breadcrumbs: Network Stack > chromium_link: Chromium chromium_os_link: Chromium OS home_link: Home page_name: network-stack-use-in-chromium title: Network Stack Use in Chromium title_hdr: |
Chromium is the primary consumer of the Chromium network stack. While there are other consumers, the main one is the Chromium browser process. For the most part, the network stack runs in the browser process's IO thread, and interfaces with the Chromium network stack via ChromeURLRequestContext
and URLRequest
/URLFetcher
.
The Chromium browser process uses several instances of ChromeURLRequestContext
, a subclass of [URLRequestContext](/developers/design-documents/network-stack#TOC-Overview)
. For a given profile, there are a number of ChromeURLRequestContext
instances:
ChromeURLRequestContext
. The majority of URLRequest
s are associated with this ChromeURLRequestContext
instance.ChromeURLRequestContext
. This one uses a different [HttpCache](/developers/design-documents/network-stack/disk-cache)
instance that is optimized for large media requests (video). It shares many objects, such as the HostResolver
, [CookieMonster](/developers/design-documents/network-stack/cookiemonster)
, etc. with the main ChromeURLRequestContext
.ChromeURLRequestContext
. It shares many objects with the main ChromeURLRequestContext
. This instance helps service chrome-extension:// requests.There are also some other ChromeURLRequestContext
s, such as sync's HttpBridge::RequestContext
, the ConnectionTester
‘s ChromeURLRequestContext
, etc. These are not tied to the profile, although with a multiple profile Chromium, sync’s HttpBridge::RequestContext
likely would have to be tied to a profile.
Note that the “off the record” (Incognito) profile uses a special OTR ChromeURLRequestContext
, in addition to the media and extension contexts, that shares some objects, although notably not the HttpCache
or CookieMonster
etc that are important to be separated for an off the record profile. The HostResolver
is shared.
Many of the objects in ChromeURLRequestContext
use scoped_refptr
s to share ownership. This causes a lot of problems with object destruction ordering and reference cycles. Many of these members are being moved towards being owned externally, by the IOThread
, although that choice may have to change to ProfileImpl
or something to support multiple profiles in Chromium.
Note that currently ChromeURLRequestContext contains a lot of extra members that don‘t exist in URLRequestContext
. These are being moved out, because they have nothing to do with URLRequestContext
and were only placed in ChromeURLRequestContext
because it was a convenient per-profile object, and originally there weren’t many ChromeURLRequestContext objects.
ChromeURLRequestContext
is constructed and destroyed on the same thread. However, sometimes other threads need to hold references to them, even before they get constructed. ChromeURLRequestContextGetter
is a handle for ChromeURLRequestContext
that will lazily construct it on the first attempt to use it. Accesses to the contained ChromeURLRequestContext
are only allowed on the IO thread.
ResourceDispatcherHost
creates URLRequests
when the renderer or plugin processes issue resource requests. It retains pointers to all the ChromeURLRequestContextGetters
.URLFetcher
is another frontend for URLRequest
. It allows any thread with access to the appropriate ChromeURLRequestContextGetter
to proxy requests to the IO thread. It provides a simplified delegate interface that returns the full response body as a single string. There are many users of URLFetcher
in the Chromium browser process, including the omnibox, extensions, sync (which uses its own ChromeURLRequestContext
), etc. Usually these objects use the “main” ChromeURLRequestContextGetter object.It uses URLRequest::Interceptor
to hook into all URLRequests
to watch them come and go, so it can analyze the referers to learn the subdomains used in subsequent requests. It uses this knowledge to predict network requests. For likely requests, it keeps around the pointer to the “main” ChromeURLRequestContext
's HostResolver
, so it can perform DNS prefetching. For requests that are almost guaranteed to happen, it goes even further and uses the “main” ChromeURLRequestContext
's HttpStreamFactory
to preconnect a socket.