What is a cluster and a node oriented to containers?

Question

Sorry for this question, but I just started with Docker and Docker Compose and I really didn't need any of this until I read that I need to use Docker Swarn or Kuebernetes to have more stability in production. I started reading about Docker Swarn and they mentioned nodes and clusters.

I was really happy not knowing about this as I understood docker-compose:

Is that I could manage my services/containers from a single file and only have to run several commands to launch, build, delete, etc. all my services based on the docker-compose configuration.

But now the nodes and cluster have come out and I've really gone a bit crazy, and that's why if you can help me understand this next step in the life of containers. I've been googling and it's not very clear to me.

I hope you can help me and explain it to me in a way that I can understand.

Thank you!

Answer 1

By default Docker Compose runs a set of containers on a single system. If you need to run more containers than fit on one system, or you're just afraid of that system crashing, you need more than one system to do it. The cluster is the group of all of the systems (physical computers, virtual machines, cloud instances) that are working together to run the containers. Each of those individual systems is a node.

The other important part of the cluster container setups is that you can generally run multiple replicas of a give container, and you don't care where in the cluster they run. Say you have five nodes, and a Web server container, and you'd like to run three copies of it for redundancy. Instead of having to pick a node, ssh to it, and manually docker run there, you just tell the cluster manager "run me three of these", and it chooses a node and launches the container for you. You can also scale the containers up and down at runtime, or potentially set the cluster to do the scaling on its own based on load.

If your workload is okay running a single copy of containers on a single server, you don't need a cluster setup. (You might have some downtime during updates or if the single server dies.) Swarm has the advantages of being bundled with Docker and being able to use Docker-native tools (docker-compose can deploy to a Swarm cluster). Kubernetes is much more complex, but at this point most public cloud providers will sell you a preconfigured Kubernetes cluster, and it has better stories around security, storage management, and autoscaling. There are also a couple other less-prominent alternatives like Nomad and Mesos out there.

Answer 2

A node is just a physical or virtual machine.
In Kubernetes/Docker Swarm context each node must have the relevant binaries installed (Docker Engine, kubelet etc..)

A cluster is a grouping of one or more nodes.

If you have just been testing on your local machine you have a single node.

If you were to add a second machine and link both machines together using docker swarm/kubernetes then you would have created a 2 node cluster

You can then use docker swarm/kubernetes to run your services/containers on any or all nodes in your cluster. This allows your services to be more resilient and fault tolerant.