added checking of result when building demos

[urisagit/Stem.git] / Design / proc_notes

Stem::Proc

A major service in network management infrastructure is running and 
controlling processes.  Stem::Proc is a very simple to use cell which 
has a wide range of useful options to control the process behavior 
and its standard I/O.  The required parameters to the cell specify 
the program  and its arguments.   

The first group of configuration parameters controls how and when the 
process is actually started.  A process can be started at 
configuration time or initiated by a command message.  The message 
interface is very important as it allows process management from any 
other cell, including Stem::Cron (this will allow emulation of the OS 
cron running processes), Stem::SockMsg (this will allow emulation 
of the OS inetd), and other similar needs for remote process 
invocation.  Process monitoring is achieved by handling SIGCHLD 
signals and I/O handle shut-down detection.  A message can be sent 
upon process exiting and the process can optionally be restarted 
automatically, giving you a watchdog service.

The second group is much larger and controls the I/O behavior of the 
process.  You can enable/disable any subset of its 
stdin/stdout/stderr handles.  This is needed for processes that don't 
use all of their standard I/O handles.  Process I/O can be managed by 
the Stem::AsyncIO class and callbacks.  In addition, the cell can be 
configured to transfer its I/O data to/from other cells via Stem 
messages.  One neat feature is the ability to buffer all output from 
a process and send it via a message only upon process exit.  This 
emulates the running of a process in backquotes as supported by 
shells and Perl.

Processes that need to work with a terminal can optionally be run 
behind a pseudo-TTY device.