Learn how to capture, inspect, and debug WebSocket traffic in iOS apps using Trace. Frame-by-frame analysis, connection lifecycle tracking, and common debugging patterns.
WebSockets are essential for real-time features in modern apps—chat, live updates, collaborative editing, and more. But debugging WebSocket connections can be challenging because of their bidirectional, long-lived nature. This tutorial shows you how to use Trace to debug WebSocket traffic effectively.
Unlike HTTP, which follows a request-response pattern, WebSocket connections:
Traditional HTTP debugging tools often show the initial handshake but miss the actual message traffic. Trace treats WebSockets as first-class protocol with dedicated inspection.
WebSocket capture in Trace is automatic. When your app establishes a WebSocket connection:
Every WebSocket connection begins with an HTTP upgrade request:
GET /chat HTTP/1.1
Host: api.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==
Sec-WebSocket-Version: 13
The server responds with:
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
Trace captures both the request and response headers. The 101 status code indicates the connection was successfully upgraded to WebSocket.
After the handshake, Trace shows all WebSocket frames in chronological order:
WebSocket defines several frame types:
Trace displays each frame with:
For text frames, Trace automatically formats JSON payloads for readability:
{
"type": "message",
"user": "alice",
"content": "Hello, world!",
"timestamp": 1704470400
}For binary frames, Trace shows a hex dump with ASCII representation.
If your WebSocket connection fails to establish:
Upgrade and Connection headers present?Common failure modes:
For debugging message ordering or timing issues:
Example: If your app expects a response within 5 seconds but receives it after 10, the timeline will show the delay.
WebSocket has specific rules about frame structure and message flow. Trace helps identify violations:
Trace flags protocol violations with warnings in the UI.
In a busy capture session, filter for WebSocket traffic:
101Export WebSocket traffic for offline analysis:
Trace's request builder doesn't currently support WebSocket connections (HTTP-only), but you can:
Let's debug a chat app that occasionally misses messages:
/ws/chat)Finding: Some sent messages don't have corresponding received acknowledgment frames. This suggests either:
The frame timestamps show the issue occurs when frames arrive within 100ms of each other—potential race condition.
A few things to be aware of:
These are areas for future improvement.
WebSocket debugging is crucial for modern iOS apps with real-time features. Trace gives you complete visibility into connection lifecycle, frame content, and timing—everything you need to debug real-time communication effectively.
For more on WebSocket support, see the WebSocket documentation.