In computer science, in particular networking, a session is a semi-permanent interactive information exchange, i.e. a dialogue, a conversation or a meeting, between two or more communicating devices, or between a computer and user (see Login session). A session is set up or established at a certain point in time, and torn down at a later point in time. An established communication session may involve more than one message in each direction. A session is typically, but not always, stateful, meaning that at least one of the communicating parts need to save information about the session history in order to be able to communicate, as opposed to stateless communication, where the communication consists of independent requests with responses.
Communication sessions may be implemented as part of protocols and services at the application layer, at the session layer or at the transport layer in the OSI model.
Application layer examples:
- HTTP sessions, which may allow dynamic web pages, i.e. interactive web pages, as opposed to static web pages.
- A telnet remote login session
Session layer example:
- A Session Initiation Protocol (SIP) based Internet phone call
Transport layer example:
- A TCP session, which is synonymous to a TCP virtual circuit, a TCP connection, or an established TCP socket.
In the case of transport protocols which do not implement a formal session layer (e.g., UDP) or where sessions at the session layer are generally very short-lived (e.g., HTTP), sessions are maintained by a higher level program using a method defined in the data being exchanged. For example, an HTTP exchange between a browser and a remote host may include an HTTP cookie which identifies state, such as a unique session ID, information about the user's preferences or authorization level.
Protocol version HTTP/1.1 makes it possible to reuse the same TCP session for a sequence of service requests and responses (a sequence of file transfers) in view to reduce the session establishment time, while HTTP/1.0 only allows a single request and response during one TCP session. However, this transport layer session mechanism should not be confused with a so called HTTP session, since it is not lasting sufficiently long time, and does not provide application level interactive services such as dynamic web pages.
Communication sessions, especially stateful communication, are often implemented in software using multithreading, where a new thread is created when the computer establishes or joins a session. The advantage over a single-thread approach is relaxed complexity of the software, since each thread is an instance with its own history and variables. The disadvantage is large overhead in terms of system resources.
When a client may connect to any in a cluster of servers, a special problem is encountered in maintaining consistency when the servers must maintain session state. The client must either be directed to the same server for the duration of the session, or the servers must transmit server-side session information via a shared file system or database. Otherwise, the client may reconnect to a different server than the one it started the session with, which will cause problems when the new server does not have access to the stored state of the old one.
Reference:
http://en.wikipedia.org/wiki/Session_%28computer_science%29
