Transcript Intro to AJAX Part I

Introduction to
Learning Objectives
By the end of this lecture, you should be able to:
– Describe the difference between client-side and server-side scripting
– Name some commonly encountered web-based tasks that can only be
achieved using server-side scripting
– Describe the benefits of using AJAX
– Contrast the AJAX request-response model with the traditional requestresponse model
– Describe the 3 types of server applications needed to run most
commercial or large-scale web sites
What Isn’t AJAX?
Admittedly, this is a strange way to introduce a topic.
However, there is some confusion about what AJAX is and
is not. So let’s start with what it isn’t:
• AJAX tands for ‘Asynchronous JavaScript And XML’ – yet
this acronym is not altogether appropriate. For
example the ‘XML’ part of AJAX is really no longer used
very much at all.
• AJAX is not a programming or scripting language! It
certainly involves and requires scripting, but scripting is
only one part of AJAX.
• AJAX is not a new ‘gee-whiz’ technology. In fact, it is
really a collection of relatively “older”, but wellestablished and stable technologies.
Introduction: Okay, so what is AJAX?
• AJAX allows web pages to be updated asynchronously by exchanging small
amounts of data with the server behind the scenes. This means that it is
possible to update parts of a web page, without reloading the whole page.
• Classic web pages, must often reload the entire page if a new piece of
information is being requested from the server.
• Pretty important nowadays . In fact, AJAX is used on many, many sites that you
probably use on a daily basis.
• Examples of applications using AJAX: Google Maps, Gmail, Youtube, and
Facebook.
Client Side v.s. Server Side Scripting – The Client
• All of the scripting we have done to date is “client-side” scripting.
• Client-side script = script that is executed by the client (e.g. your web browser).
• Most popular client-side scripting language is JavaScript (and the jQuery
dialect).
• Main advantage to client-side scripting is that it is faster, and distributes the
workload.
• Example: Imagine even a very simple script that does a conversion from pounds
to kilograms. On a single client, this is extremely rapid. But imagine if this same
script was executed on a server that receives 1000 requests per minute. Many
such pages could bog down a web server.
– Much better to distribute this workload to the clients
Client Side v.s. Server Side Scripting – The Server
• Sometimes the requested information/resource simply can not be processed by
the client
• A server-side script is a script that is stored on and executed by the web server.
• Most popular server-side scripting language today is probably PHP
– Others include
• ASP.NET
• ColdFusion
• Ruby
• Python
• Many others
• Example – Amazon.com:
–
–
–
–
Request information about a product
Requires a database lookup
Convert the user's text field input into a database query (client or server side script)
Query the database (must be server side)
Scenarios that typically require server-side scripts
• Sending an email with information from a form
• Accessing any file that resides on the server such as a database, spreadsheet,
other HTML files, text documents, etc.
• One of the most common reasons for using server-side scripting is when you
want to give the user the ability to access some specific pieces of information
on a resource located on the server (e.g. querying a database) but without
giving them the ability to see the entire file/resource.
• AJAX requires server-side scripting as we will see.
AJAX and jQuery
• Characteristics of AJAX include:
– Somewhat involved in terms of code length and complexity
– Several steps involved - even in basic programs
– Prone to cross-browser compatibility issues
• AJAX is scripted using JavaScript. (In fact, JavaScript is the ‘J’ in AJAX), though
the JS code is at times a bit unwieldy.
• jQuery greatly simplifies the length, and steps.
• jQuery also handles the large majority (though not all) of browser compatibility
issues. In fact, one of the reasons that jQuery became so popular was its ability
to greatly simplify the use of AJAX.
• Other JS/AJAX libraries available besides jQuery:
– DOJO
– MochiKit (specializes in manipulating JSON data)
– YUI (Yahoo Interface Library)
What does AJAX do for us?
Waiting for pages to load takes time…
• Imagine you are viewing a company’s home page that contains numerous images, menus, buttons,
icons, etc.
• The designer of the website intends for all of these items to be visible on every page for that
company’s website.
• You wish to view their ‘Contact Us’ page. You click on the link to that page which has the following
information: Rabbits ‘R Us Inc, 222 S. Carrot Drive, Beatrix Potter, PA, Tel: 333-444-5555
• To load this contact_us.html page, you would have to wait for ALL of the site’s menus,
buttons, icons, images, and so on to reload – even though much of this information is redundant
• It would be far faster and more efficient to have only requested the key information from the
company’s ‘Contact Us’ page without having to redundantly re-load all of those web page
components (images, links, etc) that are still sitting right there in your browser window.
• What if your company receives thousands of page requests a minute (or even per second)?
• Makes for happier customers / visitors when requests for information from your web page is visible
nearly instantaneously, i.e. without having to wait for the page to reload redundant content.
AJAX In Action
•
•
•
•
•
If you visit Google’s home page and begin typing a query, you will notice that as you type, the text
field attempts to predict what you are searching for. Yet no other information on the page is
realoded.
Though you are communicating with google’s server with every character you type, much of the
underlying page does not change – because it is not being reloaded.
Google Maps: Allows you to zoom in/out, move in any direction and so on, without reloading any
non-map areas of the page.
Apple Store: Begin pricing out a new Mac, and you will see that the outer menus and so on do not
change. Only the information that needs to change is requested from the server.
Amazon: On most product pages, you will also see a section that says something along the lines of
‘Customers Who Bought this Item Also Bought:’. Scrolling shows a series of additional items appears
on the page – yet your original product page does not change.
Examples of things that are well-suited to AJAX
• Browse database information: Many shopping sites will allow you to find
additional/related information about a product – without leaving the product’s
page.
– E.g. clicking on ‘Reviews’ on Amazon.com
• Refresh new content at regular intervals: E.g. Visit the National Hockey League
and you will see scores regularly update without the entire page refreshing.
• Confirmation messages: Confirm that a form was successfully submitted –
without leaving the page.
• Voting/Rating: User clicks on stars or a poll-choice – the user’s selection can be
submitted to a database on the server without leaving the page.
That is what AJAX Does!
• Makes pages feel snappier and more responsive.
• Enables the user to retrieve new or additional content from your website
without waiting to reload the entire page.
Makes for a happier customer, as well as for a happier web server (and happier IT
staff, CEO, CFO, etc).
How Does it Work? Traditional Model
Shown here is the traditional HTTP request model:
1.
2.
3.
Web page makes a request to the server for some information (e.g. a separate
‘Contact Us’ web page).
Server sends back the desired web page.
Requesting other information (e.g. ‘Our Hours’ page) requires a new page to
be downloaded from the server.
The AJAX Model
Here is the AJAX way of doing things:
1.
2.
Web page requests some information (e.g. a ‘Contact Us’ web page) from the
server.
Server sends back ONLY the required information
– Typically loaded into a specific section (e.g. a ‘div’ section) of the ORIGINAL web
page.
3.
Requesting some other information (e.g. ‘Our Hours’ page) will again load
ONLY the relevant information.
To Summarize: With AJAX, the client requests only NEW information from the
server. The original page is simply updated to reflect new content. Nothing else in
the original page is changed.
Example
1.
2.
3.
User clicks on a button in the ‘News Headlines’ section
Request is sent to the web server for that particular item.
Info returned from the server is loaded into the ‘headlines’ div section of the
current document.
–
Remainder of the document (the header with title and logo at the top, the news headlines
box, the footer section at the bottom, etc) is never modified or reloaded.
Example: Amazon Search for ‘Seawing Gorilla Fin’
How Does it Work? -- Some Key Components
•
•
•
The XMLHttpRequest Object:
– Built into JavaScript is an ugly-sounding programming object called the
XMLHttpRequest object.
– A programming “tool” that allows JS to communicate with a web server and send
information back and forth.
– We will return to this later.
Other JavaScript:
– Open a line of communication with the web server
– Request information
– Process the information that comes back
– Insert new information into the appropriate location of our web page, etc.
Web Server:
– Receives requests from the client
– Processes the requests
– Responds appropriately
– Example: Client requests information from a database (e.g. ‘Spaulding Basketball’).
The server will search the database and will return all matching records to the
client.
Review: The Web Server
• Think of a web server as a glorified filing cabinet hosting lots of resources
(HTML pages, drivers, databases, etc, etc), ready to “serve” them up upon
request.
• Hardware or software?
– Probably best thought of as software.
– Recall that a web client (i.e. a browser) is simply software that can request pages
from a server, parse pages that come back form a server, run JavaScript, display
HTML and so on.
– A web server is also software, but it is entirely different from web-client software.
– Server software specializes in interpreting HTTP request objects that originate from
a client, retrieving the requested resource, and sending that resource back to the
client.
• However, since web server software frequently runs on a dedicated computer
whose only job in life is to run the web-server software, we frequently refer to
the computer itself as “the web server”. Technically, though, the web server is
actually a piece of software.
Web Server - Hardware
Single PC for a low-traffic site.
Numerous servers at a web-hosting service.
HARDWARE: Four Sun Fire
X4200 web servers mounted
together to serve a highertraffic site.
World’s First Web Server Computer
Note the sticker saying DO NOT POWER DOWN!
Web Server Software
As you can see, web server software is an entirely
different animal than web client software.
This is because even though they are both examples of
“software”, they have entirely different jobs to do!
“Resources”
• Not every request from a server is for an HTML document.
• Web servers can hold any digital item and ‘serve’ these items to clients upon
request.
• Examples:
–
–
–
–
–
–
an updated driver for your new printer
the latest episode of Big Bang Theory from the iTunes store
a database request to search for a book on amazon.com
a Youtube video
a request from weather.com for the latest forecast in your area
a static 15-year-old ‘HTML’ document that someone wrote giving Julia Child’s recipe
for Crème Brulee
Web Server Software: Communication
• Most web clients “speak” HTML, CSS, and JavaScript
• Because your typical web server does not need to display web documents,
there is no need to have an HTML (or CSS) parsing engine built in.
• The web server typically has specialized requirements such as the ability to
–
–
–
–
query a database
serve up a files
check log-in credentials
etc
• Therefore, we typically use languages that specialize in these kinds of features
• Common languages for server-side scripts include PHP, ASP.NET, and others.
What about databases?
• We have repeatedly referred to databases in this lecture.
• Databases are the backbone behind much of the information on the web.
• For example, every time you search for something on Amazon or any other
shopping portal, you are asking the server to query a database (via a serverside script) .
• Even resources such as a driver for your new printer, are typically stored and
located by the server using a database somewhere.
– Similarly, information relating to that driver (e.g. installation instructions) is probably linked to
the driver using a database.
A typical server setup: The Pieces
Web Server:
• A web server technically speaking, refers only to the server software.
• Examples: Apache, Microsoft’s IIS, and Google’s GWS
• Sometimes refers to the computer on which the web server software is running. (e.g.
“We need to reboot the server”).
Database Server:
• Nearly every commercial web site often includes a database application.
• Common database server applications include MySQL and Microsoft SQL Server.
Application Server:
• The computer hosting the web server must also be running some software that can
understand the any scripting languages that are intended for use.
• If you think about it, your web browser’s ability to “speak” JavaScript is built into the
browser software. Most web server’s do not come pre-packaged with the ability to
speak all of the many different server-side scripting languages that are commonly
spoken.
• Administrators must install an ‘application server’ that can “speak” the desired language
• Examples: PHP, ASP.NET, ColdFusion, etc.
(May, 2010)
Combinations
•
Example – Commercial Setup:
– Web Server: Microsoft IIS
– Database Server: SQL Server
– Application server: Microsoft's ASP.NET
•
Example – Freeware/Open Source Setup
–
–
–
–
Web Server: Apache web server
Database Server: My SQL
Application server: PHP
This is a particularly popular combination and is called 'AMP'
Running AJAX
•
•
•
AJAX does require access to a web server that is running application server software
such as PHP.
The ‘studentweb’ web server at CDM runs PHP and a couple of other application server
software applications.
In order to work with your AJAX programs, you will have to upload your documents to
the server (so that the PHP server can execute your code) every time you make a
change.
–
You will quickly find that this is a tedious step!
Easier Option: You can install AMP on your own computer as a "development server"
• When everything is ready to go, you can upload to the web server.
• For this course, we will only be dipping our toe into running AJAX, so you probably do
NOT need to bother setting up a development server such as AMP.
If You Want to Install AMP
•
•
•
While it can be a little bit time-consuming to set up and configure AMP, if you are
interested in really getting in there and working with AJAX and/or learning PHP, then
installing AMP is probably a great idea.
Good News: There are some installers out there that install all three components with
only one executable.
One of the current favorites is WampServer:
• Windows: ‘WAMP’ at http://www.wampserver.com/en/
– Video installation guide: http://uptospeedguides.com/wamp/
• Mac: ‘MAMP’ at http://www.mamp.info/en
– Video installation guide: http://uptospeedguides.com/mamp/