When implementing WebSocket in an existing system, how many layers of architecture need to be modified?

Implementing WebSocket into an existing system usually requires modifications to several layers of architecture:

1. Network Layer (Transport Layer) — configuring the server to support the WebSocket protocol over HTTP.
2. Application Layer — implementing the logic for handling WebSocket messages, managing sessions, routing messages.
3. Client Layer (Frontend or client application) — adding support for WebSocket to establish and maintain the connection.

Thus, at least three layers need changes to ensure full WebSocket functionality in the system.