Control distribution of docker swarm services across different computers? - docker

Question

Is there a way to control the distribution of services across different computers? I have one master with two workers and 5 services:
web server
database
redis
celery
s3 storage connection
I only want to outsource the celery workers and run everything else on the master. Is there a way to control that with docker swarm? I have not created a registry yet, because I am not sure if that is still necessary.
Here is my current experimental docker-compose file.
version: "3.8"
volumes:
s3data:
driver: local
services:
web:
image: localhost:5000/web
build: .
env_file:
- ./.env
environment:
- ENVIRONMENT=develop
command: python manage.py runserver 0.0.0.0:8000
volumes:
- ./app/:/app/
- ./lib/lrg_omics/:/lrg-omics/
- s3data:/datalake/
- /data/media/:/appmedia/
- /data/static/:/static/
ports:
- "8000:8000"
depends_on:
- db
- redis
- s3vol
links:
- redis:redis
restart: always
db:
image: postgres
volumes:
- /data/db/:/var/lib/postgresql/data
environment:
- POSTGRES_DB=postgres
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=postgres
redis:
restart: always
image: redis:alpine
ports:
- "6379:6379"
celery:
restart: on-failure
image: pp-celery-worker
build:
context: .
dockerfile: Dockerfile
command: bash -c "celery -A main worker -l info --concurrency 8"
env_file:
- ./.env
volumes:
- ./app/:/app/
- ./lib/lrg_omics/:/lrg-omics/
- s3data:/datalake/
environment:
- DB_HOST=db
- DB_NAME=app
- DB_USER=postgres
- DB_PASS=postgres
depends_on:
- db
- redis
- web
- s3vol
deploy:
replicas: 2
placement:
max_replicas_per_node: 1
s3vol:
image: elementar/s3-volume
command: /data s3://PQC
environment:
- BACKUP_INTERVAL=2
- AWS_ACCESS_KEY_ID=...
- AWS_SECRET_ACCESS_KEY=...
- ENDPOINT_URL=https://example.com
volumes:
- s3data:/data
When I deploy this with sudo docker stack deploy --compose-file docker-compose-distributed.yml QC
And then look at the services I get something like this:
sudo docker stack services QC
>>>
ID NAME MODE REPLICAS IMAGE PORTS
xx5hkbswipoz QC_celery replicated 0/2 (max 1 per node) celery-worker:latest
natb3trv9ngi QC_db replicated 0/1 postgres:latest
1bxpkb18ojay QC_redis replicated 1/1 redis:alpine *:6379->6379/tcp
6rsl5gfpd0oa QC_s3vol replicated 1/1 elementar/s3-volume:latest
aszkle6msmqr QC_web replicated 0/1 localhost:5000/web:latest *:8000->8000/tcp
For some reason only redis and the S3 containers run. And both of them on the master. Nothing runs on the workers.
I am quite new to docker swarm so there is probably more than one thing wrong here. Any comments on best practices are welcome.

Answer 1

To determine why the services are not starting
docker service ps QC_celery --no-trunc will show the state of the service and a message from docker.
To control placement consult the Compose file version 3 reference on placement constraints. Basically it entails adding to the deploy: node:
deploy:
replicas: 2
placement:
max_replicas_per_node: 1
constraints:
- node.role==worker
While, nominally, compose.yml and stack.yml files share a format, they support different feature subsets and for complex deployments it becomes helpful to split the deployment into discreet compose.yml files for docker compose and stack.yml files for swarm deployments.
docker stack deploy -c docker-compose.yml -c docker-stack.yml QC can merge a docker-compose.yml base file with stack specific settings, and you can keep docker compose artifacts in your docker-compose.override.yml. these artifacts include:
build: - docker swarm needs the image to be built and available in a registry, either local(swarm hosted?) or docker-hub.
depends_on:, links: - not supported by swarm, which assumes services can be restarted at any time, and will find each other using docker networks.
restart: controlled by restart_policy: under deploy: