# Intro to Mojo & Services [TOC] ## Overview This document contains the minimum amount of information needed for a developer to start using Mojo effectively in Chromium, with example Mojo interface usage, service definition and hookup, and a brief overview of the Content layer's core services. See other [Mojo & Services](/docs/README.md#Mojo-Services) documentation for introductory guides, API references, and more. ## Mojo Terminology A **message pipe** is a pair of **endpoints**. Each endpoint has a queue of incoming messages, and writing a message at one endpoint effectively enqueues that message on the other (**peer**) endpoint. Message pipes are thus bidirectional. A **mojom** file describes **interfaces**, which are strongly-typed collections of **messages**. Each interface message is roughly analogous to a single proto message, for developers who are familiar with Google protobufs. Given a mojom interface and a message pipe, one of the endpoints can be designated as a **`Remote`** and is used to *send* messages described by the interface. The other endpoint can be designated as a **`Receiver`** and is used to *receive* interface messages. *** aside NOTE: The above generalization is a bit oversimplified. Remember that the message pipe is still bidirectional, and it's possible for a mojom message to expect a reply. Replies are sent from the `Receiver` endpoint and received by the `Remote` endpoint. *** The `Receiver` endpoint must be associated with (*i.e.* **bound** to) an **implementation** of its mojom interface in order to process received messages. A received message is dispatched as a scheduled task invoking the corresponding interface method on the implementation object. Another way to think about all this is simply that **a `Remote` makes calls on a remote implementation of its interface associated with a corresponding remote `Receiver`.** ## Example: Defining a New Frame Interface Let's apply this to Chrome. Suppose we want to send a "Ping" message from a render frame to its corresponding `RenderFrameHostImpl` instance in the browser process. We need to define a nice mojom interface for this purpose, create a pipe to use that interface, and then plumb one end of the pipe to the right place so the sent messages can be received and processed there. This section goes through that process in detail. ### Defining the Interface The first step involves creating a new `.mojom` file with an interface definition, like so: ``` cpp // src/example/public/mojom/ping_responder.mojom module example.mojom; interface PingResponder { // Receives a "Ping" and responds with a random integer. Ping() => (int32 random); }; ``` This should have a corresponding build rule to generate C++ bindings for the definition here: ``` python # src/example/public/mojom/BUILD.gn import("//mojo/public/tools/bindings/mojom.gni") mojom("mojom") { sources = [ "ping_responder.mojom" ] } ``` ### Creating the Pipe Now let's create a message pipe to use this interface. *** aside As a general rule and as a matter of convenience when using Mojo, the *client* of an interface (*i.e.* the `Remote` side) is typically the party who creates a new pipe. This is convenient because the `Remote` may be used to start sending messages immediately without waiting for the InterfaceRequest endpoint to be transferred or bound anywhere. *** This code would be placed somewhere in the renderer: ```cpp // src/third_party/blink/example/public/ping_responder.h mojo::Remote ping_responder; mojo::PendingReceiver receiver = ping_responder.BindNewPipeAndPassReceiver(); ``` In this example, ```ping_responder``` is the `Remote`, and ```receiver``` is a `PendingReceiver`, which is a `Receiver` precursor that will eventually be turned into a `Receiver`. `BindNewPipeAndPassReceiver` is the most common way to create a message pipe: it yields the `PendingReceiver` as the return value. *** aside NOTE: A `PendingReceiver` doesn't actually **do** anything. It is an inert holder of a single message pipe endpoint. It exists only to make its endpoint more strongly-typed at compile-time, indicating that the endpoint expects to be bound by a `Receiver` of the same interface type. *** ### Sending a Message Finally, we can call the `Ping()` method on our `Remote` to send a message: ```cpp // src/third_party/blink/example/public/ping_responder.h ping_responder->Ping(base::BindOnce(&OnPong)); ``` *** aside **IMPORTANT:** If we want to receive the response, we must keep the `ping_responder` object alive until `OnPong` is invoked. After all, `ping_responder` *owns* its message pipe endpoint. If it's destroyed then so is the endpoint, and there will be nothing to receive the response message. *** We're almost done! Of course, if everything were this easy, this document wouldn't need to exist. We've taken the hard problem of sending a message from a renderer process to the browser process, and transformed it into a problem where we just need to take the `receiver` object from above and pass it to the browser process somehow where it can be turned into a `Receiver` that dispatches its received messages. ### Sending a `PendingReceiver` to the Browser It's worth noting that `PendingReceiver`s (and message pipe endpoints in general) are just another type of object that can be freely sent over mojom messages. The most common way to get a `PendingReceiver` somewhere is to pass it as a method argument on some other already-connected interface. One such interface which we always have connected between a renderer's `RenderFrameImpl` and its corresponding `RenderFrameHostImpl` in the browser is [`BrowserInterfaceBroker`](https://cs.chromium.org/chromium/src/third_party/blink/public/mojom/browser_interface_broker.mojom). This interface is a factory for acquiring other interfaces. Its `GetInterface` method takes a `GenericPendingReceiver`, which allows passing arbitrary interface receivers. ``` cpp interface BrowserInterfaceBroker { GetInterface(mojo_base.mojom.GenericPendingReceiver receiver); } ``` Since `GenericPendingReceiver` can be implicitly constructed from any specific `PendingReceiver`, it can call this method with the `receiver` object it created earlier via `BindNewPipeAndPassReceiver`: ``` cpp RenderFrame* my_frame = GetMyFrame(); my_frame->GetBrowserInterfaceBroker().GetInterface(std::move(receiver)); ``` This will transfer the `PendingReceiver` endpoint to the browser process where it will be received by the corresponding `BrowserInterfaceBroker` implementation. More on that below. ### Implementing the Interface Finally, we need a browser-side implementation of our `PingResponder` interface. ```cpp #include "example/public/mojom/ping_responder.mojom.h" class PingResponderImpl : example::mojom::PingResponder { public: explicit PingResponderImpl(mojo::PendingReceiver receiver) : receiver_(this, std::move(receiver)) {} PingResponderImpl(const PingResponderImpl&) = delete; PingResponderImpl& operator=(const PingResponderImpl&) = delete; // example::mojom::PingResponder: void Ping(PingCallback callback) override { // Respond with a random 4, chosen by fair dice roll. std::move(callback).Run(4); } private: mojo::Receiver receiver_; }; ``` `RenderFrameHostImpl` owns an implementation of `BrowserInterfaceBroker`. When this implementation receives a `GetInterface` method call, it calls the handler previously registered for this specific interface. ``` cpp // render_frame_host_impl.h class RenderFrameHostImpl ... void GetPingResponder(mojo::PendingReceiver receiver); ... private: ... std::unique_ptr ping_responder_; ... }; // render_frame_host_impl.cc void RenderFrameHostImpl::GetPingResponder( mojo::PendingReceiver receiver) { ping_responder_ = std::make_unique(std::move(receiver)); } // browser_interface_binders.cc void PopulateFrameBinders(RenderFrameHostImpl* host, mojo::BinderMap* map) { ... // Register the handler for PingResponder. map->Add(base::BindRepeating( &RenderFrameHostImpl::GetPingResponder, base::Unretained(host))); } ``` And we're done. This setup is sufficient to plumb a new interface connection between a renderer frame and its browser-side host object! Assuming we kept our `ping_responder` object alive in the renderer long enough, we would eventually see its `OnPong` callback invoked with the totally random value of `4`, as defined by the browser-side implementation above. ## Services Overview & Terminology The previous section only scratches the surface of how Mojo IPC is used in Chromium. While renderer-to-browser messaging is simple and possibly the most prevalent usage by sheer code volume, we are incrementally decomposing the codebase into a set of services with a bit more granularity than the traditional Content browser/renderer/gpu/utility process split. A **service** is a self-contained library of code which implements one or more related features or behaviors and whose interaction with outside code is done *exclusively* through Mojo interface connections, typically brokered by the browser process. Each service defines and implements a main Mojo interface which can be used by the browser to manage an instance of the service. ## Example: Building a Simple Out-of-Process Service There are multiple steps typically involved to get a new service up and running in Chromium: - Define the main service interface and implementation - Hook up the implementation in out-of-process code - Write some browser logic to launch a service process This section walks through these steps with some brief explanations. For more thorough documentation of the concepts and APIs used herein, see the [Mojo](/mojo/README.md) documentation. ### Defining the Service Typically service definitions are placed in a `services` directory, either at the top level of the tree or within some subdirectory. In this example, we'll define a new service for use by Chrome specifically, so we'll define it within `//chrome/services`. We can create the following files. First some mojoms: ``` cpp // src/chrome/services/math/public/mojom/math_service.mojom module math.mojom; interface MathService { Divide(int32 dividend, int32 divisor) => (int32 quotient); }; ``` ``` python # src/chrome/services/math/public/mojom/BUILD.gn import("//mojo/public/tools/bindings/mojom.gni") mojom("mojom") { sources = [ "math_service.mojom", ] } ``` Then the actual `MathService` implementation: ``` cpp // src/chrome/services/math/math_service.h #include "chrome/services/math/public/mojom/math_service.mojom.h" namespace math { class MathService : public mojom::MathService { public: explicit MathService(mojo::PendingReceiver receiver); MathService(const MathService&) = delete; MathService& operator=(const MathService&) = delete; ~MathService() override; private: // mojom::MathService: void Divide(int32_t dividend, int32_t divisor, DivideCallback callback) override; mojo::Receiver receiver_; }; } // namespace math ``` ``` cpp // src/chrome/services/math/math_service.cc #include "chrome/services/math/math_service.h" namespace math { MathService::MathService(mojo::PendingReceiver receiver) : receiver_(this, std::move(receiver)) {} MathService::~MathService() = default; void MathService::Divide(int32_t dividend, int32_t divisor, DivideCallback callback) { // Respond with the quotient! std::move(callback).Run(dividend / divisor); } } // namespace math ``` ``` python # src/chrome/services/math/BUILD.gn source_set("math") { sources = [ "math_service.cc", "math_service.h", ] deps = [ "//base", "//chrome/services/math/public/mojom", ] } ``` Now we have a fully defined `MathService` implementation that we can make available in- or out-of-process. ### Hooking Up the Service Implementation For an out-of-process Chrome service, we simply register a factory function in [`//chrome/utility/services.cc`](https://cs.chromium.org/chromium/src/chrome/utility/services.cc). ``` cpp auto RunMathService(mojo::PendingReceiver receiver) { return std::make_unique(std::move(receiver)); } void RegisterMainThreadServices(mojo::ServiceFactory& services) { // Existing services... services.Add(RunFilePatcher); services.Add(RunUnzipper); // We add our own factory to this list services.Add(RunMathService); //... ``` With this done, it is now possible for the browser process to launch new out-of-process instances of MathService. ### Launching the Service If you're running your service in-process, there's really nothing interesting left to do. You can instantiate the service implementation just like any other object, yet you can also talk to it via a Mojo Remote as if it were out-of-process. To launch an out-of-process service instance after the hookup performed in the previous section, use Content's [`ServiceProcessHost`](https://cs.chromium.org/chromium/src/content/public/browser/service_process_host.h?rcl=e7a1f6c9a24f3151c875598174a05167fb12c5d5&l=47) API: ``` cpp mojo::Remote math_service = content::ServiceProcessHost::Launch( content::ServiceProcessHost::Options() .WithDisplayName("Math!") .Pass()); ``` Except in the case of crashes, the launched process will live as long as `math_service` lives. As a corollary, you can force the process to be torn down by destroying (or resetting) `math_service`. We can now perform an out-of-process division: ``` cpp // NOTE: As a client, we do not have to wait for any acknowledgement or // confirmation of a connection. We can start queueing messages immediately and // they will be delivered as soon as the service is up and running. math_service->Divide( 42, 6, base::BindOnce([](int32_t quotient) { LOG(INFO) << quotient; })); ``` *** aside NOTE: To ensure the execution of the response callback, the `mojo::Remote` object must be kept alive (see [this section](/mojo/public/cpp/bindings/README.md#A-Note-About-Endpoint-Lifetime-and-Callbacks) and [this note from an earlier section](#sending-a-message)). *** ### Specifying a sandbox All services must specify a sandbox. Ideally services will run inside the `kService` process sandbox unless they need access to operating system resources. For services that need a custom sandbox, a new sandbox type must be defined in consultation with security-dev@chromium.org. The preferred way to define the sandbox for your interface is by specifying a `[ServiceSandbox=type]` attribute on your `interface {}` in its `.mojom` file: ``` import "sandbox/policy/mojom/sandbox.mojom" [ServiceSandbox=sandbox.mojom.Sandbox.kService] interface FakeService { ... }; ``` Valid values are those in [`//sandbox/policy/mojom/sandbox.mojom`](https://cs.chromium.org/chromium/src/sandbox/policy/mojom/sandbox.mojom). Note that the sandbox is only applied if the interface is launched out-of-process using `content::ServiceProcessHost::Launch()`. As a last resort, dynamic or feature based mapping to an underlying platform sandbox can be achieved but requires plumbing through ContentBrowserClient (e.g. `ShouldEnableNetworkServiceSandbox()`). ## Content-Layer Services Overview ### Interface Brokers We define an explicit mojom interface with a persistent connection between a renderer's frame object and the corresponding `RenderFrameHostImpl` in the browser process. This interface is called [`BrowserInterfaceBroker`](https://cs.chromium.org/chromium/src/third_party/blink/public/mojom/browser_interface_broker.mojom?rcl=09aa5ae71649974cae8ad4f889d7cd093637ccdb&l=11) and is fairly easy to work with: you add a new method on `RenderFrameHostImpl`: ``` cpp void RenderFrameHostImpl::GetGoatTeleporter( mojo::PendingReceiver receiver) { goat_teleporter_receiver_.Bind(std::move(receiver)); } ``` and register this method in `PopulateFrameBinders` function in `browser_interface_binders.cc`, which maps specific interfaces to their handlers in respective hosts: ``` cpp // //content/browser/browser_interface_binders.cc void PopulateFrameBinders(RenderFrameHostImpl* host, mojo::BinderMap* map) { ... map->Add(base::BindRepeating( &RenderFrameHostImpl::GetGoatTeleporter, base::Unretained(host))); } ``` It's also possible to bind an interface on a different sequence by specifying a task runner: ``` cpp // //content/browser/browser_interface_binders.cc void PopulateFrameBinders(RenderFrameHostImpl* host, mojo::BinderMap* map) { ... map->Add(base::BindRepeating( &RenderFrameHostImpl::GetGoatTeleporter, base::Unretained(host)), GetIOThreadTaskRunner({})); } ``` Workers also have `BrowserInterfaceBroker` connections between the renderer and the corresponding remote implementation in the browser process. Adding new worker-specific interfaces is similar to the steps detailed above for frames, with the following differences: - For Dedicated Workers, add a new method to [`DedicatedWorkerHost`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/worker_host/dedicated_worker_host.h) and register it in [`PopulateDedicatedWorkerBinders`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_interface_binders.cc;l=1126;drc=e24e0a914ff0da18e78044ebad7518afe9e13847) - For Shared Workers, add a new method to [`SharedWorkerHost`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/worker_host/shared_worker_host.h) and register it in [`PopulateSharedWorkerBinders`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_interface_binders.cc;l=1232;drc=e24e0a914ff0da18e78044ebad7518afe9e13847) - For Service Workers, add a new method to [`ServiceWorkerHost`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/service_worker/service_worker_host.h) and register it in [`PopulateServiceWorkerBinders`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/browser_interface_binders.cc;l=1326;drc=e24e0a914ff0da18e78044ebad7518afe9e13847) Interfaces can also be added at the process level using the `BrowserInterfaceBroker` connection between the Blink `Platform` object in the renderer and the corresponding `RenderProcessHost` object in the browser process. This allows any thread (including frame and worker threads) in the renderer to access the interface, but comes with additional overhead because the `BrowserInterfaceBroker` implementation used must be thread-safe. To add a new process-level interface, add a new method to `RenderProcessHostImpl` and register it using a call to [`AddUIThreadInterface`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/renderer_host/render_process_host_impl.h;l=918;drc=ec5eaba0e021b757d5cbbf2c27ac8f06809d81e9) in [`RenderProcessHostImpl::RegisterMojoInterfaces`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/renderer_host/render_process_host_impl.cc;l=2317;drc=a817d852ea2f2085624d64154ad847dfa3faaeb6). On the renderer side, use [`Platform::GetBrowserInterfaceBroker`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/public/platform/platform.h;l=781;drc=ee1482552c4c97b40f15605fe6a52565cfe74548) to retrieve the corresponding `BrowserInterfaceBroker` object to call `GetInterface` on. For binding an embedder-specific document-scoped interface, override [`ContentBrowserClient::RegisterBrowserInterfaceBindersForFrame()`](https://cs.chromium.org/chromium/src/content/public/browser/content_browser_client.h?rcl=3eb14ce219e383daa0cd8d743f475f9d9ce8c81a&l=999) and add the binders to the provided map. *** aside NOTE: if BrowserInterfaceBroker cannot find a binder for the requested interface, it will call `ReportNoBinderForInterface()` on the relevant context host, which results in a `ReportBadMessage()` call on the host's receiver (one of the consequences is a termination of the renderer). To avoid this crash in tests (when content_shell doesn't bind some Chrome-specific interfaces, but the renderer requests them anyway), use the [`EmptyBinderForFrame`](https://cs.chromium.org/chromium/src/content/browser/browser_interface_binders.cc?rcl=12e73e76a6898cb6df6a361a98320a8936f37949&l=407) helper in `browser_interface_binders.cc`. However, it is recommended to have the renderer and browser sides consistent if possible. *** ### Navigation-Associated Interfaces For cases where the ordering of messages from different frames is important, and when messages need to be ordered correctly with respect to the messages implementing navigation, navigation-associated interfaces can be used. Navigation-associated interfaces leverage connections from each frame to the corresponding `RenderFrameHostImpl` object and send messages from each connection over the same FIFO pipe that's used for messages relating to navigation. As a result, messages sent after a navigation are guaranteed to arrive in the browser process after the navigation-related messages, and the ordering of messages sent from different frames of the same document is preserved as well. To add a new navigation-associated interface, create a new method for `RenderFrameHostImpl` and register it with a call to `associated_registry_->AddInterface` in [`RenderFrameHostImpl::SetUpMojoConnection`](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/renderer_host/render_frame_host_impl.cc;l=8365;drc=a817d852ea2f2085624d64154ad847dfa3faaeb6). From the renderer, use [`LocalFrame::GetRemoteNavigationAssociatedInterfaces`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/frame/local_frame.h;l=409;drc=19f17a30e102f811bc90a13f79e8ad39193a6403) to get an object to call `GetInterface` on (this call is similar to `BrowserInterfaceBroker::GetInterface` except that it takes a [pending associated receiver](https://chromium.googlesource.com/chromium/src/+/main/mojo/public/cpp/bindings/README.md#associated-interfaces) instead of a pending receiver). ## Additional Support If this document was not helpful in some way, please post a message to your friendly [chromium-mojo@chromium.org](https://groups.google.com/a/chromium.org/forum/#!forum/chromium-mojo) or [services-dev@chromium.org](https://groups.google.com/a/chromium.org/forum/#!forum/services-dev) mailing list.