Add documentation on servicification recipes

docs/servicification.md

+# Servicification Strategies
+
+This document captures strategies, hints, and best practices for solving typical
+challenges enountered when converting existing Chromium
+code to services. It is assumed that you have already read the high-level
+documentation on [what a service is](/services).
+
+If you're looking for Mojo documentation, please see the [general
+Mojo documentation](/mojo) and/or the [documentation on converting Chrome IPC to
+Mojo](/ipc).
+
+Note that throughout the below document we link to CLs to illustrate the
+strategies being made. Over the course of time code tends to shift, so it is
+likely that the code on trunk does not exactly match what it was at the time of
+the CLs. When necessary, use the CLs as a starting point for examining the
+current state of the codebase with respect to these issues (e.g., exactly where
+a service is embedded within the content layer).
+
+[TOC]
+
+## Questions to Answer When Getting Started
+
+For the basic nuts and bolts of how to create a new service, see [the
+documentation on adding a new service](/services#Adding-a-new-service). This 
+section gives questions that you should answer in order to shape the design of 
+your service, as well as hints as to which answers make sense given your 
+situation.
+
+### Is your service global or per-BrowserContext?
+The Service Manager can either:
+
+- create one service instance per user ID or
+- field all connection requests for a given service via the same instance
+
+Which of these policies the Service Manager employs is determined by the
+contents of your service manifest: the former is the default, while the latter
+is selected by informing the Service Manager that your service requires the
+service_manager:all_users capability([example](https://cs.chromium.org/chromium/src/services/device/manifest.json)).
+
+In practice, there is one user ID per-BrowserContext, so the question becomes:
+Is your Service a global or keyed by BrowserContext?  In considering this
+question, there is one obvious hint: If you are converting per-Profile classes
+(e.g., KeyedServices), then your service is almost certainly going to be
+per-user. More generally, if you envision needing to use *any* state related to
+the user (e.g., you need to store files in the user's home directory), then your
+service should be per-user.
+
+Conversely, your service could be a good fit for being global if it is a utility
+that is unconcerned with the identity of the requesting client (e.g., the [data
+decoder service](/services/data_decoder), which simply decodes untrusted data in
+a separate process.
+
+### Will you embed your service in //content, //chrome, or neither?
+
+At the start (and potentially even long-term), your service will likely not
+actually run in its own process but will rather be embedded in the browser
+process. This is especially true in the common case where you are converting
+existing browser-process code.
+
+You then have a question: Where should it be embedded? The answer to this
+question hinges on the nature and location of the code that you are converting:
+ 
+- //content is the obvious choice if you are converting existing //content code
+  (e.g., the Device Service). Global services
+  are embedded by [content::ServiceManagerContext](https://cs.chromium.org/chromium/src/content/browser/service_manager/service_manager_context.cc?type=cs&q=CreateDeviceService), 
+  while per-user services are naturally embedded by [content::BrowserContext](https://cs.chromium.org/chromium/src/content/browser/browser_context.cc?type=cs&q=CreateFileService).
+
+- If your service is converting existing //chrome code, then you will need
+  to embed your service in //chrome rather than //content. Global services
+  are embedded by [ChromeContentBrowserClient](https://cs.chromium.org/chromium/src/chrome/browser/chrome_content_browser_client.cc?type=cs&q=CreateMediaService), 
+  while per-user services are embedded by [ProfileImpl](https://cs.chromium.org/chromium/src/chrome/browser/profiles/profile_impl.cc?type=cs&q=CreateIdentityService).
+
+- If you are looking to convert all or part of a component (i.e., a feature in
+  //components) into a service, the question arises of whether your new service
+  is worthy of being in //services (i.e., is it a foundational service?). If
+  not, then it can be placed in an appropriate subdirectory of the component
+  itself. See this [email
+  thread](https://groups.google.com/a/chromium.org/forum/#!topic/services-dev/3AJx3gjHbZE) and its [resulting CL](https://codereview.chromium.org/2832633002)
+  for discussion of this point, and if in doubt, start a similar email thread
+  discussing your feature.
+
+### If your service is embedded in the browser process, what is its threading model?
+
+If your service is embedded in the browser process, it will run on the IO thread
+by default. You can change that by specifying a task runner as part of the
+information for constructing your service. In particular, if the code that you
+are converting is UI-thread code, then you likely want your service running on
+the UI thread. Look at the changes to profile_impl.cc in [this
+CL](https://codereview.chromium.org/2753753007) to see an example of setting the
+task runner that a service should be run on as part of the factory for creating
+the service.
+
+### What is your approach for incremental conversion?
+
+In creating your service, you likely have two goals:
+
+- Making the service available to other services
+- 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 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](https://docs.google.com/document/d/1EPLEJTZewjiShBemNP5Zyk3b_9sgdbrZlXn7j1fubW0/edit)).
+- 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](https://docs.google.com/document/d/1JU8QUWxMEXWMqgkvFUumKSxr7Z-nfq0YvreSJTkMVmU/edit#heading=h.19gc5b5u3e3x)).
+- Build out the new service piece-by-piece by picking a given
+  bite-size piece of functionality and entirely servicifying that functionality
+  ([Device Service](https://docs.google.com/document/d/1_1Vt4ShJCiM3fin-leaZx00-FoIPisOr8kwAKsg-Des/edit#heading=h.c3qzrjr1sqn7)).
+
+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](https://codereview.chromium.org/2643713002) 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](https://codereview.chromium.org/2690963002).
+(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](#Coupling-to-UI).
+
+### iOS
+The high-level [servicification design doc](https://docs.google.com/document/d/15I7sQyQo6zsqXVNAlVd520tdGaS8FCicZHrN0yRu-oU/edit) gives the motivations and 
+vision for supporting services on iOS (in particular, search for the mentions 
+of iOS within the doc).
+
+Services are not *yet* supported, but this support is being actively being 
+worked on; see [this bug](crbug.com/705982) for the current status. If you have 
+a use case or need for services on iOS, contact blundell@chromium.org.
+
+## Client-Specific Issues
+
+### Services and Blink
+Connecting to services directly from Blink is fully supported. [This
+CL](https://codereview.chromium.org/2698083007) gives a basic example of
+connecting to an arbitrary service by name from Blink (look at the change to
+SensorProviderProxy.cpp as a starting point).
+
+Below, we go through strategies for some common challenges encountered when
+servicifying features that have Blink as a client.
+
+#### Mocking Interface Impls in JS
+It is a common pattern in Blink's layout tests to mock a remote Mojo interface
+in JS. [This CL](https://codereview.chromium.org/2643713002) illustrates the
+basic pattern for porting such mocking of an interface hosted by
+//content/browser to an interface hosted by an arbitrary service (see the
+changes to mock-battery-monitor.js).
+
+#### 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](https://codereview.chromium.org/2415083002) 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](https://groups.google.com/a/chromium.org/forum/#!topic/services-dev/CSnDUjthAuw),
+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.
+
+In the longer term, this will still be the basic model, only with "the browser"
+replaced by "the Navigation Service" or "the web permissions broker".
+
+## 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](https://codereview.chromium.org/2734943003).
+
+### 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](https://groups.google.com/a/chromium.org/forum/#!topic/services-dev/Y9FKZf9n1ls) for more discussion of this issue.
+
+### Tests that muck with service internals
+It is often the case that browsertests reach directly into what will become part
+of the internal service implementation to either inject mock/fake state or to
+monitor private state.
+
+This poses a challenge: As part of servicification, *no* code outside the
+service impl should depend on the service impl. Thus, these dependencies need to
+be removed. The question is how to do so while preserving testing coverage.
+
+To answer this question, there are several different strategies. These
+strategies are not mutually-exclusive; they can and should be combined to
+preserve the full breadth of coverage.
+
+- Blink client-side behavior can be tested via [layout tests](https://codereview.chromium.org/2731953003)
+- To test service impl behavior, create [service tests](https://codereview.chromium.org/2774783003).
+- To preserve tests of end-to-end behavior (e.g., that when Blink makes a
+  request via a Web API in JS, the relevant feature impl receives a connection
+  request), we are planning on introducing the ability to register mock 
+  implementations with the Service Manager.
+
+To emphasize one very important point: it is in general necessary to leave
+*some* test of end-to-end functionality, as otherwise it is too easy for bustage
+to slip in via e.g. changes to how services are registered. See [this thread](https://groups.google.com/a/chromium.org/forum/#!topic/services-dev/lJCKAElWz-E)
+for further discussion of this point.