Docker Swarm — the lightweight K8s alternative

When 3-5 hosts need to coordinate but you don't want K8s, Swarm is the sweet spot. Init, join, rolling upgrades, monitoring.

Docker Swarm (3-10 nodes) 90 min

Why Swarm is still useful#

Docker downplays Swarm but still maintains it. Sweet spot:

3-10 hosts; you want load balancing + rolling upgrades + service discovery
Team isn’t fluent in K8s
Run docker-compose.yml directly (with a few deploy fields)

Init the cluster#

# First host (manager)
docker swarm init --advertise-addr <PUBLIC_IP>

# On other hosts:
docker swarm join --token <TOKEN> <MANAGER_IP>:2377

Deploy a stack#

Add deploy fields to your compose:

version: '3.8'
services:
  chatwoot-rails:
    image: chatwoot/chatwoot:v4.x
    deploy:
      replicas: 3
      update_config:
        parallelism: 1
        delay: 10s
        order: start-first
      restart_policy:
        condition: on-failure
    networks: [ai-support]
    secrets: [chatwoot_env]

  postgres:
    image: postgres:16
    deploy:
      placement:
        constraints: [node.labels.role == db]
    volumes:
      - pg-data:/var/lib/postgresql/data

networks:
  ai-support:
    driver: overlay

secrets:
  chatwoot_env:
    external: true

Deploy:

docker stack deploy -c stack.yml ai-support

Concepts#

Concept	Meaning
Service	A long-running container group
Replica	How many copies per service
Overlay network	Cluster-wide internal network
Secret	Encrypted sensitive value
Placement constraint	Pin a service to certain nodes

Rolling upgrade#

docker service update --image chatwoot/chatwoot:v4.5.1 ai-support_chatwoot-rails

update_config.order: start-first brings up new versions before tearing down old — zero downtime.

Observability#

docker service ls
docker service ps ai-support_chatwoot-rails
docker service logs -f ai-support_chatwoot-rails

Layer Prometheus + cAdvisor + Grafana on top.

Swarm vs K8s#

Situation	Pick
3-10 hosts	Swarm
> 20 nodes / multi-team	K8s
Need Helm / Operator ecosystem	K8s
Docker-fluent, K8s-curious	Swarm
Heavy GitOps CI/CD	K8s (ArgoCD etc. are mature)