> ## 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.
# Swarm in HA mode
> This guide explains how to configure a Docker Swarm cluster in high-availability mode with multiple manager and worker nodes.
In this guide, you’ll learn how to configure a Docker Swarm cluster in HA mode with three manager nodes and three worker nodes. We’ll cover initialization, adding managers and workers, testing quorum, and restoring cluster health.
## Prerequisites
* Six CentOS 7.6 VMs (2 vCPU, 4 GB RAM) on AWS
* Docker 19.03 installed on all nodes
* Swarm ports open (2377, 7946 TCP/UDP, 4789 UDP)
* Fully qualified hostnames and name resolution for all nodes
| Hostname | IP Address | Role (after setup) |
| ------------ | ------------- | ------------------ |
| managerone | 172.31.42.232 | Manager (Leader) |
| managertwo | 172.31.42.xxx | Manager (Replica) |
| managerthree | 172.31.42.xxx | Manager (Replica) |
| workerone | 172.31.39.115 | Worker |
| workertwo | 172.31.39.xxx | Worker |
| workerthree | 172.31.39.xxx | Worker |
Verify OS and Docker:
```bash theme={null}
[root@managerone ~]# cat /etc/centos-release
CentOS Linux release 7.6.1810 (Core)
[root@managerone ~]# nproc
2
[root@managerone ~]# docker version --format '{{.Server.Version}}'
19.03.8
```
Verify network connectivity:
```bash theme={null}
[root@managerone ~]# ping -c2 workerone
PING workerone (172.31.39.115): 56 data bytes
64 bytes from workerone: icmp_seq=1 ttl=64 time=0.45 ms
```
## 1. Initialize the Swarm
On **managerone**, initialize the Swarm and advertise its IP:
```bash theme={null}
[root@managerone ~]# docker swarm init --advertise-addr 172.31.42.232
Swarm initialized: current node (kvbht...) is now a manager.
```
Confirm Swarm status:
```bash theme={null}
[root@managerone ~]# docker info --format '{{.Swarm.LocalNodeState}}'
active
```
The `--advertise-addr` flag sets the manager’s reachable IP for new nodes.
## 2. Add Additional Managers
### 2.1 Retrieve Manager Join Token
On **managerone**:
```bash theme={null}
[root@managerone ~]# docker swarm join-token manager --quiet
SWMTKN-1-xxxxx-xxxx
```
### 2.2 Join `managertwo` and `managerthree`
On each additional manager node:
```bash theme={null}
[root@managertwo ~]# docker swarm join --token SWMTKN-1-xxxxx-xxxx 172.31.42.232:2377
This node joined a swarm as a manager.
```
Repeat on **managerthree**.
### 2.3 Verify Managers
From **managerone**:
```bash theme={null}
[root@managerone ~]# docker node ls
ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS ENGINE VERSION
* kvbht... managerone Ready Active Leader 19.03.8
s2zym... managertwo Ready Active Reachable 19.03.8
u81im... managerthree Ready Active Reachable 19.03.8
```
## 3. Add Worker Nodes
### 3.1 Retrieve Worker Join Token
On **managerone**:
```bash theme={null}
[root@managerone ~]# docker swarm join-token worker --quiet
SWMTKN-1-yyyyy-yyyy
```
### 3.2 Join `workerone`, `workertwo`, `workerthree`
On each worker:
```bash theme={null}
[root@workerone ~]# docker swarm join --token SWMTKN-1-yyyyy-yyyy 172.31.42.232:2377
This node joined a swarm as a worker.
```
Verify all nodes:
```bash theme={null}
[root@managerone ~]# docker node ls
ID HOSTNAME ROLE MANAGER STATUS AVAILABILITY ENGINE VERSION
* kvbht... managerone Manager Leader Active 19.03.8
s2zym... managertwo Manager Reachable Active 19.03.8
u81im... managerthree Manager Reachable Active 19.03.8
38oeh... workerone Worker — Active 19.03.8
1pqdd... workertwo Worker — Active 19.03.8
k4gc5... workerthree Worker — Active 19.03.8
```
## 4. Testing Manager Quorum
Docker Swarm requires a majority of manager nodes to maintain a leader.
### 4.1 Simulate One Manager Failure
On **managertwo**:
```bash theme={null}
[root@managertwo ~]# systemctl stop docker
```
Check from **managerone**:
```bash theme={null}
[root@managerone ~]# docker node ls
… managertwo Down Active Unreachable 19.03.8
```
The cluster remains healthy (2 of 3 managers online).
### 4.2 Simulate Two Manager Failures
On **managerthree**:
```bash theme={null}
[root@managerthree ~]# systemctl stop docker
```
Then on **managerone**:
```bash theme={null}
[root@managerone ~]# docker node ls
Error response from daemon: … The swarm does not have a leader …
```
With only one manager online, the cluster loses quorum and cannot elect a leader.
Never simulate quorum loss in production. You will lose control over scheduling and service updates.
## 5. Restoring Quorum
1. Start Docker on **managertwo**:
```bash theme={null}
[root@managertwo ~]# systemctl start docker
```
2. Wait for the node to rejoin (quorum 2/3 restored).
3. Start Docker on **managerthree**:
```bash theme={null}
[root@managerthree ~]# systemctl start docker
```
4. Verify:
```bash theme={null}
[root@managerone ~]# docker node ls
```
All managers should show `Leader` or `Reachable`.
## Conclusion
You’ve now deployed a Docker Swarm in HA mode, added managers and workers, tested quorum behavior, and restored cluster health. This setup ensures fault tolerance and continuous service availability.
## Links and References
* [Docker Swarm Mode Overview](https://docs.docker.com/engine/swarm/)
* [Swarm Init](https://docs.docker.com/engine/reference/commandline/swarm_init/)
* [Swarm Join](https://docs.docker.com/engine/reference/commandline/swarm_join/)