构建实时用户体验：实操指南

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What Is Realtime?

Since the explosion of the Web, developers have been inclined to think in terms of

building websites. Even in this book, I’ve spent a good deal of time writing about

building websites. But make no mistake about it, a realtime user experience does not

exist entirely inside a web browser.

The original web browsers were designed to load and display web pages. The idea of

a web page is quite similar to the printed page. It is a static document, stored in a

computer, but a static document nonetheless. The interface of web browsers evolved

to work within this paradigm. There is a Next button and a Back button designed

to take you from the current page to the page that you viewed previously. That

makes perfect sense when you’re working with documents. However, the Web is quite

quickly shifting away from a document-based paradigm to a web-based form of

communication.

It used to be that a web page was more or less published to the Web. A page was created

and given a specific URI, and when the user went to that page, it was pretty clear what

to expect. Now we have sites like Facebook where the same URL is not only different

for each of the hundreds of millions of different users, but it changes moments after a

user loads the page.

Changing Interactions

In the past, the interaction between a user and a web application was very simple. When

a user wanted content, she would load up her browser, point it at a URL, and get the

content (see Figure 1-1). If she wanted to write a blog post, she’d load up her browser,

fill out a form, and press submit. When she wanted to see comments, it was much the

same.

This has changed. No longer can a website wait for users to navigate to the right URL;

the website must contact the user wherever that user may be. The paradigm has shifted

from a website-centric model, where the website was at the center of the interaction,

to a user-centric model. Now all interactions start and end at the user (see Fig-

ure 1-2), whether she is visiting the website or sending in Short Message Service (SMS)

updates.

A truly realtime experience exists anywhere the user is at a given moment. If the user

is interacting with the web browser, then that’s the place to contact her. If she’s got her

instant messenger program open, she’d better be able to interact with your app from

that window. When she’s offline and your application has an important message for

her, send it via SMS. Naturally, you’ll need to ask the user’s permission before you do

some of these things, but your application needs to offer them.

2 | Chapter 1: Introduction

I mentioned SMS, but the mobile experience does not end there. These days, users have

phones in their pockets with full-fledged web browsers that in some cases offer more

functionality than their desktop-based brethren. Among other things, mobile browsers

can handle offline data storage, GPS sensors, and touch-based interfaces. Their im-

pressive featureset, coupled with nearly ubiquitous wireless broadband, means they

cannot be treated as second class Internet citizens. Applications and user experiences

simply must be built with mobile devices in mind.

Push Versus Pull

For about as long as the Web has been around, there have been two main ways of

getting content to a user: push and pull. Pull is the method in which most interactions

have worked—the user clicks a link and the browser pulls the content down from the

server. If the server wants to send additional messages to the user after the data has

been pulled down, it just waits and queues them up until the client makes another

request. The idea behind push technology is that as soon as the server has a new message

for the user, it sends it to him immediately. A connection is maintained between the

server and the client and new data is sent as needed.

In the scheme of the Internet, push technology is not a new development. Throughout

the years there have been different standards dictating how it should work. Each pro-

posed standard has had varying levels of support amongst browser makers and different

requirements on the server side.

The differing behaviors and requirements of the two technologies have led many de-

velopers to use one or the other. This has meant that many sites wanting to offer dy-

namic updates to their users had to resort to Ajax timers polling the site every X seconds

to check for new content. This increased amount of requests is taxing on the server and

provides a far less graceful user experience than it should have.

Pushing content out to the user as it happens gives the user a much more engaging

experience and uses far less resources on the server. Fewer requests means less band-

width is used and less CPU consumed, because the server is not constantly checking

and responding to update requests (see Figure 1-3).

Prerequisites

This book assumes the reader is comfortable with most aspects of web development.

The example code in this text uses Java, JavaScript, PHP, and Python. You are encour-

aged to use the technologies that make the most sense to you. If you’re more comfort-

able with PostgreSQL than MySQL, please use what you’re more familiar with. Many

of the command-line examples assume you’re using something Unix-like (Mac OS X,

Linux, etc.), but most of this software runs on Windows, and I’m confident that you

can translate any commands listed so that they work in your environment.

4 | Chapter 1: Introduction

Another prerequisite for this book is a Google account. Several of the examples require

a Google account for App Engine, whereas others use it for authentication. Where it’s

used for authentication, you could fairly easily drop in authentication from another

third-party site.

Python

Python is the most-used language in this book for a couple of reasons. Whether or not

you know Python, it’s a very simple language to read on the page. It also has a couple

of terrific libraries that are used throughout this book. During the process of writing

this text, one of the pioneers on the realtime web, FriendFeed, was sold to Facebook.

After the acquisition, Facebook released a core piece of FriendFeed’s infrastructure as

open source software called Tornado, which is both a collection of libraries and a web

server.

JavaScript

This book uses JavaScript in many of the chapters. In most chapters I use a library that

helps with some cross-browser issues and enables the examples to contain less code by

wrapping up some common activities into simple function calls. This was done to save

space on the page and make things easier. If you have a preference of mootools.com or

any other JavaScript library over jQuery, please go ahead and use those. These examples

are not based around the languages.

JavaScript Object Notation

This book heavily uses JavaScript Object Notation (JSON) in a number of the examples.

JSON is a simple, lightweight data interchange format. It’s often used as the payload

from Application Programming Interface (API) calls to external services, but in this

book it’s also used to send messages to the server and back to the browser.

Google’s App Engine

Another technology that is used in several chapters is Google’s App Engine platform.

This service is Google’s entry into the cloud computing services industry. It’s a fairly

unique way of looking at serving applications, and the developer does not have to think

about scaling up by adding servers. It’s useful here because it gives us a lot of standard

features for free. There is a datastore, authentication, and integration with other serv-

ices, all without paying a cent or writing much complicated code. It was also picked

because it requires almost no configuration for the developer. If you’re not familiar with

the service, that is no problem, because we go through the process of setting up an

account in the text.

6 | Chapter 1: Introduction

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构建实时用户体验：实操指南

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