
  • No new messages should be sent, though some buffered messages might still be processed.

Gracefully Closing a Connection

You can explicitly close a WebSocket connection using the ws.close() method. This will transition the connection through the CLOSING state (readyState = 2) before finally becoming CLOSED (readyState = 3).

Observe the state changes when closing:

const WebSocket = require('ws');
const ws = new WebSocket('ws://localhost:8080');

ws.onopen = () => {
  console.log(`Connection OPEN. State: ${ws.readyState}`); // 1
  console.log('Initiating connection close...');
  ws.close(1000, 'Client requested shutdown'); // Code 1000: normal closure
  console.log(`State after calling close(): ${ws.readyState}`); // Expected: 2 (CLOSING)
};

ws.onclose = (event) => {
  console.log(`Connection CLOSED. State: ${ws.readyState}`); // 3
  console.log(`Close Code: ${event.code}, Reason: ${event.reason}`);
};

ws.onerror = (error) => {
  console.error('Error during connection:', error.message);
};

The CLOSED State (3)

The final state for a WebSocket connection is CLOSED.

  • Its numeric value is 3.
  • The connection has been completely terminated.
  • No further communication can occur on this particular WebSocket object.
  • If you need to reconnect, you must create a new WebSocket instance.

Checking Connection Status

You can always check the current state of a WebSocket connection using its readyState property. This property returns one of the numeric values (0, 1, 2, 3) or their corresponding named constants:

  • WebSocket.CONNECTING (0)
  • WebSocket.OPEN (1)
  • WebSocket.CLOSING (2)
  • WebSocket.CLOSED (3)

Using these constants makes your code more readable!

Quick Check: Connection States

The readyState property indicates the current status of a WebSocket connection.

Recap: WebSocket Lifecycle

Great job! You've learned about the four essential states of a WebSocket connection:

  • CONNECTING (0): Handshake in progress.
  • OPEN (1): Connection established, ready for data.
  • CLOSING (2): Close handshake initiated.
  • CLOSED (3): Connection terminated.

Understanding these states is fundamental for building robust and reliable realtime applications.

よくある質問

「接続のライフサイクルと状態」レッスンは無料ですか？

はい。「接続のライフサイクルと状態」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、WebSockets & Realtime Systems Programmingコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 WebSockets & Realtime Systems Programmingコースには全4レッスンが含まれています。

「接続のライフサイクルと状態」で何を学びますか？

WebSocket接続が開始してから終了するまでのさまざまな状態と、その管理方法を学びます。 ブラウザで直接実行するハンズオンコードでWebSockets & Realtime Systems Programmingを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

WebSockets & Realtime Systems Programmingを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのWebSockets & Realtime Systems Programmingは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン3/4です。

「接続のライフサイクルと状態」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このWebSockets & Realtime Systems Programmingレッスンでコードを書いて実行できますか？

はい。すべてのWebSockets & Realtime Systems Programmingレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. WebSocketハンドシェイクの仕組み
  2. WebSocketのデータフレーミングとメッセージ
  3. 接続のライフサイクルと状態
  4. サブプロトコル、拡張機能、圧縮
← WebSockets & Realtime Systems Programmingに戻る