Bug 1330151 - Push Architecture docs recovered from Bug 1207714 r=nalexander

Nick's original docs copy-pasted verbatim.

MozReview-Commit-ID: PPVXwoL6He

--HG--
extra : rebase_source : 3e59b34b2e1aceb9672dc5f5ec5be8461d8ed6a7

mobile/android/docs/index.rst

@@ -17,6 +17,7 @@ Contents:
    defaultdomains
    bouncer
    shutdown
+   push
 
 Indices and tables
 ==================

mobile/android/docs/push.rst

@@ -0,0 +1,156 @@
+.. -*- Mode: rst; fill-column: 80; -*-
+
+=======================================================
+ The architecture of the Fennec Webpush implementation
+=======================================================
+
+Overview of the Push API
+========================
+
+The *Push API* is a Web API that allows web applications to "wake up" the User
+Agent (the browser, Fennec), even when the application is not visible (or even
+loaded in a tab).  The Push API can only be used by secure sites that register a
+Service Worker.
+
+There are four major components in Push:
+
+1. A *web application*;
+2. The User Agent (Fennec);
+3. An *app server* associated to the web application;
+4. A *push service backend*.
+
+These are listed roughly in the order that they appear in a successful push
+message.  First, the web application registers a Service Worker and requests a
+*push subscription*.  Fennec arranges a push channel and returns a subscription,
+including a User-Agent-specific *push endpoint* URL to the web application.  The
+web application then provides that push endpoint URL to its app server.  When
+the app server wishes to push a message to the web application, it posts the
+message and some additional meta-data to the provided push endpoint URL.  The
+message is received by the *push service backend* and delivered by some
+unspecified out-of-band mechanism to the User Agent.
+
+Two notes on terminology
+------------------------
+
+"Push notifications" mean many things to many people.  In this system, "push
+notifications" may or may not notify the user.  Therefore, we use the term "push
+messages" exclusively.  This avoids confusing the push system with the "Web
+Notification" API, which provides the familiar pop-up and system notification
+based user interface.
+
+Throughout, we use Fennec to refer to the Java-and-Android application shell,
+and Gecko to refer to the embedded Gecko platform.
+
+Overview of the Fennec Push implementation
+==========================================
+
+Fennec uses the Google Cloud Messaging (GCM) service to deliver messages to the
+User Agent.  Fennec registers for Google Cloud Messaging directly, like any
+other Android App; however, consumers do not interact with GCM directly.  To
+provide a uniform interface to all consumers across all implementations, Fennec
+intermediates through the Mozilla-specific *autopush* service.  Autopush
+maintains User Agent identification and authentication, provides per-web
+application messaging channels, and bridges unauthenticated push messages to the
+GCM delivery queue.
+
+The Fennec Push implementation is designed to address the following technical
+challenge: **GCM events, including incoming push messages, can occur when Gecko
+is not running**.
+
+In the future, Fennec will be able to start Gecko (more accurately, a particular
+Gecko profile) in order to service incoming push messages.  Right now, if a push
+message is received and Gecko is not running (with the correct profile) to
+service the message, it will be dropped on the floor (with
+helpful-but-unsatisfying diagnostic messages).  In particular, **neither the
+sending application server nor the receiving web application are informed of the
+failed delivery at any time!** (We may, at some point, choose to queue messages
+on the device until Gecko can service them; and we may add support for
+*receipts* to allow web applications to acknowledge successful message
+delivery.)
+
+It's worth noting that Fennec uses push to implement internal functionality like
+Sync and Firefox Accounts, and that these background services are *not* tied to
+Gecko being available.
+
+Therefore, the principal constraints and requirements are:
+
+1) Fennec must be able to service GCM events, including incoming push messages,
+independently of Gecko.
+
+2) Gecko must be able to maintain push subscriptions across its entire
+life-cycle.
+
+3) Fennec must be able to use push messages for non-Gecko purposes independently
+of Gecko.
+
+Significant previous experience building Fennec background services has shown
+that configuring such services across the Gecko-Fennec interface is both
+valuable and difficult.  Therefore, we add the following requirement:
+
+4) Gecko must own the push configuration details where appropriate and possible.
+
+We explicitly do not care to support push messages across multiple processes.
+This means that we **do not support the Push API in APK-based Web Apps that run
+in a separate process**.  (Such Web Apps are scheduled to be removed from Fennec
+in the Nightly 46 development cycle.)
+
+Push component architecture
+===========================
+
+Fennec components
+-----------------
+
+The two major components are the `PushManager` and the associated
+`PushManagerStorage`.  The `PushManager` interacts with the GCM system on the
+device and the autopush service, updating the `PushManagerStorage` as the system
+state changes.
+
+There is a unique `PushManagerStorage` instance per-App that may only be
+accessed by the main process.  The `PushManagerStorage` maintains two mappings.
+The first is a one-to-one mapping from a Gecko profile to a `PushRegistration`:
+a datum of User Agent state.  The `PushManager` maintains each profile's
+registration across Android life-cycle events, Gecko events, and GCM events.
+Each `PushRegistration` includes:
+
+* autopush server configuration details;
+* debug settings;
+* profile details;
+* access tokens and invalidation timestamps.
+
+The second mapping is a one-to-many mapping from push registrations to
+`PushSubscription` instances.  A push subscription corresponds to a unique push
+message channel from the autopush server to Fennec.  Each `PushSubscription`
+includes:
+
+* a Fennec service identifier, like "webpush" or "Sync" or "Firefox Accounts";
+* an associated Gecko profile;
+* a unique channel identifier.
+
+The `PushManager` uses the `PushSubscription` service and profile maintained in
+the `PushManagerStorage` to determine how to deliver incoming GCM push messages.
+
+Each `PushRegistration` corresponds to a unique *uaid* (User-Agent ID) on the
+autopush server.  Each *uaid* is long-lived; a healthy client will maintain the
+same *uaid* until the client's configuration changes or the service expires the
+registration due to inactivity or an unexpected server event.  Each
+`PushSubscription` is associated to a given *uaid* and correponds to a unique
+(per-*uaid*) *chid* (Channel ID) on the autopush server.  An individual *chid*
+is potentially long-lived, but clients must expect the service to expire *chid*s
+as part of regular maintainence.  The `PushManager` uses an `AutopushClient`
+instance to interact with the autopush server.
+
+Between the `PushManager`, the `PushManagerStorage`, and assorted GCM event
+broadcast receivers, push messages that do not target Gecko can be implemented.
+
+Gecko components
+----------------
+
+The Gecko side of the architecture is implemented in JavaScript by the
+`PushServiceAndroidGCM.jsm` module.  This registers a PushService, like the Web
+Socket and HTTP2 backed services, which simply delegates to the Fennec
+`PushManager` using `Messaging.jsm` and friends.
+
+There are some complications: first, Gecko must maintain the autopush
+configuration; and second, it is possible for the push system to change while
+Gecko is not running.  Therefore, the communication is bi-directional
+throughout, so that Gecko can react to Fennec events after-the-fact.