This section describes the session concept used by Web server side applications. A Web session represents a series of HTTP requests and responses exchanged between a browser and a Web server. Usually, a cookie is used to carry the ID of a Web session.
What Is a Session? A session is an abstract concept to represent a series of HTTP requests and
responses exchanged between a specific Web browser and a specific Web server.
The session concept is very useful for Web based applications to pass and share
information from one Web page (request) to another Web page (request).
Since the current design of HTTP protocol does not support session concept,
all Web server side scripting technologies, including PHP, have designed their
own ways to support the session concept. The key design element of session support is
about how to identify a session with an ID (identification) and how to maintain the session ID.
One common way to maintain the session ID is use the cookie technology.
The following diagram shows you how to do this:
Server Browser
ID created | <-- Request #1 --- |
| --- Response #1 --> | ID kept as cookie
| <-- Request #2 --- | ID send back to server
| --- Response #2 --> |
| <-- Request #3 --- | ID send back to server
| --- Response #3 --> |
| ...... |
The session concept should be managed by the server. When the first request comes
from a browser on a client host, the server should create a new session,
and assigns a new session ID. The session ID will be then sent back to the same browser
as a cookie. The browser will remember this ID, and send the ID back to the
server in subsequent requests. When the server receives a request containing the same
session ID, it knows that this request is a continuation of an existing session.
When the server receives a request from a browser on a new client host (request
without a session ID), the server should not only create a new session ID, it should also
create a new session object associated with the new session ID. This session object
should become the storage place for different requests of the same session to store
and share information between requests containing the same session ID.
If there is no subsequent request coming back for a long time for a particular
session ID, that session should be timed out. After the session has been timed out,
if the browser comes back again with the same session ID, the server should give
an invalid session error.
