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Multi-process Resource Loading

Background

All network communication is handled by the main browser process. This is done not only so that the browser process can control each renderer's access to the network, but also so that we can maintain consistent session state across processes like cookies and cached data. It is also important because as a HTTP/1.1 user-agent, the browser as a whole should not open too many connections per host.

Overview

Our multi-process application can be viewed in three layers. At the lowest layer is the WebKit library which renders pages. Above that are the renderer process (simplistically, one-per-tab), each of which contains one WebKit instance. Managing all the renderers is the browser process, which controls all network accesses.


WebKit

WebKit has a ResourceLoader object which is responsible for fetching data. Each loader has a ResourceHandle for performing the actual requests. The header file for this object is inside the WebKit code, and we are unwilling to fork it. Fortunately, it contains a d member which we can define. We remove the old implementation of ResourceHandleInternal and provide our own, located in webkit/glue/resource_handle_win.cc. Despite the name, it has nothing to do with Win32.

ResourceHandleInternal implements the virtual interface ResourceLoaderBridge::Peer, defined in webkit/glue/resource_loader_bridge.h. This is the callback interface used by the renderer to dispatch data and other events to WebKit.

ResourceHandleInternal inside WebKit talks to the renderer (to initiate or cancel requests) via the ResourceLoaderBridge virtual interface. An implementation of this interface is retrieved by calling the static function ResourceLoaderBridge::Create, which the renderer implements. The test shell uses a different resource loader, so provides a different implementation, non-IPC version of ResourceLoaderBridge, located in webkit/tools/test_shell/simple_resource_loader_bridge.

Renderer

The renderer's implementation of ResourceLoaderBridge, called IPCResourceLoaderBridge, is located in renderer/resource_dispatcher. It uses the global ResourceDispatcher singleton object (one for each renderer process) to create a unique request ID and forward the request to the browser via IPC. Responses from the browser will reference this request ID, which can then be converted back to the ResourceLoaderBridge::Peer object (inside WebKit) by the resource dispatcher.

The conversion between resource requests and their data is all handled by common/resource_loader_ipc. The renderer and the browser both share this code so that the conversions are in sync.

Browser

The RenderProcessHost objects inside the browser receive the IPC requests from each renderer. It forwards these requests to the global ResourceDispatcherHost, using a pointer to the render process host (specifically, an implementation of ResourceDispatcherHost::Receiver) and the request ID generated by the renderer to uniquely identify the request.

Each request is then converted into a URLRequest object, which in turn forwards it to it's internal URLRequestJob that implements the specific protocol desired. When the URLRequest generates notifications, its ResourceDispatcherHost::Receiver and request ID are used to send the notification to the correct RenderProcessHost for sending back to the renderer. Since the ID generated by the renderer is preserved, it is able to correlate all responses with a specific request first generated by WebKit.

Cookies

All cookies are handled by our CookieMonster object in /net/base. We do not share cookies with WinInet. The cookie monster lives in the browser process which handles all network requests because cookies need to be the same across all tabs.

Pages can request cookies for a document via document.cookie. When this occurs, we send a synchronous message from the renderer to the browser requesting the cookie. While the browser is processing the cookie, the thread that WebKit works on is suspended. When the renderer's I/O thread receives the response from the browser, it un-suspends the thread and passes the result back to the JavaScript engine.

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Brett Wilson,
Jul 30, 2008, 7:50 PM
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Mike Jeffrey,
Jul 25, 2008, 2:17 PM
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