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Docker : Use Docker Swarm
2017/12/21
 
Configure Docker Swarm to create Docker Cluster with multiple Docker nodes.
On this example, Configure Swarm Cluster with 3 Docker nodes like follows.
There are 2 roles on Swarm Cluster, those are [Manager nodes] and [Worker nodes].
This example shows to set those roles like follows. 
 -----------+---------------------------+--------------------------+------------
            |                           |                          |
        eth0|10.0.0.51              eth0|10.0.0.52             eth0|10.0.0.53
 +----------+-----------+   +-----------+----------+   +-----------+----------+
 | [ node01.srv.world ] |   | [ node02.srv.world ] |   | [ node03.srv.world ] |
 |       Manager        |   |        Worker        |   |        Worker        |
 +----------------------+   +----------------------+   +----------------------+
[1]
This example is based on the environment that SELnux is Permissive or Disabled and also Firewalld is disabled.
[2]
Install and run Docker service on all nodes, refer to here.
[3] Configure Swarm Cluster on Manager Node.
[root@node01 ~]#
docker swarm init 

Swarm initialized: current node (no8wxh1rbetehdt7xslsxn3ik) is now a manager.

To add a worker to this swarm, run the following command:

    docker swarm join \
    --token SWMTKN-1-3oo361b6e9nzooxnm59r5o39res06mdy1iaaldytgo2ka366c4-crl0p2toqjhorkpykou5dzcx5 \
    10.0.0.51:2377

To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.
[4] Join in Swarm Cluster on all Worker Nodes.
It's OK to run the command which was shown when running swarm init on Manager Node.
[root@node02 ~]#
docker swarm join \
--token SWMTKN-1-3oo361b6e9nzooxnm59r5o39res06mdy1iaaldytgo2ka366c4-crl0p2toqjhorkpykou5dzcx5 \
10.0.0.51:2377

This node joined a swarm as a worker.
[5] Verify with a command [node ls] that worker nodes could join in Cluster normally.
[root@node01 ~]#
docker node ls 

ID                           HOSTNAME          STATUS  AVAILABILITY  MANAGER STATUS
5o8chkmix2l536vjje7x320vt    node03.srv.world  Ready   Active
no8wxh1rbetehdt7xslsxn3ik *  node01.srv.world  Ready   Active        Leader
pa1w0zbch7j3q781155xkpvyq    node02.srv.world  Ready   Active
[6]
After creating Swarm Cluster, next, configure services that the Swarm Cluster provides.
Create the same container image on all Nodes for the service first.
On this exmaple, use a Container image which provides http service like an example of the link.
[7] Configure service on Manager Node.
After successing to configure service, access to the Manager node's Hostname or IP address to verify it works normally. By the way, requests to worker nodes are load-balanced with round-robin like follows.
[root@node01 ~]#
docker images 

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
web_server          latest              4e9cb1f9e9d9        10 minutes ago      473 MB
docker.io/fedora    latest              422dc563ca32        5 weeks ago         252 MB
# create a service with 2 repricas

[root@node01 ~]#
docker service create --name swarm_cluster --replicas=2 -p 80:80 web_server:latest

x7zl0nmojs73syt21gzrije8j
# show service list

[root@node01 ~]#
docker service ls 

ID            NAME           MODE        REPLICAS  IMAGE
x7zl0nmojs73  swarm_cluster  replicated  2/2       web_server:latest
# inspect the service

[root@node01 ~]#
docker service inspect swarm_cluster --pretty 

ID:             x7zl0nmojs73syt21gzrije8j
Name:           swarm_cluster
Service Mode:   Replicated
 Replicas:      2
Placement:
UpdateConfig:
 Parallelism:   1
 On failure:    pause
 Max failure ratio: 0
ContainerSpec:
 Image:         web_server:latest
Resources:
Endpoint Mode:  vip
Ports:
 PublishedPort 80
  Protocol = tcp
  TargetPort = 80
# show service state

[root@node01 ~]#
docker service ps swarm_cluster 

ID            NAME             IMAGE              NODE              DESIRED  CURRENT STATE         ...
h8ovakrnrw6y  swarm_cluster.1  web_server:latest  node01.srv.world  Running  Running 49 seconds ago
ncjffinn2hr5  swarm_cluster.2  web_server:latest  node03.srv.world  Running  Running 49 seconds ago
# verify it works normally

[root@node01 ~]#
curl http://node01.srv.world/

node03
[root@node01 ~]#
curl http://node01.srv.world/

node01
[root@node01 ~]#
curl http://node01.srv.world/

node03
[root@node01 ~]#
curl http://node01.srv.world/

node01
[8] If you'd like to change the number of repricas, configure like follows.
# change repricas to 3

[root@node01 ~]#
docker service scale swarm_cluster=3

swarm_cluster scaled to 3
[root@node01 ~]#
docker service ps swarm_cluster 

ID     NAME             IMAGE              NODE              DESIRED STATE  CURRENT STATE          ...
yhm... swarm_cluster.1  web_server:latest  node01.srv.world  Running        Running 29 seconds ago
icc... swarm_cluster.2  web_server:latest  node03.srv.world  Running        Running 28 seconds ago
e4n... swarm_cluster.3  web_server:latest  node02.srv.world  Running        Preparing 2 seconds ago
# verify working

[root@node01 ~]#
curl http://node01.srv.world/

node02
[root@node01 ~]#
curl http://node01.srv.world/

node03
[root@node01 ~]#
curl http://node01.srv.world/

node01
 
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