Swarm in HA mode

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:

[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:

[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:

[root@managerone ~]# docker swarm init --advertise-addr 172.31.42.232
Swarm initialized: current node (kvbht...) is now a manager.

Confirm Swarm status:

[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:

[root@managerone ~]# docker swarm join-token manager --quiet
SWMTKN-1-xxxxx-xxxx

2.2 Join `managertwo` and `managerthree`

On each additional manager node:

[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:

[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:

[root@managerone ~]# docker swarm join-token worker --quiet
SWMTKN-1-yyyyy-yyyy

3.2 Join `workerone`, `workertwo`, `workerthree`

On each worker:

[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:

[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:

[root@managertwo ~]# systemctl stop docker

Check from managerone:

[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:

[root@managerthree ~]# systemctl stop docker

Then on managerone:

[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

Start Docker on managertwo:

[root@managertwo ~]# systemctl start docker

Wait for the node to rejoin (quorum 2/3 restored).

Start Docker on managerthree:

[root@managerthree ~]# systemctl start docker

Verify:
[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.

Introduction

Docker Engine

Docker Image Management

Docker Engine - Security

Docker Engine - Networking

Docker Engine - Storage

Docker Compose

Docker Swarm

Kubernetes

Docker Engine Enterprise

Docker Trusted Registry

Disaster Recovery

Prerequisites

1. Initialize the Swarm

2. Add Additional Managers

2.1 Retrieve Manager Join Token

2.2 Join `managertwo` and `managerthree`

2.3 Verify Managers

3. Add Worker Nodes

3.1 Retrieve Worker Join Token

3.2 Join `workerone`, `workertwo`, `workerthree`

4. Testing Manager Quorum

4.1 Simulate One Manager Failure

4.2 Simulate Two Manager Failures

5. Restoring Quorum

Conclusion

Links and References

Watch Video

​Prerequisites

​1. Initialize the Swarm

​2. Add Additional Managers

​2.1 Retrieve Manager Join Token

​2.2 Join managertwo and managerthree

​2.3 Verify Managers

​3. Add Worker Nodes

​3.1 Retrieve Worker Join Token

​3.2 Join workerone, workertwo, workerthree

​4. Testing Manager Quorum

​4.1 Simulate One Manager Failure

​4.2 Simulate Two Manager Failures

​5. Restoring Quorum

​Conclusion

​Links and References

Watch Video

Prerequisites

1. Initialize the Swarm

2. Add Additional Managers

2.1 Retrieve Manager Join Token

2.2 Join `managertwo` and `managerthree`

2.3 Verify Managers

3. Add Worker Nodes

3.1 Retrieve Worker Join Token

3.2 Join `workerone`, `workertwo`, `workerthree`

4. Testing Manager Quorum

4.1 Simulate One Manager Failure

4.2 Simulate Two Manager Failures

5. Restoring Quorum

Conclusion

Links and References