Websocket channel support for Workhorse

In some cases, GitLab can provide the following through a WebSocket:

  • In-browser terminal access to an environment: a running server or container, onto which a project has been deployed.
  • Access to services running in CI.

Workhorse manages the WebSocket upgrade and long-lived connection to the websocket connection, which frees up GitLab to process other requests. This document outlines the architecture of these connections.

Introduction to WebSockets

Websockets are an “upgraded” HTTP/1.1 request. They permit bidirectional communication between a client and a server. Websockets are not HTTP. Clients can send messages (known as frames) to the server at any time, and vice versa. Client messages are not necessarily requests, and server messages are not necessarily responses. WebSocket URLs have schemes like ws:// (unencrypted) or wss:// (TLS-secured).

When requesting an upgrade to WebSocket, the browser sends a HTTP/1.1 request like this:

GET /path.ws HTTP/1.1
Connection: upgrade
Upgrade: websocket
Sec-WebSocket-Protocol: terminal.gitlab.com
# More headers, including security measures

At this point, the connection is still HTTP, so this is a request. The server can send a normal HTTP response, such as 404 Not Found or 500 Internal Server Error.

If the server decides to permit the upgrade, it sends a HTTP 101 Switching Protocols response. From this point, the connection is no longer HTTP. It is now a WebSocket and frames, not HTTP requests, flow over it. The connection persists until the client or server closes the connection.

In addition to the sub-protocol, individual websocket frames may also specify a message type, such as:

  • BinaryMessage
  • TextMessage
  • Ping
  • Pong
  • Close

Only binary frames can contain arbitrary data. The frames are expected to be valid UTF-8 strings, in addition to any sub-protocol expectations.

Browser to Workhorse

Using the terminal as an example:

  1. GitLab serves a JavaScript terminal emulator to the browser on a URL like https://gitlab.com/group/project/-/environments/1/terminal.
  2. This URL opens a websocket connection to wss://gitlab.com/group/project/-/environments/1/terminal.ws. This endpoint exists only in Workhorse, and doesn’t exist in GitLab.
  3. When receiving the connection, Workhorse first performs a preauthentication request to GitLab to confirm the client is authorized to access the requested terminal:
    • If the client has the appropriate permissions and the terminal exists, GitLab responds with a successful response that includes details of the terminal the client should be connected to.
    • Otherwise, Workhorse returns an appropriate HTTP error response.
  4. If GitLab returns valid terminal details to Workhorse, it:
    1. Connects to the specified terminal.
    2. Upgrades the browser to a WebSocket.
    3. Proxies between the two connections for as long as the browser’s credentials are valid.
    4. Send regular PingMessage control frames to the browser, to prevent intervening proxies from terminating the connection while the browser is present.

The browser must request an upgrade with a specific sub-protocol:


This sub-protocol considers TextMessage frames to be invalid. Control frames, such as PingMessage or CloseMessage, have their usual meanings.

  • BinaryMessage frames sent from the browser to the server are arbitrary text input.
  • BinaryMessage frames sent from the server to the browser are arbitrary text output.

These frames are expected to contain ANSI text control codes and may be in any encoding.


This sub-protocol considers BinaryMessage frames to be invalid. Control frames, such as PingMessage or CloseMessage, have their usual meanings.

  • TextMessage frames sent from the browser to the server are base64-encoded arbitrary text input. The server must base64-decode them before inputting them.
  • TextMessage frames sent from the server to the browser are base64-encoded arbitrary text output. The browser must base64-decode them before outputting them.

In their base64-encoded form, these frames are expected to contain ANSI terminal control codes, and may be in any encoding.

Workhorse to GitLab

Using the terminal as an example, before upgrading the browser, Workhorse sends a normal HTTP request to GitLab on a URL like https://gitlab.com/group/project/environments/1/terminal.ws/authorize. This returns a JSON response containing details of where the terminal can be found, and how to connect it. In particular, the following details are returned in case of success:

  • WebSocket URL to connect** to, such as wss://example.com/terminals/1.ws?tty=1.
  • WebSocket sub-protocols to support, such as ["channel.k8s.io"].
  • Headers to send, such as Authorization: Token xxyyz.
  • Optional. Certificate authority to verify wss connections with.

Workhorse periodically rechecks this endpoint. If it receives an error response, or the details of the terminal change, it terminates the websocket session.

Workhorse to the WebSocket server

In GitLab, environments or CI jobs may have a deployment service (like KubernetesService) associated with them. This service knows where the terminals or the service for an environment may be found, and GitLab returns these details to Workhorse.

These URLs are also WebSocket URLs. GitLab tells Workhorse which sub-protocols to speak over the connection, along with any authentication details required by the remote end.

Before upgrading the browser’s connection to a websocket, Workhorse:

  1. Opens a HTTP client connection, according to the details given to it by Workhorse.
  2. Attempts to upgrade that connection to a websocket.
    • If it fails, an error response is sent to the browser.
    • If it succeeds, the browser is also upgraded.

Workhorse now has two websocket connections, albeit with differing sub-protocols, and then:

  • Decodes incoming frames from the browser, re-encodes them to the channel’s sub-protocol, and sends them to the channel.
  • Decodes incoming frames from the channel, re-encodes them to the browser’s sub-protocol, and sends them to the browser.

When either connection closes or enters an error state, Workhorse detects the error and closes the other connection, terminating the channel session. If the browser is the connection that has disconnected, Workhorse sends an ANSI End of Transmission control code (the 0x04 byte) to the channel, encoded according to the appropriate sub-protocol. To avoid being disconnected, Workhorse replies to any websocket ping frame sent by the channel.

Workhorse only supports the following sub-protocols:

Supporting new deployment services requires new sub-protocols to be supported.


Used by Kubernetes, this sub-protocol defines a simple multiplexed channel.

Control frames have their usual meanings. TextMessage frames are invalid. BinaryMessage frames represent I/O to a specific file descriptor.

The first byte of each BinaryMessage frame represents the file descriptor (fd) number, as a uint8. For example:

  • 0x00 corresponds to fd 0, STDIN.
  • 0x01 corresponds to fd 1, STDOUT.

The remaining bytes represent arbitrary data. For frames received from the server, they are bytes that have been received from that fd. For frames sent to the server, they are bytes that should be written to that fd.


Also used by Kubernetes, this sub-protocol defines a similar multiplexed channel to channel.k8s.io. The main differences are:

  • TextMessage frames are valid, rather than BinaryMessage frames.
  • The first byte of each TextMessage frame represents the file descriptor as a numeric UTF-8 character, so the character U+0030, or “0”, is fd 0, STDIN.
  • The remaining bytes represent base64-encoded arbitrary data.