> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Demo Swarm Cluster Setup
> Learn to bootstrap a resilient Docker Swarm cluster on CentOS 7.6 and manage node roles effectively.
In this step-by-step guide, you will learn how to bootstrap a resilient Docker Swarm cluster on CentOS 7.6, manage node roles, control availability, and gracefully remove nodes. By the end of this tutorial, you’ll have a three-node swarm (one manager, two workers) and know how to:
1. Initialize the swarm
2. Join worker nodes
3. Promote and demote managers
4. Drain and reactivate nodes
5. Remove nodes from the cluster
This tutorial assumes all nodes can resolve each other by hostname, have open swarm ports, and run Docker Engine v19.03.8 or later.
* Three CentOS 7.6 servers (`managerone`, `workerone`, `workertwo`)
* 2 CPU cores and 4 GB RAM each
* Hostname resolution (e.g., via `/etc/hosts` or DNS)
* Open ports:
* TCP 2377 for cluster management
* TCP/UDP 7946 for node discovery
* UDP 4789 for overlay networking
* Docker Engine installed and running
## System Overview
| Node | OS Release | CPUs | Memory (MiB) | Docker Version |
| ---------- | ------------------------------------ | ---- | ------------ | -------------- |
| managerone | CentOS Linux release 7.6.1810 (Core) | 2 | 3787 | 19.03.8 |
| workerone | CentOS Linux release 7.6.1810 (Core) | 2 | 3787 | 19.03.8 |
| workertwo | CentOS Linux release 7.6.1810 (Core) | 2 | 3787 | 19.03.8 |
## 1. Initialize the Swarm on `managerone`
1. Confirm Swarm is inactive:
```bash theme={null}
docker system info | grep -i swarm
# Swarm: inactive
```
2. Initialize with the manager’s advertise address:
```bash theme={null}
docker swarm init --advertise-addr 172.31.42.232
```
After initialization, note the worker join command output:
```bash theme={null}
docker swarm join --token 172.31.42.232:2377
```
## 2. Add the First Worker (`workerone`)
On **workerone**, run the join command displayed by the manager:
```bash theme={null}
docker swarm join \
--token SWMTKN-1-3fdj9fgrjcrrj5t0pekb42n45tj96zgwxodtwd4ujv4qnhl-cop40spyhgc1tmzythfss49xn \
172.31.42.232:2377
# This node joined a swarm as a worker.
```
Back on **managerone**, verify:
```bash theme={null}
docker node ls
# ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS ENGINE VERSION
# ... managerone Ready Active Leader 19.03.8
# ... workerone Ready Active 19.03.8
```
## 3. Add the Second Worker (`workertwo`)
### 3.1. Prepare `workertwo`
Ensure OS, CPU, memory, Docker version, and connectivity:
```bash theme={null}
cat /etc/centos-release
nproc
free -m
docker --version
systemctl status docker
ping -c3 managerone
ping -c3 workerone
```
### 3.2. Retrieve and Run the Join Token
On **managerone**, print the worker join command:
```bash theme={null}
docker swarm join-token worker
```
Copy the `docker swarm join --token …` line and execute it on **workertwo**. Then confirm on **managerone**:
```bash theme={null}
docker node ls
# ... workertwo Ready Active 19.03.8
```
## 4. Promote a Worker to Manager
1. List current nodes:
```bash theme={null}
docker node ls
```
2. Promote **workerone**:
```bash theme={null}
docker node promote workerone
# Node workerone promoted to a manager in the swarm.
```
3. Verify the new manager status:
```bash theme={null}
docker node ls
# workerone now shows MANAGER STATUS: Reachable
```
4. (Optional) Inspect **workerone** in detail:
```bash theme={null}
docker node inspect workerone --pretty
```
## 5. Demote a Manager back to Worker
If you need to revert **workerone** to a purely worker role:
```bash theme={null}
docker node demote workerone
docker node ls
# workerone returns to no MANAGER STATUS
```
## 6. Drain and Reactivate a Node
To prevent new tasks from scheduling on **workerone**:
```bash theme={null}
docker node update --availability drain workerone
docker node ls
# AVAILABILITY: Drain
```
When you’re ready to allow tasks again:
```bash theme={null}
docker node update --availability active workerone
docker node ls
# AVAILABILITY: Active
```
## 7. Remove a Node from the Swarm
1. Drain **workerone**:
```bash theme={null}
docker node update --availability drain workerone
```
2. Attempt removal (will fail if the node is still up):
```bash theme={null}
docker node rm workerone
# Error: node is not down and can't be removed
```
3. On **workerone**, leave the swarm:
```bash theme={null}
docker swarm leave
# Node left the swarm.
```
4. Back on **managerone**, remove the node:
```bash theme={null}
docker node rm workerone
docker node ls
# Only managerone and workertwo remain.
```
Congratulations! You’ve successfully created, scaled, and managed a Docker Swarm cluster on CentOS 7.6.
***
## References and Further Reading
* [Docker Swarm Overview](https://docs.docker.com/engine/swarm/)
* [Get Started with Swarm Mode](https://docs.docker.com/get-started/swarm/)
* [Docker Engine Installation Guide](https://docs.docker.com/engine/install/centos/)