I'm struggling to understand the idea of replica instances in Docker Swarm Mode. I've read that it's a feature that helps with high availability.
However, Docker automatically starts up a new task on a different node if one node goes down even with 1 replica defined for the service, which also provides high availability.
So what's the advantage of having 3 replica instances rather than 1 for an arbitrary service? My assumption was that with more replicas, Docker spends less time creating a new instance on another node in the event of failure, which aids performance. Is this correct?
What Makes a System Highly Available?
One of the goals of high availability is to eliminate single points of failure in your infrastructure. A single point of failure is a component of your technology stack that would cause a service interruption if it became unavailable.
Let's take your example of a replica that consists of a single instance. Now let's suppose there is a failure. Docker Swarm will notice that the service failed and restart it. The service restarts, but a restart isn't instant. Let's say the restart takes 5 seconds. For those 5 seconds your service is unavailable. Single point of failure.
What if you had a replica that consists of 3 instances. Now when one of them fails (no service is perfect), Docker Swarm will notice that one of the instances is unavailable and create a new one. During that time you still have 2 healthy instances serving requests. To a user of your service, it appears as if there was no down time. This component is no longer a single point of failure.
ROMANARMY answer is very good and i just wanted to mention that the replicas can be on different nodes, so if one of your servers goes down(become unavailable) the container(replica) on the other server can be run without problem.