servicification.md

Servicifying Chromium Features

[TOC]

Overview

Much to the dismay of Chromium developers, practicing linguists, and keyboard operators everywhere, the term servicificificification [sic] has been egregiously smuggled into the Chromium parlance.

Lots of Chromium code is contained in reasonably well-isolated component libraries with some occasionally fuzzy boundaries and often a surprising number of gnarly runtime interdependencies among a complex graph of components. Y implements one of Z's delegate interfaces, while X implements one of Y's delegate interfaces, and now it's possible for some ridiculous bug to creep in where W calls into Z at the wrong time and causes a crash in X. Yikes.

Servicification embodies the ongoing process of servicifying Chromium features and subsystems, or refactoring these collections of library code into services with well-defined public API boundaries and very strong runtime isolation via Mojo interfaces.

The primary goals are to improve maintainability and extensibility of the system over time, while also allowing for more flexible runtime configuration. For example, with the Network Service in place we can now run the entire network stack either inside or outside of the browser process with the flip of a command-line switch. Client code using the Network Service stays the same, independent of that switch.

This document focuses on helpful guidelines and patterns for servicifying parts of Chromium, taking into account some nuances in how Chromium models its core services as well as how it embeds and configures the Service Manager. Readers are strongly encouraged to first read some basic Service Manager documentation, as it will likely make the contents of this document easier to digest.

Also see general Mojo & Services documentation for other introductory guides, API references, etc.

Setting Up The Service

There are three big things you must decide when building and hooking up a shiny new service:

• Where should the service live in the tree?
• Do you need an instance of your service per BrowserContext?
• Can Content depend on your service, or must Content embedders like Chrome do so independently?

This section aims to help you understand and answer those questions.

Where in the Tree?

Based on the service development guidelines, any service which could be reasonably justified as a core system service in a hypothetical, well-designed operating system may belong in the top-level //services directory. If that sounds super hand-wavy and unclear, that's because it is! There isn't really a great universal policy here, so when in doubt, contact your favorite local [email protected] mailing list and start a friendly discussion.

Other common places where developers place services, and why:

• //components/services for services which haven't yet made the cut for //services but which are either used by Content directly or by multiple Content embedders.
• //chrome/services for services which are used exclusively within Chrome and not shared with other Content embedders.
• //chromeos/services for services which are used on Chrome OS by more than just Chrome itself (for example, if the ash service must also connect to them for use in system UI).

Inside Content or Not?

The next decision you need to make is whether or not Content will wire in your service directly -- that is, whether or not your service is necessary to support some subsystem Content makes available to either the web platform or to Content embedders like Chrome, Android WebView, Cast Shell, and various third-party applications.

For example, Content cannot function at all without the Network Service being available, because Content depends heavily on the Network Service to issue and process all of its network requests (imagine that, right?). As such, the Network Service is wired up to the Service Manager from within Content directly. In general, services which will be wired up in Content must live either in //services or //components/services but ideally the former.

Conversely there are a large number of services used only by Chrome today, such as the unzip service which safely performs sandboxed unpacking of compressed archive files on behalf of clients in the browser process. These can always be placed in //chrome/services.

Per-BrowserContext or Not?

Now that you've decided on a source location for your service and you know whether it will be wired into Content or hooked up by Content embedder code, all that's left left is to decide whether or not you want an instance of your service per BrowserContext (i.e. per user profile in Chrome).

The alternative is for you to manage your own instance arity, either as a singleton service (quite common) or as a service which supports multiple instances that are not each intrinsically tied to a BrowserContext. Most services choose this path because BrowserContext coupling is typically unnecessary.

As a general rule, if you're porting a subsystem which today relies heavily on BrowserContextKeyedService, it's likely that you want your service instances to have a 1:1 correspondence with BrowserContext instances.

Putting It All Together

Let's get down to brass tacks. You're a developer of action. You've made all the important choices you need to make and you've even built a small and extremely well-tested prototype service with the help of this glorious guide. Now you want to get it working in Chromium while suffering as little pain as possible.

You're not going to believe it, but this section was written just for YOU.

For services which are are not isolated per BrowserContext and which can be wired directly into Content:

• Include your service's manifest in the content_packaged_services manifest directly, similar to these ones.
• If you want to run your service embedded in the browser process, follow the examples using RegisterInProcessService here.
• If you want to run your service out-of-process, update out_of_process_services like so and hook up your actual private Service implementation exactly like the many examples here.

For services which are are isolated per BrowserContext and which can be wired directly into Content:

• Include your service's manifest in the content_browser manifest directly, similar to these ones.
• If you want to run your service embedded in the browser process, follow the example here
• If you want to run your service out-of-process, you are doing something that hasn't been done yet and you will need to build a new thing.

For services which are not isolated per BrowserContext but which can not be wired directly into Content:

• Include your service's manifest in Chrome's content_packaged_services manifest overlay similar to these ones
• If you want to run your service embedded in the browser process, follow the examples in ChromeContentBrowserClient::HandleServiceRequest here
• If you want to run your service out-of-process, modify ChromeContentBrowserClient::RegisterOutOfProcessServices like the examples here and hook up your Service implementation in ChromeContentUtilityClient::HandleServiceRequest like the ones here.

For services which are isolated per BrowserContext but which can not be wired directly into Content:

• Include your service's manifest in Chrome's content_browser manifest overlay similar to these ones
• If you want to run your service embedded in the browser process, follow the examples in ProfileImpl::HandleServiceRequest here
• If you want to run your service out-of-process, you are doing something that hasn't been done yet and you will need to build a new thing.

*** aside The non-Content examples above are obviously specific to Chrome as the embedder, but Chrome's additions to supported services are all facilitated through the common ContentBrowserClient and ContentUtilityClient APIs that all embedders can implement. Mimicking what Chrome does should be sufficient for any embedder.

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Incremental Servicification

For large Chromium features it is not feasible to convert an entire subsystem to a service all at once. As a result, it may be necessary for the subsystem to spend a considerable amount of time (weeks or months) split between the old implementation and your beautiful, sparkling new service implementation.

In creating your service, you likely have two goals:

• Making the service available to its consumers
• Making the service self-contained

Those two goals are not the same, and to some extent are at tension:

• To satisfy the first, you need to build out the API surface of the service to a sufficient degree for the anticipated use cases.

• To satisfy the second, you need to convert all clients of the code that you are servicifying to instead use the service, and then fold that code into the internal implementation of the service.

Whatever your goals, you will need to proceed incrementally if your project is at all non-trivial (as they basically all are given the nature of the effort). You should explicitly decide what your approach to incremental bringup and conversion will be. Here are some approaches that have been taken for various services:

• Build out your service depending directly on existing code, convert the clients of that code 1-by-1, and fold the existing code into the service implementation when complete (Identity Service).
• Build out the service with new code and make the existing code into a client library of the service. In that fashion, all consumers of the existing code get converted transparently (Preferences Service).
• Build out the new service piece-by-piece by picking a given bite-size piece of functionality and entirely servicifying that functionality (Device Service).

These all have tradeoffs:

• The first lets you incrementally validate your API and implementation, but leaves the service depending on external code for a long period of time.
• The second can create a self-contained service more quickly, but leaves all the existing clients in place as potential cleanup work.
• The third ensures that you're being honest as you go, but delays having the breadth of the service API up and going.

Which makes sense depends both on the nature of the existing code and on the priorities for doing the servicification. The first two enable making the service available for new use cases sooner at the cost of leaving legacy code in place longer, while the last is most suitable when you want to be very exacting about doing the servicification cleanly as you go.

Platform-Specific Issues: Android

As you servicify code running on Android, you might find that you need to port interfaces that are served in Java. Here is an example CL that gives a basic pattern to follow in doing this.

You also might need to register JNI in your service. That is simple to set up, as illustrated in this CL. (Note that that CL is doing more than just enabling the Device Service to register JNI; you should take the register_jni.cc file added there as your starting point to examine the pattern to follow).

Finally, it is possible that your feature will have coupling to UI process state (e.g., the Activity) via Android system APIs. To handle this challenging issue, see the section on Coupling to UI.

Platform-Specific Issues: iOS

*** aside WARNING: Some of this content is obsolete and needs to be updated. When in doubt, look approximately near the recommended bits of code and try to find relevant prior art.

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Services are supported on iOS, with the usage model in //ios/web being very close to the usage model in //content. More specifically:

• To embed a global service in the browser service, override WebClient::RegisterServices. For an example usage, see ShellWebClient and the related integration test that connects to the embedded service.
• To embed a per-BrowserState service, override BrowserState::RegisterServices. For an example usage, see ShellBrowserState and the related integration test that connects to the embedded service.
• To register a per-frame Mojo interface, override WebClient::BindInterfaceRequestFromMainFrame. For an example usage, see ShellWebClient and the related integration test that connects to the interface. Note that this is the equivalent of ContentBrowserClient::BindInterfaceRequestFromFrame(), as on iOS all operation "in the content area" is implicitly operating in the context of the page's main frame.

If you have a use case or need for services on iOS, contact [email protected]. For general information on the motivations and vision for supporting services on iOS, see the high-level servicification design doc. In particular, search for the mentions of iOS within the doc.

Client-Specific Issues

Mocking Interface Impls in JS

It is a common pattern in Blink's web tests to mock a remote Mojo interface in JS so that native Blink code requests interfaces from the test JS rather than whatever would normally service them in the browser process.

The current way to set up that sort of thing looks like this.

Feature Impls That Depend on Blink Headers

In the course of servicifying a feature that has Blink as a client, you might encounter cases where the feature implementation has dependencies on Blink public headers (e.g., defining POD structs that are used both by the client and by the feature implementation). These dependencies pose a challenge:

• Services should not depend on Blink, as this is a dependency inversion (Blink is a client of services).
• However, Blink is very careful about accepting dependencies from Chromium.

To meet this challenge, you have two options:

1. Move the code in question from C++ to mojom (e.g., if it is simple structs).
2. Move the code into the service's C++ client library, being very explicit about its usage by Blink. See this CL for a basic pattern to follow.

Frame-Scoped Connections

You must think carefully about the scoping of the connection being made from Blink. In particular, some feature requests are necessarily scoped to a frame in the context of Blink (e.g., geolocation, where permission to access the interface is origin-scoped). Servicifying these features is then challenging, as Blink has no frame-scoped connection to arbitrary services (by design, as arbitrary services have no knowledge of frames or even a notion of what a frame is).

After a long discussion, the policy that we have adopted for this challenge is the following:

CURRENT

• The renderer makes a request through its frame-scoped connection to the browser.
• The browser obtains the necessary permissions before directly servicing the request.

AFTER SERVICIFYING THE FEATURE IN QUESTION

• The renderer makes a request through its frame-scoped connection to the browser.
• The browser obtains the necessary permissions before forwarding the request on to the underlying service that hosts the feature.

Notably, from the renderer's POV essentially nothing changes here.

Strategies for Challenges to Decoupling from //content

Coupling to UI

Some feature implementations have hard constraints on coupling to UI on various platforms. An example is NFC on Android, which requires the Activity of the view in which the requesting client is hosted in order to access the NFC platform APIs. This coupling is at odds with the vision of servicification, which is to make the service physically isolatable. However, when it occurs, we need to accommodate it.

The high-level decision that we have reached is to scope the coupling to the feature and platform in question (rather than e.g. introducing a general-purpose FooServiceDelegate), in order to make it completely explicit what requires the coupling and to avoid the coupling creeping in scope.

The basic strategy to support this coupling while still servicifying the feature in question is to inject a mechanism of mapping from an opaque "context ID" to the required context. The embedder (e.g., //content) maintains this map, and the service makes use of it. The embedder also serves as an intermediary: it provides a connection that is appropriately context-scoped to clients. When clients request the feature in question, the embedder forwards the request on along with the appropriate context ID. The service impl can then map that context ID back to the needed context on-demand using the mapping functionality injected into the service impl.

To make this more concrete, see this CL.

Shutdown of Singletons

You might find that your feature includes singletons that are shut down as part of //content's shutdown process. As part of decoupling the feature implementation entirely from //content, the shutdown of these singletons must be either ported into your service or eliminated:

• In general, as Chromium is moving away from graceful shutdown, the first question to analyze is: Do the singletons actually need to be shut down at all?
• If you need to preserve shutdown of the singleton, the naive approach is to move the shutdown of the singleton to the destructor of your service
• However, you should carefully examine when your service is destroyed compared to when the previous code was executing, and ensure that any differences introduced do not impact correctness.

See this thread for more discussion of this issue.

Additional Support

If this document was not helpful in some way, please post a message to your friendly local [email protected] or [email protected] mailing list.