Sys::RunAlone can make sure only one invocation of a script is active at a time.
# code of which there may only be on instance running on system
Provide a simple way to make sure the script from which this module is loaded, is only running once on the server.
THEORY OF OPERATION
The functionality of this module depends on the availability of the DATA handle in the script from which this module is called (more specifically: in the "main" namespace).
At INIT time, it is checked whethere there is a DATA handle: if not, it exits with an error message on STDERR and an exit value of 2.
If the DATA handle is available, and it cannot be flocked, it exits with an error message on STDERR and an exit value of 1.
If there is a DATA handle, and it could be flocked, execution continues without any further interference.
Execution of scripts that are (sym)linked to another script, will all be seen as execution of the same script, even though the error message will only show the specified script name. This could be considered a bug or a feature.
changing a running script
If you change the script while it is running, the script will effectively lose its lock on the file. Causing any subsequent run of the same script to be successful, causing two instances of the same script to run at the same time (which is what you wanted to prevent by using Sys::RunAlone in the first place). Therefore, make sure that no instances of the script are running (and won't be started by cronjobs while making changes) if you really want to be 100% sure that only one instance of the script is running at the same time.