Despite cron being an age old *nix technology, it alway seems like scheduling tasks is an annoying thing.

Note: Cron is the standard *nix technology for running tasks on a recurring basis like "every monday at 5 am execute this program".

We are now doing things like running tasks within containers and that raises the question of where does cron exist:

  • within the container?
  • at the OS level?
  • what about multiple instances of the container across different machines?

I'm not going to try and address the bigger picture questions here like multiple instances but I can show you how to use cron at the OS level to drive a rake task within a container.

How Do You Run Something Within in a Container

The first thing to understand is that you can easily execute a shell with in a container, from the outside, with this syntax:

docker exec -it CONTAINER_HASH /bin/bash

The CONTAINER_HASH is the docker equivalent of a *nix pid (process identifier).

How to Run a Rake Task

And if you can run a shell like /bin/bash within a container from the outside then, drum roll please, you can easily run anything from outside including a Rake task. And if you have that then you can easily schedule it. Here's an example:

docker exec -it 8d76da4ab481  bundle exec rake data_source:update_or_initial_load --trace

The 8d76da4ab481, shown above, is an example of the CONTAINER_HASH.

But Docker Container Hashes Change…

If you think about this a bit there is one real problem – that damn CONTAINER_HASH isn't consistent. It changes every time the container is restarted. The trick to making a cron job that runs your rake task is to use a shell script which pulls the container hash dynamically. And to figure that you we are going to start with my dockerbash blog post.

Here's the shell script from the dockerbash post:

if [ -z $1 ]; then
  echo "You need to specify the name of the container you want to get into like:"
  echo "dockerbash police"
  pid=`docker ps | grep $1 | awk '{print $1}'`
  docker exec -it $pid /bin/bash

Let's create another shell script calling it, say, docker_datasource_update:

pid=`docker ps | grep police | awk '{print $1}'`
docker exec -it $pid bundle exec rake data_source:update_or_initial_load --trace

Note: This is for an application which has the word 'police' in its name hence the above grep for the word police.

Let's say that this is now stored in /usr/local/bin/docker_datasource_update. You then need to write a cron job to call it:

# m h  dom mon dow   command
0 7 * * * /usr/local/bin/docker_datasource_update

Note: Add the crontab above with crontab -e.