Gaia/Architecture Proposal

Architecture Proposal

This is a proposal for a new and unified application architecture for Gaia apps. This version of the proposed architecture is not finalized yet, and still under heavy development.

This documentation tries to explain the core ideas in more details, and how each pieces works and interacts with others.

If you are looking for a prettier and higher level introduction to the architecture goals, it can be found at: http://arcturus.github.io/v3-architecture/presentation/#/

Note: This proposal is not about using framework x, y or z. Not even about using libraries x, y and z for the internals. Such discussions could and should append in a separated proposal, or on https://etherpad.mozilla.org/fxos-engineering-most-wanted

High level overview

Web App

Web Applications architecture is built based on the pattern described in this document.

The pattern is not tied to specific versions of a library nor a framework. It is intended to expose to the browser some of the application logic in order to benefits from the browser architecture.

As a result it enforces a platform level encapsulation, by having different compartments per logical pieces of code. For more informations on compartments, please see [1] [2] [3] [4]

In this document 'logical pieces of code' is often referred to as a Client or a Server. But more specific examples for applications can be a particular View, or the view application logic, a main-thread-only WebAPI wrapper, etc. In order to not create confusions with Modules, nor Web Components, they will be called capsules.

Also all applications are hosted web applications, running offline through the use of Service Worker.

Service Worker

Applications are no longer packaged. Instead they are web applications cached locally by Service Worker. For more informations on Service Worker, see [5]

Applications are not glued together and have independent updates. As a result every application lives into its own repository.

Telemetry

Every capsule has its own set of telemetry reports. So the telemetry reports for an application is a set of capsule reports. Those reports contains user data such as the time to load a specific capsule, the memory consume by this capsule as well as any capsule specific datas the developer has asked to be reported.

Those datas will be collected to a remote server, if the user has opt-in, in order to have tools for decision making.

For an idea about what is telemetry, please see [6]

Data Sync

Applications data are now synchronized over a remote service. The storage back-end is not decided yet, but the idea is to ensure users data are always available.

Those data can be displayed, using a mobile device, or any other front-end built for the desktop browser.

So one application may have multiple front-end used to access its data.

Service Worker

As describe previously, Service Worker is used as a replacement for our current packaging solution. This change implies a new Security Model that is currently under investigation.

Each application has its own Service Worker. Individual Service Worker, usually offers one store for caching the application resources. In this proposal it is often referred to as the Offline Store.

But a Service Worker can have an indefinite numbers of stores. The current proposal is to leverage this capability to add 2 new stores. More can be added in the future if needed.

The 2 additional stores are:

- Custom store
- Render store

And so, the proposal contains 3 stores:

- Offline store
- Custom store
- Render store

Each of those stores has a different purposes. See the individual section for each store.

Those stores are ordered in the following way:

Render store -> Custom store -> Offline store

So when an application fetch a resource (js, css, html, images, locales, etc...), it iterates over stores to see if there is a match, or falls back on the network.

fetch -> Render store -> Custom store -> Offline store -> Network

Offline store

The Offline store is used to perform a local copy of the source code of the hosted application.

Note: For applications that are shipped by default, the local copy will be inserted at build time.

The application Service Worker can be awaken on a timer in order to check for updates. If any, instead of performing a raw fetch (the default for Service Worker) a client side library will try to perform a delta update.

Application updates are independent of other applications updates. So one application can ship an update when there is a new feature fully finished and validated by QA, or in order to fix a Security issue, etc...

If possible, and if the update does not affect one of the visible capsule for the user, the client side library will try to perform a 'restartless' delta update.

As a more concrete example, if one update is fixing the code of one of the View that is not directly visible to the user, it can be updated in the background without having to restart the application. The next time the user will access this view it will be updated.

Another example is if one update affect some of the view specific logic. Even if this logic is the one for the current view, there are cases where this logic can be shutdown at runtime, and updated transparently for the user.

Custom store

As its name stands, the Custom store is intended for customizations purposes.

Because there might be multiple customizations sources, we can have multiple Custom stores. Also a Custom store can have independent updates from the Offline store.

Some examples about how to use the Custom stores are:

• Partners customizations
• Users customizations
  • Replacing one resource in order to fix a bug, or to fix a color you don't like
  • Locally fix a bug if the fix has not been released yet
• A/B testing with telemetry reports
  • UX/UI concept
  • Framework comparison
  • Impact of a change
• ...

In order to fully leverage the Custom Store, a remote infrastructure will be needed. If so, it should be possible to distribute changes to a group of users and observe the impact of this change through telemetry reports.

Render store

The Render store is intended to save/restore a serialized version of a particular view, mostly for performance purposes.

As an example, if one view has been pre-translated at build-time to en-US, and if the user changes the locale to fr-FR, then the new serialize version of the html content can be saved into this store.
Next time the user will access this view, it will correctly localized by default (pre-translated) without having to run l10n.js during the view startup.

The Render store can also contains pure virtual files.
As an example, if in the contact application the user look at Fernando's contact details. The specific contact details page can be serialized into the render store. So the next time you will access this view, it will be served over the network as pure html/css, as if it was part of the original source code.

Sometimes the cached information will be out-of-date. The specific cache eviction strategy is up to the application. A save/restore API will be exposed to any specific views, and it is up to the view logic to manage its own cache.

Telemetry Overview

Telemetry is a remote service, with real users datas reports.

Those reports can then be used for decisions making.

As mentioned previously, each capsule has its own report. It offers a wide variety of opportunities to observe the application usage in a granular way.

Reports can contains:

• Startup time per panel
  • Each capsule has its own instance of the performance.timing API
    • Global informations such as precise details of what happens during the startup of a capsule
    • Custom informations defined via the various Performance interfaces [1] [2] [3] [4]
• about:memory per capsule
  • performance.memory API (chrome Only). But for now it still does not reports enough informations to us. Basically it reports only the JS Heap size, while additional datas such as DOM, CSS, Images, ... consumptions would be valuable.
• Lags
  • Communication lags between capsule (See the Bridge section for more details)
  • Event loop lags (available in Gecko, need to find a way to expose it to our telemetry report).
• Various other datas
  • Heatmap
  • How many times a capsule has been used

Those reports, if formatted and exploited correctly could help for various types of decisions making:

• A/B testing results for marketing, UX, UI
• A/B testing results when investigating a new framework
• Blockers/Approvals decisions
• QA validation by releasing new feature to a small set of users first (via Custom store).
• ...

Data Sync Overview

Applications datas has a copy on the device, as well as a synchronized copy over a remote service. Datas that will be copy remotely are first encrypted on the client side.

This remote service is accessible by any apps using Firefox Accounts. The encryption token is derived from Firefox Accounts in order to share it between multiple devices.

The remote storage back-end is not yet defined. One suggestion that is currently under investigation is to have a proxy offering an HTTP API in order to abstract the specificity of the remote storage back-end.

High Level App Overview

Application front-end and application back-end are independent pieces of code. And so the application front-end and the application back-end lives onto different repositories.

Application back-end and application front-end are both a set of capsules with strong encapsulations.

A single front-end team could then be created in order to own all front-ends. It should makes it easier to unify all our applications front-end and to ensure the front-end and the back-end are not tied in a way that makes it hard for the front-end to evolve.

As a result, the working version of an application will be the union of 3 changesets:

- Gecko revision
- App back-end revision
- App front-end revision

The back-end repository will not contains any html, css nor localization files. The front-end repository will contains html, css, localization and js files.

Front-end and back-end are intended to runs on separate threads. Both should also be able to runs as independent standalone applications.

The integration of the front-end and the back-end is enforce by a strict contract established between capsules, following a Client/Server approach. This contract has a version in order to keep the compatibility between newest version of the server and its clients.

Basically the contract defined a set of APIs available remotely.

Front-End

The frond-end is the part responsible for perceived performance.

As mentioned in the introduction, every view is an independent capsule, living in its own compartment.

Note: As fairly common mistake is to try to compare this high-level decoupling with existing framework. Those are solving orthogonal problems, and face-to-face comparison does not really makes sense.
The main idea here is to expose some of the application logic to the web browser in order to benefits from the browser internal machinery as well as beeing able to get low level informations for the exposed part of the application.
Also the model is not about using x, y or z. It is technology agnostic and use very basic primitives of the Web, and so various technologies can be put on top of that (module UI, React, Web Components, etc.) and can be compared with real data from users using Telemetry.

Technically the compartment split is implemented using a separated <iframe> for each view. Those views are wrapped into a container responsible for the application navigation as well as transitions.

Note: It seems to be a common negative feeling about <iframe>s. Please note that <iframe>s are not the goal, but a tool to achieve this compartment splitting which results into exposing the app internals to the Web Browser. So does <iframe>s are the future of the Web. Probably not, but a high level encapsulation seems definitively needed, and the only thing that provide this level of encapsulation for the moment is an <iframe>.

Features:

• High-Level Content encapsulation aka no collision between views for:
  • DOM
    • DOM per view. When the DOM needs to be traversed for any restyle/reflow/repaint operations, it makes it cheaper.
  • CSS
    • CSS Per view. When the CSS rules needs to be traversed, it makes it cheaper.
  • JavaScript
    • This high-level encapsulation is not a replacement for a module loader.
    • Smaller JS Heap Size. When the mark-and-sweep algorithm used for GC has to run, it does not need to iterate over the whole Object tree.
  • Locales
• Scoped regressions. A change in a view should not affect other views.
• Per-View state
  • Performance API
  • Visibility State (visible, hidden, prerendered)
  • about:memory
  • Telemetry reports
• Prioritization/Deprioritization of views on the event loop. (Since they all run on the UI thread).
• Safe load/unload mechanism for views

Back-End

As described previously the back-end is a set of capsules, specialized to resolve specific needs. Those needs can be related to a specific panel, or the needs of an other back-end capsule.

None of the back-end code is allowed to touch the DOM. The DOM is purely own by the front-end side, and since the back-end can run in a Worker accessing the DOM directly is not an option.

DOM changes are driven by the front-end that can remotely calls methods of the back-end, or subscribe to some events. For example the front-end can subscribe to any contacts change, and react in order to update its rendering, possibly calling some of the methods available in the back-end.

Back-end capsules are loaded on-demand. Initially the back-end does not run, but the front-end can ask for a part of the back-end to serve it, because it either needs to call a method, or to subscribe to a specific event.

So back-ends capsule are lazy-loaded, based on the UI needs.

The back-end is also intended to be shutdown at any time if the UI does not need it anymore, or if the app is going into background. This is intended to save resources on low-end devices, that may not be able to runs too many apps at the same time.

Bridge

The bridge component is an helper to facilitate the communication between capsules. An example is between the views of the front-end with the various pieces of the back-end or even back-end intra communication.

The bridge is designed around a Client/Server architecture, where one server can have multiple clients.

The clients can call remote methods onto the server, and can subscribe to events. The server API is defined in a separated strict contract file.

The client does not need to know who is going to resolve the contract. As a result servers can either be Windows, Workers, SharedWorkers or even a ServiceWorker.

It offers the ability for a Worker to access Main-thread only WebAPIs via the bridge channel.

The contract defined between a client and a server define the methods and events available, has strong types, offer a place to record the communication for debugging purpose, can measure the latency of responses and fire a telemetry report, ...

The contract for a specific service can have multiple versions in order to allow older clients to works with the newly server code.

Contracts are defined in JS, and lives next to the code that is going to resolve this contract. The type of contexts where this code will run can be decided at runtime, which offers dynamic threading model (See the Threading Model section).

Contract example:

contracts['update'] = { 
  methods: {
    checkForUpdate: {
      args: []
    }   
  },  
  events: {
    updatefound: 'undefined'
  }
};

Client side usage example:

var c = new Client('update');
c.checkForUpdate().then(function() {
  ...
};
c.addEventListener('updatefound', function(e) {
  ...
};

Note: The contract resolution is asynchronous since the server may not run when the client ask for a service. But the API is abstracting that so you can call methods even if the server is not running yet.

Server side usage example:

var s = new Server(contracts['update'], {
  checkForUpdate: function() {
    return lookForRemoteUpdate();
  }
});

s.broadcast('updatefound');

Interactions

Front-End / Back-End

This schema represents how the front-end and the back-end collaborates together.

Front-End / Back-End with main-thread-only WebAPIs

It happens that a Worker needs to access a main-thread-only API. In such cases a server capsule will be introduced in the front-end content wrapper, and the worker will use it as a server to access the main-thread-only API.

Front-End / Back-end. Multiple Windows

While on low-end devices most of the application will be shutdown when the application is in background, on high-end devices memory is not the big bottlenext anymore and so we can favor the user experience.

In such cases, if the application is already opened in the background, and the user opens a bookmark to a specific panel, starts a WebActivity resolving to the app, etc.., there is no need to restart the whole application logic, the bridge will just connect the 2 windows in a transparent fashion.

Memory Management

While one of the goal of this architecture is to free the UI thread and reserve it for UI related tasks only, and sharing the related logic for instant bookmarks, actions, activities, there may be times where the memory limitation of the device are the bottleneck.

For such devices, the model offers a macro memory management. So when the application goes in background, most of the user non-visible parts can be shutdown (in red here) in order to safely recover memory.

Then when the app is going to foreground again, those can be restored to maximize the user-experience.

Threading Model

One of the goal of the architecture is to provide an easy way to create multi-threaded applications via bridge abstractions and the avaibility of main-thread-only APIs for worker threads.

That said this is hard to predict which threading model will fit better for which devices. So the architecture is intended to be able to run on different threading models based on runtime informations such as the available number of cores, and the available memory of the device.

This should let us use different threading model on a per hardware basis based on a configuration file per app.

Single-thread

Double-threads

Multi-threads

Back-End as Services

The hard split between the front-end and the back-end would let us explore alternative models, where both can runs onto separated processes, using the same bridge abstractions.

One front-end / One Service

One front-end / Multiple services

Multipe front-ends / Multiple services

Since the back-end is the one responsible to manage applications datas, the same set of datas can be shared across multiple front-ends.