Blink renderer

Blink architecture overview

See this "How Blink works" document.

blink/renderer directory structure

This section describes a high-level architecture of blink/renderer,
which contains most of the Web Platform implementation, and runs exclusively
in the renderer process.
On the other hand, common/ and public/common
also run in the browser process.

All code in blink/renderer is an implementation detail of Blink
and should not be used outside of it. Use Blink's public API
in code outside of Blink.

core/

The core/ directory implements the essence of the Web Platform defined by specs
and IDL interfaces. Due to historical reasons, core/ contains a lot of features with
complex inter-dependencies and hence can be perceived as a single monolithic entity.

modules/

The modules/ directory is a collection of self-contained, well-defined features
of the Web Platform that are factored out of a monolithic core/. These features are:

• large, tens to hundreds of files, with rare exceptions;
• self-contained with fine-grained responsibilities and README.md;
• have dependencies outlined with DEPS explicitly;
• can depend on other features under platform/, core/ or modules/,
  forming a healthy dependency tree.

modules/ OWNERS are responsible for making sure only features
that satisfy requirements above are added.

For example, modules/crypto implements WebCrypto API.

platform/

The platform/ directory is a collection of lower level features of Blink that are factored
out of a monolithic core/. These features follow the same principles as modules/,
but with different dependencies allowed:

• large, tens to hundreds of files, with rare exceptions;
• self-contained with fine-grained responsibilities and README.md;
• have dependencies outlined with DEPS explicitly;
• can depend on other features under platform/ (but not core/ or modules/),
  forming a healthy dependency tree.

platform/ OWNERS are responsible for making sure only features
that satisfy requirements above are added.

For example, platform/scheduler implements a task scheduler for all tasks
posted by Blink, while platform/wtf implements Blink-specific containers
(e.g., WTF::Vector, WTF::HashTable, WTF::String).

core vs modules vs platform split

Note that specs do not have a notion of "core", "platform" or "modules".
The distinction between them is for implementation
convenience to avoid putting everything in a single core/ entity
(which decreases code modularity and increases build time):

• features that are tightly coupled with HTML, CSS and other fundamental parts
  of DOM should go to core/;
• features which conceptually depend on the features from "core"
  should go to modules/;
• features which the "core" depends upon should go to platform/.

Note that some of these guidelines are violated (at the time of writing this),
but the code should gradually change and eventually conform.

bindings/

The bindings/ directory contains files that heavily use V8 APIs.
The rationale for splitting bindings out is: V8 APIs are complex, error-prone and
security-sensitive, so we want to put V8 API usage separately from other code.

In terms of dependencies, bindings/core and core/ are in the same link unit.
The only difference is how heavily they are using V8 APIs.
If a given file is using a lot of V8 APIs, it should go to bindings/core.
Otherwise, it should go to core/. Consult bindings/ OWNERS when in doubt.

Note that over time bindings/core should move to core/bindings and become
just a part of a larger "core".

All of the above applies to bindings/modules and modules/.

extensions/

The extensions/ directory contains embedder-specific, not-web-exposed APIs (e.g., not-web-exposed APIs for Chromium OS etc).
The directory is useful to implement embedder-specific, not-web-exposed APIs
using Blink technologies for web-exposed APIs like WebIDL, V8 bindings and Oilpan.

Remember that you should not implement web-exposed APIs in extensions/. Web-exposed APIs should go through the standardization process and be implemented in core/ or modules/. Also, per the Chromium contributor guideline, code that is not used by Chromium should not be added to extensions/.

In terms of dependencies, extensions/ can depend on modules/, core/ and platform/, but not vice versa.

controller/

The controller/ directory contains the system infrastructure
that uses or drives Blink. Functionality that implements the Web Platform
should not go to controller/, but instead reside in platform/, core/
or modules/.

If the sole purpose of higher level functionality is to drive the Web Platform
or to implement API for the embedder, it goes to controller/,
however most of the features should go to other directories.
Consult controller/ OWNERS when in doubt.

In terms of dependencies, controller/ can depend on extensions/, modules/, core/ and platform/, but not vice versa.

build/

The build/ directory contains scripts to build Blink.

In terms of dependencies, build/ is a stand-alone directory.

Dependencies

Dependencies only flow in the following order:

• public/web
• controller/
• extensions/
• modules/ and bindings/modules
• core/ and bindings/core
• platform/
• public/platform
• public/common
• //base, V8 etc.

See this diagram.

build/ is a stand-alone directory.

Type dependencies

Member variables of the following types are strongly discouraged in Blink:

• STL strings and containers. Use WTF::String and WTF containers instead.
• GURL and url::Origin. Use KURL and SecurityOrigin respectively.
• Any //base type which has a matching type in platform/wtf. The number of
  duplicated types between WTF and base is continuously shrinking,
  but always look at WTF first.

The types above could only be used at the boundary to interoperate
with //base, //services, //third_party/blink/common and other
Chromium-side or third-party code. It is also allowed to use local variables
of these types when convenient, as long as the result is not stored
in a member variable.
For example, calling an utility function on an std::string which came
from //net and then converting to WTF::String to store in a field
is allowed.

We try to share as much code between Chromium and Blink as possible,
so the number of these types should go down. However, some types
really need to be optimized for Blink's workload (e.g., Vector,
HashTable, AtomicString).

Exceptions to this rule:

• Code in //third_party/blink/common and //third_party/blink/public/common
  also runs in the browser process, and should use STL and base instead of WTF.
• Selected types in public/platform and public/web,
  whole purpose of which is conversion between WTF and STL,
  for example WebString or WebVector.

To prevent use of random types, we control allowed types by allow listing them
in DEPS and a presubmit
script.

Mojo

Blink can use Mojo and directly talk to the browser process. This allows removal of unnecessary
public APIs and abstraction layers and it is highly recommended.

Contact

If you have any questions about the directory architecture and dependencies,
reach out to platform-architecture-dev@chromium.org!