# SerenityOS Browser process architecture

*NOTE: This document is partly aspirational, in that the state of the code does not yet fully reflect what's described here. Implementation is underway.*

The SerenityOS web browser (**"Browser"**) uses a multi-process architecture to improve stability and security in the face of arbitrary (and possibly hostile) web content.

## Process overview

![](Images/processes.png)

Every instance of the **Browser** application can have one or more tabs open. Each tab has a unique **WebContent** service process spawned on its behalf.

Two important aspects of web browsing are further separated from the **WebContent** process: *network requests* and *image decoding*, segregated to the **RequestServer** and **ImageDecoder** processes respectively.

All processes are aggressively sandboxed using the `pledge()` and `unveil()` mechanisms. Furthermore, all processes except **Browser** run as an unprivileged user, separate from the primary logged-in desktop user.

### Process: WebContent

This process hosts the main HTML/CSS engine (**LibWeb**.) It also runs JavaScript (**LibJS**.) It gets input events from **Browser** and paints the web content into shared bitmaps. It can only communicate with the outside world via **RequestServer**.

### Process: RequestServer

This process can use networking protocols (like HTTP or HTTPS) to request files from the outside world. Each **WebContent** process gets its own **RequestServer** to do uploading or downloading on its behalf.

For DNS lookups, **RequestServer** asks for help from the system's global **LookupServer** service, which handles all outgoing DNS requests.

### Process: ImageDecoder

This process can decode images (PNG, JPEG, BMP, ICO, PBM, etc.) into bitmaps. Each image is decoded in a fresh **ImageDecoder** process. These are strongly sandboxed and can't do much except receive encoded bitmap data and return a bitmap to **WebContent** if decoding is successful.

### How processes are spawned

To get a fresh **WebContent** process, anyone with the suitable file system permissions can spawn one by connecting to
the socket at `/tmp/session/%sid/portal/webcontent`, with `%sid` being the current login session id. This socket is managed
by **SystemServer** and will spawn a new instance of **WebContent** for every connection.

The same basic concept applies to **RequestServer** and **ImageDecoder** as well, except that those services are spawned
by **WebContent** as needed, not by **Browser**.

## Class overview

![](Images/classes.png)

In the GUI application process, a `OutOfProcessWebView` widget is placed somewhere in a window, and it takes care of spawning all of the helper processes, etc.

Internally, the `OutOfProcessWebView` has a `WebContentClient` object that implements the client side of the **WebContent** IPC protocol.

The `WebContentClient` speaks to a `WebContent::ConnectionFromClient` in the **WebContent** process. Internally, the `WebContent::ConnectionFromClient` has a `WebContent::PageHost` which hosts the **LibWeb** engine's main `Web::Page` object.

Inside **LibWeb**, a `Web::Page` has a main `Web::Frame`, which may have subframes corresponding to `<frame>` or `<iframe>` HTML elements. Each `Web::Frame` has a `Web::Document`, which is the root node of the DOM tree.

Describing the **LibWeb** object model in detail is outside the scope of this document.