Introduction (mcron 1.2.2)

1 Introducing mcron

The mcron program represents a complete re-think of the cron concept originally found in V7 Unix, ratified by POSIX, and re-realized by Paul Vixie for the free world. The original idea was to have a daemon that wakes up every minute, scans a set of files under a special directory, and determines from those files if any shell commands should be executed in this minute.

The new¹ idea is to read the required command instructions, work out which command needs to be executed next, and then sleep until the inferred time has arrived. On waking the commands are run, and the time of the next command is computed. Furthermore, the specifications are written in Scheme, allowing at the same time simple command execution instructions and very much more flexible ones to be composed than the POSIX format allows. This has several useful advantages over the original idea.

Does not consume CPU resources when not needed. Many cron daemons only run jobs once an hour, or even just once a day. Busy computers with high memory pressure can keep the mcron program swapped out to disk more of the time.

Can easily allow for finer time-points to be specified, i.e. seconds. In principle this could be extended to microseconds, but this is not implemented.

Times can be more or less regular. For example, a job that runs every 17 hours can be specified, or a job that runs on the first Sunday of every month.

Times can be dynamic. Arbitrary Guile (Scheme) code can be provided to compute the next time that a command needs to be run. This could, for example, take the system load into consideration.

Turns out to be easy to provide complete backwards compatibility with the POSIX cron specification.

Each user looks after their own files in their own directory. They can use more than one to break up complicated cron specifications.

Each user can run their own daemon. This removes the need for suid programs to manipulate the crontabs, and eliminates many security concerns that surround all existing cron programs.

The user can obtain an advance schedule of all the jobs that are due to run.

Vixie cron is implemented in 4500 lines of C code; mcron was 2000 lines of Scheme (it has grown a little fatter since), despite the fact that it offered many more features and much more flexibility, and complete compatibility with Vixie cron and the POSIX specification.

A full discussion of the design and philosophy of mcron can be found in the white paper at http://www.gnu.org/software/mcron/design.html.

Footnotes

(1)

With retrospect, not so new: AT&T and Berkeley Unices had a cron program which also implemented this ’new’ idea, though Dale Mellor was unaware of this at the time of mcron’s conception and initial development. The Wikipedia page for cron appears quite authoritative and is a good source of information about the history of cron.