Tag: RedHat

Sometimes, you just want to learn the internal mechanics of Docker Swarm, but unfortunately there is only one Linux box at hand, and you don’t want to bother to install Virtual Machines on it. In this scenario, you certainly can build a Docker Swarm cluster on one host, and this tutorial will provide a detailed guide:

(1) Make sure the Go environment has been ready on your system, if not, please follow this document to setup it. Also remember add$GOPATH/bin into $PATH environment variable.

(2) Install Docker Swarm:

# go get -u github.com/docker/swarm

Execute swarm command to check whether Docker Swarm is well equipped:

# swarm
Usage: swarm [OPTIONS] COMMAND [arg...]

A Docker-native clustering system

Version: 1.2.3 (HEAD)

Options:
  --debug                       debug mode [$DEBUG]
  --log-level, -l "info"        Log level (options: debug, info, warn, error, fatal, panic)
  --experimental                enable experimental features
  --help, -h                    show help
  --version, -v                 print the version
......

(3) Modify the Docker configuration file. E.g., on my RHEL 7, the file is /etc/sysconfig/docker:

# systemctl show docker
......
EnvironmentFile=/etc/sysconfig/docker (ignore_errors=yes)
......

Add “-H tcp://127.0.0.1:2375” in OPTIONS field:

# cat /etc/sysconfig/docker
# /etc/sysconfig/docker

# Modify these options if you want to change the way the docker daemon runs
OPTIONS='--selinux-enabled -H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock'

Restart Docker, and check whether the new OPTIONS takes effect:

# systemctl restart docker
# systemctl status docker
● docker.service - Docker Application Container Engine
   Loaded: loaded (/usr/lib/systemd/system/docker.service; disabled; vendor preset: disabled)
   Active: active (running) since Wed 2016-06-08 12:32:19 CST; 10s ago
     Docs: http://docs.docker.com
 Main PID: 14429 (sh)
   CGroup: /system.slice/docker.service
           ├─14429 /bin/sh -c /usr/bin/docker-current daemon $OPTIONS            $DOCKER_STORAGE_OPTIONS            $DOCKER_NETWORK_OPTI...
           ├─14430 /usr/bin/docker-current daemon --selinux-enabled -H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock --add-registr...
           └─14431 /usr/bin/forward-journald -tag docker
......

(4) Run “swarm create” command to create token for the cluster:

# swarm create
d10eacbda9763b0740548a2a4c2f1a59

(5) Execute swarm join to create a Docker Swarm node:

# swarm join --addr 127.0.0.1:2375 token://d10eacbda9763b0740548a2a4c2f1a59
INFO[0000] Registering on the discovery service every 1m0s...  addr=127.0.0.1:2375 discovery=token://d10eacbda9763b0740548a2a4c2f1a59
......

You should notice that the argument of --addr option is the IP and port of the Docker engine on this host. Since we have set theOPTIONS in Docker configuration file in step 3, the IP should be 127.0.0.1 whilst port is 2375.

(6) Open a new terminal, and create the manager of the cluster. Because port 2375 is occupied by Docker engine, we use another available port:

# swarm manage -H 127.0.0.1:3375 token://d10eacbda9763b0740548a2a4c2f1a59
INFO[0000] Listening for HTTP                            addr=127.0.0.1:3375 proto=tcp
INFO[0001] Registered Engine localhost.localdomain at 127.0.0.1:2375

Through the log, you can see the node and manager have communicated successfully.

Now, you can think a Docker engine is listening on tcp://127.0.0.1:3375, but actually, there is one Docker cluster behindtcp://127.0.0.1:3375, even though the cluster has only one node. You can play docker client commands now, such as get the cluster info:

# docker -H tcp://127.0.0.1:3375 info
Containers: 0
Images: 5
Server Version: swarm/1.2.3
Role: primary
Strategy: spread
Filters: health, port, containerslots, dependency, affinity, constraint
Nodes: 1
 localhost.localdomain: 127.0.0.1:2375
  └ ID: ZUIV:BMPV:3B5R:2WBC:JXEI:2S6H:XM3H:66W5:UZQI:NJON:JY4T:HIFB
  └ Status: Healthy
  └ Containers: 0 (0 Running, 0 Paused, 0 Stopped)
  └ Reserved CPUs: 0 / 8
  └ Reserved Memory: 0 B / 12.1 GiB
  └ Labels: executiondriver=native-0.2, kernelversion=3.10.0-327.el7.x86_64, operatingsystem=Red Hat Network, storagedriver=devicemapper
  └ UpdatedAt: 2016-06-08T04:58:05Z
  └ ServerVersion: 1.9.1
Kernel Version: 3.10.0-327.el7.x86_64
......

Or run a container:

# docker -H tcp://127.0.0.1:3375 run hello-world

Hello from Docker.
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
......

Enjoy Docker Swarm now!

Reference:
Swarm docs;
Docker Swarm Tutorial and Examples.