How
do Web clients and Web servers communicate together?
One
of the protocols in the application layer of the OSI protocol stack is Hyper-Text
Transfer Protocol or HTTP for short. This
is the standard that the Web client and Web server use to communicate with
each other. If we think back to client-server architecture,
Web browser acts as the client and the Web server as the server.
Browser sends a request to the Web server and server processes the
request and returns a response to the client.
Client's request in this case is in ASCII format, however response
from the server is much more varied. Remember that browser could be requesting web
pages that consist of not only text but also images, audio, and in all hyper-media
content. Furthermore some of the content
may use other applications such as Word, Excel, PDF Reader, etc to display
the content. Response from the server
therefore is in Multipurpose Internet Mail Extensions (MIME) format. This basically means that content that comes
back from the server indicates what application should be used to view it.
Hyper-text
facility allows one to encode the URL of a Web resource in an object. This object could be a piece of string in the
text, or an image. When object is clicked,
the embedded URL is transferred to the browser and consequent request is made
to the server. It is this capability
that prompts the use of HTTP as communication standard for the Web.
An
HTTP request-response life cycle is that
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What
is the general format of a HTTP request? |
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One element of HTTP request is "method-name".
What are the different methods?
What do they do? |
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http://www.w3c.org/ is a good place to start for
finding the answers.
Going
back to the request-response life cycle,
a connection is established at the beginning.
This connection is closed after server responds to the request. In essence what this means is that in case client
makes a second request, server has no recollection of who the client is and
of the client's previous request. We
say HTTP is stateless. Web was originally
conceived to be the platform on which researchers would publish scientific
documents and articles. Such application
requires the architecture to store resources and to host services for publishing
those resources. Clients would make
requests for those resources and server would retrieve the required from storage
and serve it to the client. Keeping
state is not a requirement for such an application and the architecture of
Web was therefore not designed to keep state.
Doing
business online however does require keeping state.
Server needs to distinguish between the clients. A good example of when state management becomes
essential is the application of virtual shopping cart. Server needs to maintain that the content of
a shopping cart belong to one client and one client alone while managing connections
and potential shopping carts for a number of clients simultaneously.
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What are the different methods for
keeping state? |
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How does the system architect decide
on a method for keeping state? |
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Food
for thought:
What
is the general format of server response?
What
privacy issues can you think of when you keep state?