What's the main advantage of using replicas in Docker Swarm Mode?

Question

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?

Answer 1

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.

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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.