Threading

ChromeDriver uses 3 types of threads:

• One command thread. This is the primary thread that executes the main function and starts the ChromeDriver process.
  It is also responsible for parsing each incoming command to determine its target session and dispatch it accordingly.

• One I/O thread. Responsible for I/O with clients and with Chrome.

• Session threads, one for each session. It is responsible for executing all ChromeDriver commands that target a specific session.

Thread Interactions

As one would expect, threads cannot directly call each other. Instead, ChromeDriver uses mechanisms provided by Chromium base library to post tasks between threads.

Each thread in ChromeDriver is wrapped by a base::Thread object. This object exposes a base::TaskRunner, which provides the ability to post tasks to that thread.
When thread A wants to call a function on thread B, it finds the TaskRunner object corresponding to thread B, and calls its TaskRunner::PostTask method.
This will cause a task to be posted to thread B. When thread B is not busy, it will execute tasks posted to it, in the order they were queued.

A task cannot directly return results to the calling thread. If a response is desired, thread B can post a task back to thread A.

Conventions

Functions intended as inter-thread tasks have special names to make it clear which threads they should run on.

• A function intended for the command thread should have name ending with OnCmdThread or OnCommandThread, e.g., HandleRequestOnCmdThread.
• A function intended for the I/O thread has name ending with OnIOThread, e.g., StartServerOnIOThread.
• A function for a session thread has name ending with OnSessionThread, e.g., ExecuteSessionCommandOnSessionThread.

Scenarios

This section details thread transitions in several scenarios.

Process Initialization

The main function runs on the command thread. After some initialization, it calls RunServer.

RunServer also runs on the command thread. It does the following:

• Creates the I/O thread.
• Posts a task to run StartServerOnIOThread on the I/O thread.
• Run an event loop, and waits for incoming tasks posted from other threads.

The final part of initialization occurs in StartServerOnIOThread on the I/O thread. It creates an HttpServer, and then calls HttpServer::Start to start listening for incoming WebDriver requests.

New Request from Client

When a new request is received from a client, two things happen on the I/O thread.

• HttpServer::OnHttpRequest runs HandleRequestOnIOThread, which is stored in HttpServer::handle_request_func_.
• HandleRequestOnIOThread posts a task to run HandleRequestOnCmdThread.

Then several activities happen on the command thread.

• HandleRequestOnCmdThread verifies that the request comes from a trusted IP address.
• HttpHandler::Handle does some validation of the incoming request.
• HttpHandler::HandleCommand matches the request against a list of commands implemented by ChromeDriver, and dispatches the request to the appropriate command handler.

If the command is global (i.e., affecting all sessions), it is executed directly on the command thread. Most commands, however, target specific sessions, and are dispatched to the appropriate session thread to execute.
New session command (details below) does not target a specific session; instead it creates a new session thread for the session so that it can later be targeted by other commands.

Regardless of which thread executes the command, when the command finishes, HttpHandler::PrepareResponse runs on command thread to prepare the response, and then the I/O thread sends the response back to the client.

New Session

Before a client can do anything useful, it must first call the InitSession command to create a new session. This command is first dispatched to ExecuteCreateSession, which runs on the command thread and does the following.

• Generates a new session ID, which is defined by W3C WebDriver Standard as a 128-bit random hexadecimal string. This session ID is returned to the client, and is used to identify this session in all future requests.
• Creates a new Session object to store session-specific data.
• Creates and initializes a session thread.
• Invokes ExecuteInitSession on the newly created session thread.

Then the session thread is responsible for parsing the requested capabilities, starting Chrome, and initializing the session.

Dispatching Command to a Session

Most WebDriver protocol commands apply to a particular session. In such cases, ExecuteSessionCommand is responsible for finding the target session thread and posting a task to it.