Update image in service without downtime

Question

I am running a service on Docker Swarm. This is what I did to deploy the service:

docker swarm init
docker stack deploy -c docker-compose.yml MyApplication

Content of docker-compose.yml:

version: "3"
services:
    web:
        image: myimage:1.0
        ports:
            - "9000:80"
            - "9001:443"
        deploy:
            replicas: 3
            resources:
                limits:
                    cpus: "0.5"
                    memory: 256M
                restart_policy:
                    condition: on-failure

Let't say that I update the application and build a new image myimage:2.0. What is a proper way to deploy the new version of image to the service without the downtime?

Answer 1

A way to achieve this is:

• provide a healthcheck. That way docker will know if your new deployment has succeeded.
  • https://docs.docker.com/engine/reference/builder/#healthcheck
  • https://docs.docker.com/compose/compose-file/#healthcheck]
• control how docker will update your service with update_config
  • https://docs.docker.com/compose/compose-file/#update_config
  • pay attention to order and parallelism, for example if you choose order: stop-first + parallelism: 2 and your replicas are the same amount as parallelism, your app will stop completely when updating
• if your update doesn't succeed you probably want to rollback
  • https://docs.docker.com/compose/compose-file/#rollback_config
• don't forget the restart_policy too

I have some examples on that subject:

• Docker Swarm Mode Replicated Example with Flask and Caddy
• https://github.com/douglasmiranda/lab/tree/master/caddy-healthcheck-of-caddy-itself

---

With this you can simply run docker stack deploy... again. If there was changes in the service, it will be updated.

Answer 2

you can use the command docker service update --image but it will start a new container with a implicit scale 0/1.

The downtime depends of your application.