codebje/crocker repository overview

⁠Crocker: cron docker

crocker is an entrypoint binary for a Docker image for periodic tasks. It has two main purposes:

1. Execution of another process on a schedule.

   Docker itself has no facility for periodic execution. The usual solutions are to use host services to schedule container execution, or to embed a process for scheduled execution. This solution is an example of the latter.

2. Docker dissociates processes from init, and they must be reaped.

   It is possible to create zombie processes using Docker, when a program spawns another and doesn't reap it on exit. Usually, the init process will inherit all orphaned children and reap them, but the Docker process management approach separates processes inside a container from the host such that this no longer happens. If you only run one process, this is not a concern, but if you spawn a child and don't successfully reap it, you will wind up with zombies. For more details, see this blog post⁠.

Running cron or its variants inside the container works, but has drawbacks. These usually aren't static binaries, and usually depend on a system mailer and possibly a logging facility to work. crocker is a static binary which logs to standard out, including reporting error exit codes for the tasks it executes.

⁠Usage

crocker [--at-start|-A] <cron exrepssion> <command> [<arg> ...]

CRON expressions are the usual. If the --at-start or -A option is given, the command is immediately executed on launch.

A command will not be re-executed until it has completed its previous execution. If a scheduled execution time elapses before the command has completed, the command will be immediately re-executed on termination. If more than one scheduled execution time elapses before a command has completed, only one subsequent execution will be queued - a process running very slowly will lose executions rather than overrun an execution queue.

Here's an example Dockerfile:

FROM busybox

ADD https://github.com/APNIC-net/crocker/releases/download/v0.2.0/crocker /crocker
RUN chmod a+x /crocker

ENTRYPOINT ["/crocker", "-A", "@daily", "/bin/echo", "it is a brand new day"]

The binary is static, so you may use it in a container from scratch, but you will need to mark it executable. This means you cannot ADD it by URL.

⁠Credits

Many of the functions for handling sub-process reaping are based on the code in pid1⁠, though regrettably nothing in that code base was usable directly, as it serves a different purpose.