docker stack: Redis not working on worker node - docker

Question

I just completed the docker documentation and created two instances on aws (http://13.127.150.218, http://13.235.134.73). The first one is manager and the second one is the worker. Following is the composed file I used to deploy
version: "3"
services:
web:
# replace username/repo:tag with your name and image details
image: username/repo:tag
deploy:
replicas: 5
restart_policy:
condition: on-failure
resources:
limits:
cpus: "0.1"
memory: 50M
ports:
- "80:80"
networks:
- webnet
visualizer:
image: dockersamples/visualizer:stable
ports:
- "8080:8080"
volumes:
- "/var/run/docker.sock:/var/run/docker.sock"
deploy:
placement:
constraints: [node.role == manager]
networks:
- webnet
redis:
image: redis
ports:
- "6379:6379"
volumes:
- "/home/docker/data:/data"
deploy:
placement:
constraints: [node.role == manager]
command: redis-server --appendonly yes
networks:
- webnet
networks:
webnet:
Here the redis service has the constraint that restricts it to run only on manager node. Now my question is how the web service on worker instance is supposed to use the redis service.

Answer 1

You need to use the hostname parameter in all container, so you can use this value to access services from worker or to access from worker the services on manager.
version: "3"
services:
web:
# replace username/repo:tag with your name and image details
image: username/repo:tag
hostname: "web"
deploy:
replicas: 5
restart_policy:
condition: on-failure
resources:
limits:
cpus: "0.1"
memory: 50M
ports:
- "80:80"
networks:
- webnet
visualizer:
image: dockersamples/visualizer:stable
hostname: "visualizer"
ports:
- "8080:8080"
volumes:
- "/var/run/docker.sock:/var/run/docker.sock"
deploy:
placement:
constraints: [node.role == manager]
networks:
- webnet
redis:
image: redis
hostname: "redis"
ports:
- "6379:6379"
volumes:
- "/home/docker/data:/data"
deploy:
placement:
constraints: [node.role == manager]
command: redis-server --appendonly yes
networks:
- webnet
networks:
webnet:
In addictional if you use the portainer instead of visualizer you can control you SWARM stack with more options:
https://hub.docker.com/r/portainer/portainer
BR,
Carlos

Answer 2

Consider the stack file as per the below example -
Regardless of where it is placed manager|worker all the services in the stack file being on the same network can use the embedded DNS functionality which helps to resolve each service by the service name defined.
In this case the service web makes use of service redis by its service name.
Here is an example of the ping command able to resolve the service web from within the container associated with the redis service -
Read more about the Swarm Native Service Discovery to understand this.