24/07/2022
2 Server-side vs. Client-side
Key Points...
o) The World Wide Web is built on a number of different
technologies.
o) For most users, the web starts and ends with their choice of
web browser. The browser is said to define the client-side of
the web, with the browser, the computer it is running on, and
the user surfing the web being collectively referred to as the
client.
o ) Consider a client who has decided to visit the web site at
www.google.com. The first thing that happens is that the client
will make a request to Google’s web server for the default
page of that web site.
o) The web server is an application running on a computer
owned by Google. Like the client, the server application and
the computer on which it runs define the server-side of the
web, and are collectively referred to as the server.
o ) When the server receives the request from the client for a
particular page, its job is to retrieve the page from the
computer’s files and serve it back to the client. In many cases,
this operation is a very simple procedure involving little or no
work on the part of the server.
o) However, using a programming language like PHP, Perl or
Java, we can cause the server to either modify the page it finds
before it passes it back to the client, or even to generate the
page entirely from scratch. This is referred to as a server-side
application. The page passed back to the client looks (to the
client) exactly the same as any other page that has not been
modified.
An example of a server-side application might be to insert the
current date and time into a page. This would mean that each
time the page was requested (say, by using the browser’s
refresh button), a new time value would be added to the page.
o) Once the client has received the page from the server, it
displays the page and waits for the user to request another
page. As soon as the page reaches this state, it has moved
beyond the control of the server. No server-side application
can now alter the contents of the page without the client having
to make another trip back to the server to get a new (and
possibly updated) copy of the page.
o) However, all modern browsers allow for the running of client-
side applications. These are small applications which are
embedded within the HTML code of the page itself.
o) Server-side applications ignore any client-side applications
that they find while modifying pages to send to the client, so in
general the two types of application cannot easily “talk” to
each other.
o) However, once the client has received a client-side
application, it can begin to modify the page dynamically,
without the need to go back to the server.
o) An example of a client-side application might be a clock on a
web page that updated every second.
o) An unfortunate side effect of client-side applications is that all
the code must be sent to the client for running, which means
that the application’s inner workings are available for anyone
to see. This makes it impractical for checking passwords, or
doing anything else that could cause confidential information
to be released into the wild.
o) In addition, all modern web browsers afford the user the
opportunity to switch off client-side applications altogether.
On top of this, the way the same client-side application is run
will vary from browser type to browser type.
o) Despite these drawbacks, client-side applications (or scripts,
as they are better known due to their general brevity) remain
the best way to provide web users with a rich environment
when developing web applications.
o) In short, the two technologies each have their strengths and
weaknesses:
o) Client-side scripts allow the developer to alter pages
dynamically, and to respond to user actions immediately rather
than having to wait for the server to create a new version of the
page. However, there are security and cross-browser
compatibility issues to be aware of, and these are often non-
trivial.
o) Server-side applications allow the developer to keep her code
secure and secret, thus allowing for more powerful
applications to be created. In addition, since the server running
the code is always a known quantity, applications that run
successfully in one browser will run successfully in all
browsers. However, despite all this power, there is no direct
way for a server-side application to alter a page without
having to force the client-side to load another page. This
makes it completely impractical for things like drop-down
menus, pre-submission form checking, timers, warning alerts
and so forth
