What is Docker Swarm?

#What is Docker Swarm?

After this lesson you'll know:

• what Swarm Mode is
• the difference between Manager and Worker nodes
• how Swarm provides high-availability services

#A Cluster from your machine

So far you've run Docker on a single host. Swarm turns it into a cluster — multiple machines acting as one Docker host.

Imagine: Your app runs on three servers. One fails. Swarm restarts the containers on the other two. No downtime. No manual intervention.

#Manager vs Worker

Manager = Boss         (manages the cluster, assigns tasks)
Worker  = Worker       (runs containers)

• Manager nodes maintain cluster state (in a Raft database) and assign tasks
• Worker nodes run the containers — they're interchangeable
• A cluster should have 3 or 5 managers (odd number for Raft quorum)

#Activating Swarm

# On the first server (becomes manager)
docker swarm init --advertise-addr 192.168.1.10

# Output shows the join token for workers:
docker swarm join --token SWMTKN-... 192.168.1.10:2377

# On additional servers:
docker swarm join --token SWMTKN-xyz 192.168.1.10:2377

docker node ls  # See all nodes in the cluster

#Why Swarm and not Kubernetes?

• Simplicity — Swarm is built into Docker, no extra tools needed
• Compatible — docker-compose.yml works almost 1:1 with docker stack deploy
• Lightweight — runs on 3 small VMs, no resource hog
• Perfect for teams that know Docker but don't need K8s

Kubernetes is more powerful. Swarm is simpler. For 80% of use cases, Swarm is more than enough.

#✋ Try it out

• docker swarm init — make your machine a Swarm Manager. What does docker node ls show?
• Add a second node (simulate on the same machine: docker swarm join-token worker shows the command)
• docker swarm leave on the worker, then docker swarm leave --force on the manager — destroy the cluster

#📌 Summary

• Swarm turns multiple Docker hosts into one cluster
• Managers manage, Workers execute — 3 or 5 managers for high availability
• Swarm is the simple alternative to Kubernetes — built-in and Compose compatible