Docker Swarm : cronjobs
One of the shortcomings that Docker Swarm has compare to Kubernetes, is the possibility to schedule jobs natively.
Fortunately a project called swarm-cronjob from crazy-max allow you to schedule jobs in Docker Swarm.
Setup
Swarm-cronjob get deployed using the following stack file (swarm-cronjob.yaml):
version: "3.4"
services:
swarm-cronjob:
image: crazymax/swarm-cronjob:latest
networks:
- backend
volumes:
- "/var/run/docker.sock:/var/run/docker.sock"
environment:
- "TZ=Europe/Paris"
- "LOG_LEVEL=info"
- "LOG_JSON=false"
deploy:
placement:
constraints:
- node.role == manager
networks:
backend:
external: true
Once swarm-cronjob is successfully deployed, it is time to schedule a job.
Let’s say you want a job that hit an API endpoint every morning at 08:00.
The stack file will then looks like this (cronjob-hit-an-api.yaml):
version: "3.4"
services:
cronjob-hit-an-api:
image: ellerbrock/alpine-bash-curl-ssl:latest
networks:
- backend
command: curl -X POST https://reqbin.com/echo/post/json
deploy:
mode: replicated
replicas: 0
labels:
- "swarm.cronjob.enable=true"
- "swarm.cronjob.schedule=00 08 * * *"
- "swarm.cronjob.skip-running=false"
restart_policy:
condition: none
networks:
backend:
external: true
Swarm-cronjob will then detect via the docker socket that a new service (cronjob-hit-an-api) was added.
It is done by sniffing specific labels, similair to what Traefik is doing for example.
Tailing swarm-cronjob service log files you can see that the job was added.