In other technologies we have clusters, in docker, we have Swarm. Same concept, different name :)

A swarm in docker is set of servers who act as one and each server can be either: Manager or Worked. It is important to note, that manager is also a worked, but the reverse isn't so. Managers manage workers and workers obey, nothing different than clusters.

Below, you can see basic overview of the swarm:

Let's set it up. We will have 3 servers:

• 1 Manager
• 2 Workers

You can see the automatic creation in the Vagrant section: Multiple Servers Configuration with Vagrant

After the setup is done, please be sure that workers, have joined, we can create the service as follows:

The creation of the VMs and installation of docker-ce will not be discussed as it has been done in at least 10 other sections. So let's check how to setup manager and 2 workers:

The initiation of the Swarm is done from one of the managers as follow:

docker swarm init --listen-addr 10.100.199.200:2377 --advertise-addr 10.100.199.200:2377

That command will initialize a swarm with 1 manager who will listen on IP: 10.100.199.200:2377 for docker swarm related activities (advertise-addr) and income traffic (listen-addr)

To join a swarm a server must have the swarm's token. The token can be taken as follows:

[root@mgr1 ~]# docker swarm join-token manager
To add a manager to this swarm, run the following command:

    docker swarm join --token SWMTKN-1-4mqe4htimsl9wrlzstzx6c18qmibjfjuuihdp3o91pnmssbvml-69b13glzhdgfzh58hbg7jwn69 10.100.199.200:2377

[root@mgr1 ~]# docker swarm join-token worker
To add a worker to this swarm, run the following command:

    docker swarm join --token SWMTKN-1-4mqe4htimsl9wrlzstzx6c18qmibjfjuuihdp3o91pnmssbvml-1h1yrqixgrme7aypq6c4m0npj 10.100.199.200:2377

[root@mgr1 ~]#

Now, it is good idea to ALWAYS add the advertise and listener address to the server:

Manually, workers and manager can be joined to the swarm as follows:

docker swarm join --token SWMTKN-1-4mqe4htimsl9wrlzstzx6c18qmibjfjuuihdp3o91pnmssbvml-1h1yrqixgrme7aypq6c4m0npj \
manager_advertise_ip:2377 \
--advertise-addr worker_advert_ip_addr:2377 \ 
--listener-addr worker_listen_ip_addr:2377

Insert the correct token depending on, if you are adding worker or a manager. The above is for worker.

The creation of the service is rather easy:

[root@mgr1 ~]# docker service create --name httpDemoService --replicas 3 -p 80:1234 andonovj/httpserverdemo
nz82mserbonjcoccp7iir7hfn
overall progress: 3 out of 3 tasks
1/3: running   [==================================================>]
2/3: running   [==================================================>]
3/3: running   [==================================================>]
verify: Service converged
[root@mgr1 ~]# docker service ls
ID                  NAME                MODE                REPLICAS            IMAGE                            PORTS
q8s7iiv4rwu7        httpDemoService     replicated          3/3                 andonovj/httpserverdemo:latest
[root@mgr1 ~]#
[root@mgr1 ~]#
[root@mgr1 ~]#
[root@mgr1 ~]#
[root@mgr1 ~]# docker node ls
ID                            HOSTNAME            STATUS              AVAILABILITY        MANAGER STATUS      ENGINE VERSION
occzvuwmny5rm8x8an4bctdc1 *   mgr1                Ready               Active              Leader              19.03.7
7oubfm1qdun533xw9s14lt3pu     worker01            Ready               Active                                  19.03.7
sdcosdbt1f94rrvbr8m9ng30s     worker02            Ready               Active                                  19.03.7
[root@mgr1 ~]#
[root@mgr1 ~]# docker service ps httpDemoService
ID                  NAME                IMAGE                            NODE                DESIRED STATE       CURRENT STATE            ERROR               PORTS
avcanadogs3e        httpDemoService.1   andonovj/httpserverdemo:latest   mgr1                Running             Running 27 seconds ago
6pgfba2xdg0q        httpDemoService.2   andonovj/httpserverdemo:latest   worker01            Running             Running 9 seconds ago
me2174rye7nu        httpDemoService.3   andonovj/httpserverdemo:latest   worker02            Running             Running 8 seconds ago
[root@mgr1 ~]#