cronolizer

A simple command line daemon running one command scheduled using a five field CRON syntax via Rob Figueiredo's Golang CRON implementation available at https://github.com/robfig/cron.

Synopsis

The cronolize static binary was developed to schedule background executions inside long-running containers as a sidecar process where a normal sidecar container pattern or cronjob was not possible. It is a light-weight alternative to crond and also a narrow alternative to AWS Event Rules if you want to conserve those (now limited to 100 per account without requesting more, but was 50).

cronolize can run as a sidecar container (only as PID 1 using the foreground option) in an AWS ECS/Fargate task (cost is per task, not per container) or K8s pod very easily without changing permissions/rules (using the primary app's permissions). CronJobs in K8s can however be fairly easily configured for this task and would be the preferred way in most cases.

Usage stories

One usage scenario involved rotating log files and cleaning temp files that overgrew a long-running container's ephemeral storage (log rotation or temp removal was not handled by the app in this case). Running a sidecar container for this purpose was not easily accomplished here within our time-constraints.

Another scenario involved a proprietary docker build solution where the app suffered from degradation and needed to be restarted frequently. The app was deployed during container boot (which took some time), why a restart without cycling the container was much faster (save to resolve the leak in the app). As the use involved restarting an application inside a container, it was not possible to run as a sidecar and cronolize was much simpler to setup than installing cronie or similar cron daemon onto the image.

Prerequisites

You need Go to compile and produce the cronolize binary and make (for example: apt-get install build-essential) if you want to use the Makefile (recommended). Download Go from https://go.dev/.

Building

git clone github.com/sa6mwa/cronolizer
cd cronolizer
make clean
make

Binary will be called cronolize and will be compiled for the local OS and architecture. The Makefile happily cross-compiles for several OS/architecture combinations...

# build amd64, arm64, arm6, arm7 and 386 for linux...
make linux

# build darwin/amd64 and darwin/arm64
make darwin

# build amd64 and arm64 for freebsd, netbsd and openbsd...
make freebsd netbsd openbsd

Usage

See https://pkg.go.dev/github.com/robfig/cron/v3 for reference how to format the CRON schedule. The internal syntax help covers most usage...

$ ./cronolize
Welcome to cronolize 0.1 (C) 2022 SA6MWA https://github.com/sa6mwa/cronolizer

Syntax: ./cronolize [options] cronSpec command

Usage of ./cronolize:
  -fg
        Run cron in the foreground instead of as a background daemon process
  -log string
        Log output from stdout and stderr to this file (default "/dev/null")
  -q    Quiet, don't print the PID message at the end or the log entry in the log file
  -shell string
        Full path to shell used to execute command (default "/bin/sh")
  -shellCommandOption string
        Command option used by the shell, usually -c (default "-c")
  -truncate
        Truncate instead of appending to the log file

cronSpec is a five field CRON expression. See below or refer to
https://pkg.go.dev/github.com/robfig/cron/v3 for details.

command is the command string to execute via /bin/sh -c (by default). See -h
for more information.

Examples:
cronolize -log /var/log/minute.log "* * * * *" 'date ; echo Hello world'
cronolize -shell /bin/bash "@hourly" 'echo "Last run on $(date)" > /var/opt/output'
cronolize -log out.log "CRON_TZ=Europe/Stockholm 37 13 * * *" 'touch /var/opt/touchable ; echo "Touched file at $(date)"'
cronolize -log /var/logs/nightlyRestart.log "@daily" "echo \"Restarting myservice\" ; supervisorctl restart myservice"

Cron format:

Field name   | Mandatory? | Allowed values  | Allowed special characters
----------   | ---------- | --------------  | --------------------------
Minutes      | Yes        | 0-59            | * / , -
Hours        | Yes        | 0-23            | * / , -
Day of month | Yes        | 1-31            | * / , - ?
Month        | Yes        | 1-12 or JAN-DEC | * / , -
Day of week  | Yes        | 0-6 or SUN-SAT  | * / , - ?

Predefined schedules:

Entry                  | Description                                | Equivalent To
-----                  | -----------                                | -------------
@yearly (or @annually) | Run once a year, midnight, Jan. 1st        | 0 0 1 1 *
@monthly               | Run once a month, midnight, first of month | 0 0 1 * *
@weekly                | Run once a week, midnight between Sat/Sun  | 0 0 * * 0
@daily (or @midnight)  | Run once a day, midnight                   | 0 0 * * *
@hourly                | Run once an hour, beginning of hour        | 0 * * * *

The parent process will start a copy of itself in the background and exit while
the copy (child process) will run cron (unless the -fg option is issued) and
block indefinitely until killed.

Author

SA6MWA Michel Blomgren, email: sa6mwa@gmail.com