In Docker Swarm mode is there any point in replicating a service more than the number of hosts available?

Question

I have been looking into the new Docker Swarm mode that will be available in Docker 1.12. In this Docker Swarm Mode Walkthrough video, they create a simple Nginx service that is composed of a single Nginx container. In the video, they have 4 nodes in the Swarm cluster. During the scaling demonstration, they increase the replication factor to 10, thus creating 10 copies of the Nginx container across all 4 machines in the cluster.

I get that the video is just a demonstration, but in the real world, what is the point of creating more replicas of a container (or service) than there are nodes in the Swarm cluster? It seems to be pointless since two containers on the same machine would be sharing that machines finite computing resources anyway. I don't get what the benefit is.

So my question is, is there any real world benefit to replicating a Docker service or container beyond the number of nodes in the Swarm cluster?

Thanks

Answer 1

It depends on how the application handles threading and multiple requests. A single threaded application, or job that only handles one request at a time, may use a fraction of the OS resources and benefit from running multiple instances on a single host. An application that's been tuned to process requests concurrently and which fully utilizes the OS will see no benefit and will in fact incur a penalty of taking away resources to run multiple instances of the application.

Answer 2

One advantage can be performing live zero-downtime software updates. See the Docker 0.12rc2 Swarm tutorial on rolling updates