One of the most important aspects of a mission-critical application that needs frequent updates is the ability to do updates in a fashion that requires no outage at all. We call this a zero downtime deployment. At all times, the application which is updated is fully operational.
There are various ways how this can be achieved. Some of them are as follows:
- Rolling updates
- Blue-green deployments
- Canary releases
Docker Swarm supports rolling updates out of the box. The other two types of deployments can be achieved with some extra effort from our side.
In a mission-critical application, each application service has to run in multiple replicas. Depending on the load, that can be as few as two to three instances and as many as dozens, hundreds, or thousands of instances. At any given time, we want to have a clear majority of all service instances running. So, if we have three replicas, we want to have at least two of them up and running...