Monday, November 23, 2015

Simple Clustering with Docker Swarm and Nginx

Bringing up your own cluster has never been easier. The recent 1.0 release of Docker Swarm signals that the Docker team feel that Swarm is ready for production.

I've been running a bunch of applications on Docker for a while now, but I have managed the containers on the single machine level instead of as a cluster. With the release of Swarm 1.0, I believe it is time to start clustering my machines.

Spinning Up the Swarm

How to spin up a Swarm for development is described well in the Docker documentation and I'm not going to describe it in depth here. I'll settle for the commands and extra documentation when I feel that it may be called for.

I'm using the Swarm for development with VirtualBox here, but it is simple to substitute any of the supported docker-machine providers.

Create a Token

Create a token with the Docker Hub discovery service. When running this in production you should probably setup an alternate discovery backend to avoid the external dependency.

# Create and save a token, using the Docker-Hub discovery service, default
$ token=$(docker run swarm create)

Create a Swarm Manager

The swarm manager will be used to control the swarm. It should be protected from access from anyone but you. I'll simulate this here by setting --engine-label public=no. This is just a tag and you would have to make sure that you setup the manager protected from public access. It is possible to use multiple labels to tag the engine with all the qualities of this machine.

# Create a swarm manager using the token
$ docker-machine create \
  -d virtualbox \
  --swarm \
  --swarm-master \
  --swarm-discovery token://$token \
  --engine-label public=no \

Create a Publicly Accessible Machine

In this demo I'm only spinning up another VirtualBox machine and I'm giving it the --engine-label public=yes to allow me to discover this box in the swarm.

# Create a new node named frontend and label it public
$ docker-machine create \
  -d virtualbox \
  --swarm \
  --swarm-discovery token://$token \
  --engine-label public=yes \

Create a Couple of Additional Non-Public Machines

Here I start a couple of machines with an additional --engine-label. One with model=high-memory and one with model=large-disk

# Create two more nodes named backend1 and backend2, with label public=no
$ docker-machine create \
  -d virtualbox \
  --swarm \
  --swarm-discovery token://$token \
  --engine-label public=no \
  --engine-label model=high-memory \

$ docker-machine create \
  -d virtualbox \
  --swarm \
  --swarm-discovery token://$token \
  --engine-label public=no \
  --engine-label model=large-disk \

List the Swarm

# List your machines
$ docker-machine ls
NAME           ACTIVE   DRIVER       STATE     URL                         SWARM
backend1       -        virtualbox   Running   tcp://   swarm-master
backend2       -        virtualbox   Running   tcp://   swarm-master
frontend       -        virtualbox   Running   tcp://   swarm-master
swarm-master   -        virtualbox   Running   tcp://   swarm-master (master)

Connect to the Swarm

Configure the docker client to connect to it.

# List the environment needed to connect to the swarm
$ docker-machine env --swarm swarm-master
export DOCKER_HOST="tcp://"
export DOCKER_CERT_PATH="/Users/andersjanmyr/.docker/machine/machines/swarm-master"
export DOCKER_MACHINE_NAME="swarm-master"
# Run this command to configure your shell:
# eval "$(docker-machine env --swarm swarm-master)"

# Configure docker to use the swarm-master
$ eval $(docker-machine env --swarm swarm-master)

# List information about the cluster, output is trimmed
$ docker info
Containers: 4
Images: 4
Role: primary
Strategy: spread
Filters: health, port, dependency, affinity, constraint
Nodes: 4
   Containers: 1
   Reserved CPUs: 0 / 1
   Reserved Memory: 0 B / 1.021 GiB
   Labels: model=high-memory, provider=virtualbox, public=no, storagedriver=aufs
  Containers: 5
  Reserved CPUs: 0 / 1
  Reserved Memory: 0 B / 1.021 GiB
  Labels: model=large-disk, provider=virtualbox, public=no, storagedriver=aufs
   Containers: 1
   Reserved CPUs: 0 / 1
   Reserved Memory: 0 B / 1.021 GiB
   Labels: provider=virtualbox, public=yes, storagedriver=aufs
   Containers: 2
   Reserved CPUs: 0 / 1
   Reserved Memory: 0 B / 1.021 GiB
   Labels: provider=virtualbox, public=no, storagedriver=aufs
CPUs: 4
Total Memory: 4.086 GiB
Name: fa2d554280ff

Starting the Containers

Now it is time to start the containers. The plan is to bring up two database containers, Postgres and Redis, two counter web-services, and one proxy to front the whole cluster, like this.

Alright, let's start some containers!


According to the picture above I want to put the Redis container on the machine named backend1, but I don't want to address it by name, instead I'm going to target it by its labels.

I also want to start a Postgres container on a machine with a constraint:model==large-disk.

Starting Redis

# Start Redis on a non-public machine with high-memory.
$ docker run -d --name redis \
  --env constraint:public!=yes \
  --env constraint:model==high-memory \

In this case, constraint:public!=yes is not needed but I like to add it to avoid mistakes.

Starting Postgres

# Start Postgres on a non-public machine with large-disk
$ docker run -d --name postgres \
  --env constraint:public!=yes \
  --env constraint:model==large-disk \

If this was not a VirtualBox machine I would also mount a volume, -v /var/pgdata:/var/lib/postgresql/data, for the database, but this does not work with VirtualBox.

OK, let's see what we have.

# List running containers, output slightly trimmed
$ docker ps
CONTAINER ID     IMAGE       COMMAND                  PORTS            NAMES
aa1679b3da5c     postgres    "/docker-entrypoint.s"   5432/tcp         backend2/postgres
ffa41d90f414     redis       "/ redis"   6379/tcp         backend1/redis

Nice, two running databases on the designated machines.

Starting the Reverse Proxy

Nginx is one of my favorite building blocks when it comes to building reliable web services. Nginx provides an official Docker image, but in this case, when I want to automatically configure Nginx when new containers are started, I prefer to use an alternative image called nginx-proxy.

A container started from the nginx-proxy image, listens to events generated by the docker engine. The engine generates events for all kinds of events but all we care about here is when a container is started and stopped. If you want to see what events are triggered from the CLI, run docker events in one terminal and start and stop a few containers in another.

When nginx-proxy receives an event that a container has been started it checks if the container has any ports EXPOSEd, if it does it also checks for a VIRTUAL_HOST environment variable. If both these conditions are fulfilled nginx-proxy re-configures its Nginx server and reloads the configuration.

When you now access the VIRTUAL_HOST, Nginx proxies the connection to your web service. Cool!

Naturally, you will have to configure your DNS to point to your Nginx server. The easiest way to do this is to configure all your services to point to it with a wildcard record. Something like this:

*     Host (A)    Default

In this case, we are using VirtualBox and we can settle for changing the /etc/hosts file with the IP-number of our frontend.

# /etc/hosts

What is even more cool is that events works with Swarm and it is possible to use the nginx-proxy to listen to services that are started on different machines. All we have to do is configure it correctly.

Starting Nginx-Proxy

nginx-proxy is started with configuration read from the docker client environment variables. All the environments variables were automatically configured when you configured the docker client to access the Swarm, above.

# Start nginx-proxy configured to listen to swarm events, published on port 80.
$ docker run -d --name nginx \
  -p "80:80" \
  --env constraint:public==yes \
  --env DOCKER_HOST \

OK, we are almost done. Now it is time to start the web services.

Starting Web Services

As a web service I'm going to use a simple counter image since it can use both Postgres and Redis as backend. I want to start the web services on the same server as the databases since this allows me to use --link to connect to the container and it will speed up the data access. To do this I can use an affinity constraint: --env affinity:container==*redis*.

# Start a counter close to the container named redis and link to it.
$ docker run -d --name redis-counter \
  -p 80 \
  --link redis \
  --env affinity:container==*redis* \
  --env REDIS_URL=redis:6379 \
  --env VIRTUAL_HOST=redis-counter.docker \

The affinity constraint is not really necessary since affinity constraints are automatically generated by Swarm when --link is present as you can see when we start the postgres-counter.

# Start a counter close to the container named postgres and link to it.
$ docker run -d --name postgres-counter \
  -p 80 \
  --link postgres \
  --env POSTGRES_URL=postgres://postgres@postgres \
  --env VIRTUAL_HOST=postgres-counter.docker \

Browse to http://redis-counter.docker or http:/postgres-counter.docker and you should see your services up and running.


Here's an illustration of our current setup:

And here is a listing of all the containers on their respective machines.

$ docker ps
CONTAINER ID        IMAGE                  COMMAND                  PORTS                                NAMES
b3869a89e76c        andersjanmyr/counter   "/counter-linux">80/tcp         backend2/postgres-counter
cff69b6f970d        andersjanmyr/counter   "/counter-linux">80/tcp         backend1/redis-counter
64af31135c26        jwilder/nginx-proxy    "/app/docker-entrypoi"   443/tcp,>80/tcp   frontend/nginx
aa1679b3da5c        postgres               "/docker-entrypoint.s"   5432/tcp                             backend2/postgres,backend2/postgres-counter/postgres
ffa41d90f414        redis                  "/ redis"   6379/tcp                             backend1/redis,backend1/redis-counter/redis

May the Swarm be with you! :D


Unknown said...

Thanks for a nice tutorial!
When you mention that the hosts file should be updated, on which host should that be done?

Anders Janmyr said...

On the host you use to access the cluster.
It is needed for the correct Host header to be set.
And it allows nginx to choose the correct backend.

Jacob Gadikian said...

Indeed it was a well-done tutorial! Thanks a bunch-- I'm off to go and give this a whirl on scaleway nodes :).

Anders Janmyr said...

Thanks Jacob, I'm glad you liked it :)