An introduction to session management
Daniel J. Bernstein
Draft 2, 1991-10-04

Session basics

session, n. ... 4. any period of activity.

When a user sits down in front of a multi-user computer, gives his login name and password, and starts typing, he's begun a session. Within that session he might read mail, edit files, compile and run programs, and even play games. What happens if the screen explodes?

A typical Unix system lets a user log in through a network, or over a phone connection between the user's modem and the computer's modem. But if the phone line suddenly becomes too noisy, or the network doesn't work, the user's connection may drop. The system will then terminate the login session. Any programs run by the user will be signalled that the connection was hung up. If they ask the user for input, they won't get any. If they produce output, the output will vanish.

Other operating systems are more helpful. VMS, for instance, keeps the session running inside the computer. When the user establishes another connection and logs in again, the computer asks if he'd like to reconnect to the original session. Programs started under the session won't notice what happened. This is a primitive form of session management. (It is also an abuse of terminology, as the "session" — the "period of activity" — really consists of two periods. On the other hand, from the point of view of the programs under the session, there hasn't been any interruption.)

This paper describes a session management system which the author has developed for BSD Unix and its variants. In section 2 we describe the three fundamental operations making up the system. In section 3 we show how the system appears to novice users. In section 4 we give several examples of more complex session management techniques which cannot be performed under VMS.

sess, disconnect, reconnect

The session management system consists of three basic operations. To illustrate the operations we use pictures:

   user ==== tty ==== csh                                         (1)

A double line, such as ====, represents a two-way flow of information. Here the user types characters, which the Unix kernel processes inside a tty device and then sends to the user's shell, csh. What csh prints is then sent back through the tty to the user.

When csh starts a job, the picture changes:

               /               \
   user ==== tty ==== csh      who
               \                |
                \              sort
                 \              |
                  ------------ more

Here the user has started a pipeline, who | sort | more. To simplify the illustration somewhat we pretend that all I/O from the pipeline is sent through the shell:

   user ==== tty ==== csh ---- who
                         \      |
                          \    sort
                           \    |
                            -- more

A job consisting of only one process looks like this:

   user ==== tty ==== csh ==== vi

If the user's connection dies, the tty effectively disappears:

                      csh ==== vi

Without a place to send input and output, csh and vi receive the HUP signal and die a messy death.

The first session management operation is sess. When the user, starting from picture (1), types "sess ksh", he creates a session:

                               /------\      +------+
   user ==== tty ==== csh ==== | conn | ==== | sess | === ksh
                               \------/      +------+

The session consists of the square box, called the master because it's in charge of session management, with ksh running below it. The session is connected to the rounded box, also called the signaller because it signals csh when the session is finished. Normally both boxes are completely transparent, and the user can start jobs under the session:

                               /------\      +------+
   user ==== tty ==== csh ==== | conn | ==== | sess | === ksh ==== vi     (2)
                               \------/      +------+

(In fact, from ksh's point of view, the sess box looks just like a tty. It is actually a pseudo-tty, also known as a pseudo-terminal. Further discussion of pseudo-terminals is beyond the scope of this paper.)

What happens if the user's connection drops? As before, the original tty disappears, and csh sputters and dies. The signaller disappears too:

                                             | sess | === ksh ==== vi

However, the session keeps running. sess doesn't mind being disconnected. It waits for a reconnect, as described below. Meanwhile, ksh doesn't have the slightest inkling that the connection is gone.

The user can in fact force a disconnect manually. This is the second fundamental operation. Starting from picture (2), if the user types "disconnect" to ksh, the connection will be severed:

   user ==== tty ==== csh

                                             | sess | === ksh ==== vi     (3)

The signaller exits quietly, and the user can now type commands to csh.

How does the user reconnect to a disconnected session? He uses the third fundamental operation, the reconnect command. The system assigns each session a name. If the user starts a new session, it might look like this:

                               /------\      +------+
   user ==== tty ==== csh ==== | conn | ==== | sess | === qsh
                               \------/      +--p7--+

Here "p7" is this session's name. Say the disconnected session was named "qb":

                               /------\      +------+
   user ==== tty ==== csh ==== | conn | ==== | sess | === qsh
                               \------/      +--p7--+

                                             | sess | === ksh ==== vi

If the user types "reconnect qb" to qsh, nothing happens immediately, but the signaller remembers his reconnect:

                               /------\      +------+
   user ==== tty ==== csh ==== | conn | ==== | sess | === qsh
                               \-+qb+-/      +--p7--+

                                             | sess | === ksh ==== vi

As soon as session p7 exits (or is disconnected), the reconnect takes effect:

                               /------\      +------+
   user ==== tty ==== csh ==== | conn | ==== | sess | === ksh ==== vi
                               \------/      +--qb--+

Now it's just as if session qb had been connected all along. The user can type commands to vi. If the connection disappears again, he can reconnect again, any number of times.

Why doesn't reconnect take effect immediately? There are several reasons. Perhaps the most important is that before reconnecting the user may want to terminate the currently connected session — or he may want to disconnect it, so that he can reconnect to it later. This reconnect interface doesn't force one choice or the other. The user may also want to perform some lengthy operations before reconnecting, and it's easier to schedule the reconnect once and do it at his leisure than to have to worry about the reconnect after all the operations. Finally, it is very simple to simulate an immediate reconnect using the above operations, as detailed in section 4.

Session management for users

Most users will never have to learn anything about the sess, disconnect, or reconnect commands. If a connection dies, its owner will be shown a message like this when he logs in again:

You have a disconnected session on /dev/ttyra, with these processes:
shmoe    17790  0.0  0.9  176  504 ra T    12:47   0:01 vi paper.3
shmoe    17331  0.0  0.7   72  384 ra I    12:31   0:00 -bin/csh (csh)
shmoe    17330  0.0  0.5  104  264 ra S    12:31   0:00 pty -ds argv0 /bin/csh
Would you like to reconnect to that session?
If so, type its two-character extension, ra.
To instead start a new session as usual, just press return:

The user will recognize the "paper.3" he was editing, type ra, and be reconnected to his session. The program which handles this is called "sessmenu". While perhaps not as friendly as the reconnect facilities under some operating systems, sessmenu isolates even novices from the trauma of losing important work.

All sessions also keep track of a long name assigned by the user. The "sessname" command displays or sets the current session name:

csh% sessname
session p7
csh% sessname 'writing net test program'
csh% sessname
session p7: writing net test program

The user can see all his sessions with "sesslist":

csh% sesslist
session p7 pid 16614 slave 16615 connected: writing net test program
session ra pid 8045 slave 8046 disconnected: finishing up paper.3

Two other useful commands are "sesswho", to show all sessions on the system and their owners, and "sesswhere", to show where those sessions are currently connected to. (The system administrator may disable these commands for security reasons.)

By combining the sess, disconnect, and reconnect commands in simple ways, a user can perform surprisingly powerful session operations. In this section we give some such combinations. The reader may enjoy drawing pictures to see how the commands work.

A user can reconnect to a disconnected session without finishing the current session with "sess reconnect xx". Another method for doing an immediate reconnect is "reconnect xx; disconnect". The second method is symmetric: if the original session was yy, the user can flip back and forth with

csh-yy% reconnect xx; disconnect
ksh-xx% reconnect yy; disconnect
csh-yy% reconnect xx; disconnect

and so on. To immediately reconnect to another session and finish the current session, he can use "reconnect xx; exit" or "exec reconnect xx".

Say a user is in the middle of something — running a crucial computation, perhaps, or "talk"ing to another user — and decides to record what's happening. With sess, reconnect, and disconnect, he can do this without starting the program again:

[talk session going on, user types ^Z]
csh% sess sh
$ sessname; disconnect
session rf
csh% reconnect rf; disconnect
reconnect: will connect to session rf when session p8 is done
pty: info: reconnecting to rf
pty: info: successfully connected to rf
$ sess reconnect p8 | tee talk-record
reconnect: will connect to session p8 when session q0 is done
pty: info: reconnecting to p8
pty: info: successfully connected to p8
csh% fg
[talk session continues as before]

Now everything in session p8 is recorded in talk-record. The recording is completely transparent and does not require that "talk" be terminated or restarted.

Where to find it

The session management system described here is freely available as part of the pty 4.0 package. pty 4.0 works on BSD-derived Unix systems.

The author's session management model is based on Bellovin's "Session Tty" Manager [1], which is not as powerful as pty but is better integrated into the login system.


[1] Bellovin, S. M. The "Session Tty" Manager. In USENIX Conference Proceedings, San Francisco, Summer 1988, 339-354.

© Copyright 1991 Daniel J. Bernstein. Presumably.