I have been using docker for a couple of months now, and am working on dockerizing various different server images. One consistent issue is that many servers need to run cron jobs. There is a lot of discussion about that online (including on Stackoverflow), but I don't completely understand the mechanics of it.
Currently, I am using the host's cron and docker exec into each container to run a script. I created a convention about the script's name and location; all my containers have the same script. This avoids having the host's cron depending on the containers.
Basically, once a minute, the host's cron does this:
for each container
docker exec -it <containername> /cronscript/minute-script
That works, but makes the containers depend on the host.
What I would like to do is create a cron container that kicks off a script within each of the other containers - but I am not aware of an equivalent to "docker exec" that works from one container to the other.
The specific situations I have right now are running a backup in a MySQL container, and running the cron jobs Moodle requires to be run every minute. Eventually, there will be additional things I need to do via cron. Moodle uses command-line PHP scripts.
What is the "proper" dockerized way to kick off a script from one container in another container?
Update: maybe it helps to mention my specific use cases, although there will be more as time goes on.
Currently, cron needs to do the following:
- Perform a database dump from MySQL. I can do that via mysqldump TCP link from a cron container; the drawback here is that I can't limit the backup user to host 127.0.0.1. I might also be able to somehow finagle the MySQL socket into the cron container via a volume.
- Perform regular maintenance on a Moodle installation. Moodle includes a php command line script that runs all of the maintenance tasks. This is the biggie for me. I can probably run this script through a volume, but Moodle was not designed with that situation in mind, and I would not rule out race conditions. Also, I do not want my moodle installation in a volume because it makes updating the container much harder (remember that in Docker, volumes are not reinitialized when you update the container with a new image).
- Future: perform routine maintenance on a number of other of my servers, such as cleaning out email queues, etc.