Google Gears – Future of Web Application

06 Oct

How to take your Web Application Offline with Google Gears ?

• It will be hard to determine what is
and isn’t a web app
• The web will be everywhere
• Mobile (iPhone)
• Desktop
• Widgets
• TV
• The browser will be fast


• Reliability
• 1% of downtime can hurt at the wrong time
• Performance
• Local acceleration
• Convenience
• Not having to find a connection
• You are offline more than you think!

Offline Web via Open Web

• Why just solve this problem for Google?
• Why not solve it for others?
• Solution: Make it open source with a liberal license
• New BSD

What is the philosophy?

• One application, one URL
• Seamless transitions between online and offline
• Ability to use local data, even when online
• Available to all users on all platforms
• … and a pony

Do for offline what XMLHttpRequest did for web apps

Ajax Architecture

Offline Architecture

• Read and write using local store
• Changes are queued for later synchronization
• Server communication is completely decoupled from UI actions, happens periodically whenever there is a connection

What are the pieces?


Embedded using SQLite Contributed Full Text Search

var db = google.gears.factory.create('beta.database', '1.0');'database-demo');
db.execute('create table if not exists Demo (Phrase varchar(255),
Timestamp int)');
db.execute('insert into Demo values (?, ?)', [phrase, currTime]);
var rs = db.execute('select * from Demo order by Timestamp desc');

GearsDB – (Abstract over the API)

var bob = {id: 3, name: 'Bob', url: '', description: 'whee'};
db.insertRow('person', bob);
db.insertRow('person', bob, 'name = ?', ['Bob']);
db.selectAll('select * from person', null, function(person) {
document.getElementById('selectAll').innerHTML += ' ' +;
var person = db.selectRow('person', 'id = 1');
// update = 'Harry';
db.updateRow('person', person);
person = db.selectRow('person', 'id = 1');
// force = 'Sally';
db.forceRow('person', person);
person = db.selectRow('person', 'id = 1');
db.deleteRow('person', bob);

GearsORM – Are we going to get a GearsHibernate?

var Person = new GearsOrm.Model("Person", {
firstName: GearsOrm.Fields.String({maxLength:25}),
lastName: GearsOrm.Fields.String({maxLength:25})
GearsORM.Transaction(function() {
new Person({name:"John"}).save();
new Person({name:"Doe"}).save();
});"firstName = 'Uriel'");
Person.count("lastName = ?",["Katz"])

“While developing transaction support for GearsORM i had to write a test, in that test it execute 100 inserts and 100 updates, this test take about 15 seconds  for the inserts and about 10 seconds for the updates without transactions,when using transactions for each set it takes about 377ms for the inserts and 200ms for the updates that is about 39 times faster!”

GearShift – DB Migrations for Gears

Gearshift.rules[1] = {
// create the demo table
up: function() {
return this.e("CREATE TABLE demo_table (
name VARCHAR(30),
movie VARCHAR(30)
down: function() {
return this.e("DROP TABLE demo_table").success;

Database Tools

Alignment with AIR –

The APIs for AIR and Google Gears are nothing alike. In fact, AIR’s SQLite database API is 100% asynchronous via events while Gears API is all synchronous with results coming immediately on execution. So was created to abstract both of these APIs into a single API to access both.

Local Server  – A mini-web server that groks 200 and 304

ResourceStore  Manually Capturing

var pageFiles = [
try {
localServer = google.gears.factory.create('beta.localserver', '1.0');
} catch (e) {
alert('Could not create local server: ' + e.message);
var store = localServer.openStore(this.storeName) ||
store.capture(pageFiles, function(url, success, captureId) {
console.log(url + ' capture ' + (success ? 'succeeded' : 'failed'));

ManagedResourceStore – Capture entire applications

• List application resources in a separate manifest
• Gears captures and updates the list atomically
• Gears auto-updates automatically on each view (within reason)
• Supports multiple users per application

Sample Code

var localserver = google.gears.factory.create('beta.localserver', '1.0');
var store = localserver.createManagedStore('mystore');
store.manifestUrl = '';


// version of the manifest file format
"betaManifestVersion": 1,
// version of the set of resources described in this manifest file
"version": "my_version_string",
// optional
// If the store specifies a requiredCookie, when a request would hit
// an entry contained in the manifest except the requiredCookie is
// not present, the local server responds with a redirect to this URL.
"redirectUrl": "login.html",
// URLs to be cached (URLs are given relative to the manifest URL)
"entries": [
{ "url": "main.html", "src": "main_offline.html" },
{ "url": ".", "redirect": "main.html" },
{ "url": "main.js" }
{ "url": "formHandler.html", "ignoreQuery": true },

Gears Manifest Generator – Ruby Me

json = do |m|
m.version = 'MyNewVer'
m.add_entry({ :url => 'main.html', :src => 'foo.html' })
m.add_extra_info :to => 'main.html', :redirect => 'foo_redirect.html'
m.find_entries :in => '.', :ignore =>

HTML 5 – Offline in General

There’s a concept of an application cache. An application cache is a group of resources, the group being identified by a URI (which typically happens to resolve to a manifest). Resources in a cache are either top-level or not; top-level resources are those that are HTML or XML and when parsed with scripting disabled have with the value of the attribute pointing to the same URI as identifies the cache.

When you visit a page you first check to see if you have that page in a cache as a known top-level page.

Worker Pool – JavaScript needs threads after all?

Run JavaScript in the background

• Provides thread-like functionality to JavaScript
• No more blocking the browser UI
• Communication is via IPC, no shared state or threading primitives

Worker Pool Code

function nextPrime(n) {
// TODO: New top-secret prime-finding algorithm goes here.
var pool = google.gears.factory.create('beta.workerpool', '1.0');
pool.onmessage = function(message) {
alert('next prime is: ' + message);
var worker = pool.createWorker(String(nextPrime) + '; nextPrime()');

Worker Pool Improved!
• Cross-origin API allows Gears apps from different sites to work together
• WorkerPool improvements:
• createWorkerFromUrl()
• onerror allows handling exceptions thrown by workers
• New HttpRequest module allows fetching from WorkerPools
• New Timer module allows timer events in WorkerPools
• Implements the WhatWG Timer specification

var timer = google.gears.factory.create("beta.timer", "1.0");
timer.setTimeout(function() { alert("Hello, from the future!"); },1000);

How about Encryption?

dojox.sql("INSERT INTO CUSTOMERS VALUES (?, ?, ENCRYPT(?))", "Google", "Gear", "123-4561")

The Digg Oracle

Full Text Search

• Gears added FTS2 to SQLite
• Create the database db.execute(‘CREATE VIRTUAL TABLE recipe USING fts2(dish, ingredients)’);
• Search the database db.execute(‘SELECT dish FROM recipe WHERE recipe MATCH ?’, [‘tomatoes’]);
Fun queries: dish:stew tomatoes
Find rows with ‘stew’ in the dish field, and ‘tomatoes’ in any field.

What didn’t you see here?
Sync, sync, sync

Syncing is hard

• Read only, small data
• Read write, small data
• Read only, huge data
• Read write, huge data

Offline UI

When to ask for user input?

Working with and without Gears

content = hasGears() ? new GearsBaseContent() : new CookieBaseContent();

Two Versions? Really? – Only in the extreme

‘url’: ‘main.html’,
‘src’: ‘main_offline.html’

Debugging is a Pain ?

• Add Helper Code
• To clear out the DB
• Remove captured files
• Disable the cache
• Use Firebug / Lite

GearsBaseContent.prototype.clearServer = function() {
if (this.localServer.openStore(this.storeName)) {
this.localServer.removeStore(this.storeName); = null;
GearsBaseContent.prototype.clearTables = function() {
if (this.db) {'delete from BaseQueries');'delete from BaseFeeds');

DOJO Offline – small. easy. open.

content = hasGears() ? new GearsBaseContent() : new CookieBaseContent();

GWT and Gears

try {
db = new Database("database-demo");
db.execute("create table if not exists Demo (Phrase varchar(255), Timestamp int)");
} catch (GearsException e) { ... }
button.addClickListener(new ClickListener() {
public void onClick(Widget sender) {
try {
String phrase = input.getText();
if (phrase.length() > 0) {
db.execute("insert into Demo values (?, ?)", new String[] {
phrase, Integer.toString((int) System.currentTimeMillis())});
} catch (DatabaseException e) {

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Posted by on October 6, 2008 in Google Gears


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