What is Docker Swarm? | DevOps Dictionary

Docker Swarm — Built-In Multi-Host Container Orchestration

What Is Docker Swarm in Simple Terms?

Docker Swarm is a clustering mode built into Docker that lets you treat multiple Docker hosts as a single entity. Where Compose runs services on one machine, Swarm distributes services across many machines — with built-in load balancing, rolling updates, and secrets management.

Swarm sits between Compose (single host, simple) and Kubernetes (multi-host, complex). For teams that need multi-host deployment but are not ready for Kubernetes's operational complexity, Swarm is a practical middle ground.

◈ DIAGRAM

+------------------------------------------+
| Docker Swarm Cluster                     |
|                                          |
| Manager Node 1 (leader)                  |
| Manager Node 2 (backup)                  |
| Manager Node 3 (backup)                  |
|                                          |
| Worker Node 1   Worker Node 2            |
| Worker Node 3   Worker Node 4            |
|                                          |
| payment-api service: 6 replicas          |
| Swarm distributes across worker nodes    |
| Built-in load balancing across replicas  |
+------------------------------------------+

Setting Up Docker Swarm

Bash

# Initialize Swarm on the first manager node
docker swarm init --advertise-addr 10.0.1.50
# Output: docker swarm join --token SWMTKN-... 10.0.1.50:2377
 
# Join worker nodes (run on each worker)
docker swarm join \
  --token SWMTKN-1-abc123... \
  10.0.1.50:2377
 
# Verify cluster
docker node ls
# ID        HOSTNAME    STATUS  AVAILABILITY  MANAGER STATUS
# abc123 *  manager-1   Ready   Active        Leader
# def456    worker-1    Ready   Active
# ghi789    worker-2    Ready   Active

Deploying Services in Swarm

Bash

# Create a service (like a Deployment in Kubernetes)
docker service create \
  --name payment-api \
  --replicas 6 \
  --publish 8080:8080 \
  --network payment-overlay \
  registry.razorpay.in/payment-api:v3.1.0
 
# Scale a service
docker service scale payment-api=10
 
# Rolling update (zero downtime)
docker service update \
  --image registry.razorpay.in/payment-api:v3.2.0 \
  --update-parallelism 2 \
  --update-delay 10s \
  payment-api
 
# List services
docker service ls
docker service ps payment-api  # see which nodes each replica is on

Deploy with Stack (docker-compose.yml for Swarm)

YAML

# docker-compose.yml with Swarm deploy config
version: "3.8"
services:
  api:
    image: payment-api:v3.1.0
    deploy:
      replicas: 6
      update_config:
        parallelism: 2
        delay: 10s
        failure_action: rollback
      restart_policy:
        condition: on-failure
    networks:
      * payment-overlay
 
networks:
  payment-overlay:
    driver: overlay

Bash

# Deploy as a stack
docker stack deploy -c docker-compose.yml payment-stack

Swarm vs Kubernetes

Feature	Swarm	Kubernetes
Setup complexity	Low (5 minutes)	High (hours/days)
Learning curve	Low	Steep
Multi-host	Yes	Yes
Auto-scaling	No built-in	Yes (HPA)
Ecosystem	Small	Enormous
Production adoption	Declining	Dominant

REMEMBER THIS
**Remember:** Docker Swarm adoption has been declining since Kubernetes became the industry standard. For new projects, Kubernetes is the better long-term investment even though Swarm is simpler to start. Swarm is still a good choice for teams with existing Swarm deployments or very simple multi-host needs.