cowley-tech/content/blog/playing-with-docker-swarm-mode/index.md

date	title	category
2016-07-10	Playing with Docker Swarm Mode	devops

The big announcement of the recent DockerCon was 1.12 integrating Swarm. As far the as the ecosystem goes that is quite a game changer, but I will not be dwelling on that. I am just going to regurgitate what others have said and add a few bit of my own.

I am going to build a simple cluster that looks like this:

What we have here is 2 nodes running Centos 7 which run Docker 1.12-rcX in swarm mode. I am actually only going to create a single manager and a worker. For a bonus, I am going to touch on a subject that has been ignored a little: storage.

Many will say that your application should use S3, or whatever. The fact is that POSIX storage is really useful though. So, we need a way of having storage at the same place on each node that can be used as a volume in our application containers. In keeping with the principles of Swarm, this storage needs to be distributed, replicated and scalable. In the Open Source world we have 2 major players (and plenty of others) in Ceph and Gluster. They are both awesome, scalable, stable, blahblahblah. Either would be a great choice, but I am going to use Gluster because, err, reasons. Basically I flipped a coin.

I will make each of the 2 nodes a Gluster server with a replicated volume. I will then mount the volume on each node.

I am assuming that you have a pair of clean, and up-to-date, CentOS 7 nodes. I also assume that they have a single NIC (not best-practise, but we are playing here) and can resolve each other by name (either by DNS or /etc/hosts).

Gluster

See the main docs for more info, but the very basic process is (on each node):

sudo yum install centos-release-gluster epel-release
sudo yum install glusterfs-server
sudo systemctl enable glusterd
sudo systemctl start glusterd

Now create the cluster. From node1 run:

sudo gluster peer probe node2

Next, from node2 run

sudo gluster peer probe node1

Now, we have our storage cluster setup we need to create a volume. On each node create a folder to store the data:

mkdir -pv /data/brick1/gv0

As an aside, I mount a seperate disk at /data/brick1 formatted with XFS. This is not essential for our purposes though.

Now, from node1 (because it will be our master, so my OCD dictates it is also our admin node) create the volume:

sudo gluster volume create gv0 replica 2 node1:/data/brick1/gv0 node2:/data/brick1/gv0
sudo gluster volume start gv0

Now on each node mount the volume:

echo "$(hostname -s):/gv0 /mnt glusterfs defaults,_netdev 0 0" | sudo tee -a /etc/fstab
sudo mount /mnt

Docker

We'll install Docker using docker-machine. I do it from an admin machine, but you could use one of your cluster nodes. You need to have passwordless root SSH access to your nodes.

Install docker-engine and docker-machine as root on the admin node:

curl -fsSL https://experimental.docker.com/ | sudo sh
curl -L https://github.com/docker/machine/releases/download/v0.8.0-rc2/docker-machine-`uname -s`-`uname -m` > docker-machine && \
  chmod +x docker-machine
sudo mv -v docker-machine /usr/local/bin/

Now you can go ahead and install docker-engine on the cluster nodes:

docker-machine create --engine-install-url experimental.docker.com \
    -d generic --generic-ip-address=node-1 \
    --generic-ssh-key /root/.ssh/id_rsa node-1
docker-machine create --engine-install-url experimental.docker.com \
    -d generic --generic-ip-address=node-2 \
    --generic-ssh-key /root/.ssh/id_rsa node-2

Now you can enable Swarm mode:

eval $(docker-machine env node1)
docker swarm init
eval $(docker-machine env node2)
docker swarm join node1:2377

That is it - you now have a swarm cluster. I have another post coming where I will describe a method I use for collecting metrics about the cluster. That will also include deploying an application on the cluster too.