zero downtime for upgrades

[pcrespov]

• New version is deployed.
• The deployer updates the stack
• The user gets interrupted because the service is down and sometimes gets a gateway error
  • Instead, the old service should not be shut-down until the new stack is in place

Possible cause:

The swarm is already configured to have zero downtime per service (i.e. a given service gets turned off ONLY when the new one is started). The problem might be that even if services are ready, the state between services is not ready. For example, the new webserver is updated correctly but traffik proxy has still not detected it. That would cause a wrong gateway failure on a front-end request

Ideas to solve this problem

• Incorporate more conditions on the "healtcheck" validation function (e.g. traffik has discovered all backend services)
• Might be to deploy the entire stack separately first, have a set of rules to validate (e.g. all services healthy, all services connected, traffik routings ready) and then switch.
• Notify the frontend that a new version was deployed Front-end notifies of a new version available #2128

[pcrespov]

duplicated from #2212