The experimental build of Docker has an entirely new networking system, which enables secure communication between containers on multiple hosts. In combination with Docker Swarm and Docker Compose, you can now run multi-container apps on multi-host clusters with the same tooling and configuration format you use to develop them locally.
Note: This functionality is in the experimental stage, and contains some hacks and workarounds which will be removed as it matures.
Before you start, you’ll need to install the experimental build of Docker, and the latest versions of Machine and Compose.
To install the experimental Docker build on a Linux machine, follow the instructions here.
To install the experimental Docker build on a Mac, run these commands:
$ curl -L https://experimental.docker.com/builds/Darwin/x86_64/docker-latest > /usr/local/bin/docker $ chmod +x /usr/local/bin/docker
To install Machine, follow the instructions here.
To install Compose, follow the instructions here.
You’ll also need a Docker Hub account and a Digital Ocean account.
Set the DIGITALOCEAN_ACCESS_TOKEN environment variable to a valid Digital Ocean API token, which you can generate in the API panel.
export DIGITALOCEAN_ACCESS_TOKEN=abc12345
Start a consul server:
docker-machine --debug create \
-d digitalocean \
--engine-install-url="https://experimental.docker.com" \
consul
docker $(docker-machine config consul) run -d \
-p "8500:8500" \
-h "consul" \
progrium/consul -server -bootstrap
(In a real world setting you’d set up a distributed consul, but that’s beyond the scope of this guide!)
Create a Swarm token:
export SWARM_TOKEN=$(docker run swarm create)
Next, you create a Swarm master with Machine:
docker-machine --debug create \
-d digitalocean \
--digitalocean-image="ubuntu-14-10-x64" \
--engine-install-url="https://experimental.docker.com" \
--engine-opt="default-network=overlay:multihost" \
--engine-opt="kv-store=consul:$(docker-machine ip consul):8500" \
--engine-label="com.docker.network.driver.overlay.bind_interface=eth0" \
swarm-0
Usually Machine can create Swarms for you, but it doesn‘t yet fully support multi-host networks yet, so you’ll have to start up the Swarm manually:
docker $(docker-machine config swarm-0) run -d \
--restart="always" \
--net="bridge" \
swarm:latest join \
--addr "$(docker-machine ip swarm-0):2376" \
"token://$SWARM_TOKEN"
docker $(docker-machine config swarm-0) run -d \
--restart="always" \
--net="bridge" \
-p "3376:3376" \
-v "/etc/docker:/etc/docker" \
swarm:latest manage \
--tlsverify \
--tlscacert="/etc/docker/ca.pem" \
--tlscert="/etc/docker/server.pem" \
--tlskey="/etc/docker/server-key.pem" \
-H "tcp://0.0.0.0:3376" \
--strategy spread \
"token://$SWARM_TOKEN"
Create a Swarm node:
docker-machine --debug create \
-d digitalocean \
--digitalocean-image="ubuntu-14-10-x64" \
--engine-install-url="https://experimental.docker.com" \
--engine-opt="default-network=overlay:multihost" \
--engine-opt="kv-store=consul:$(docker-machine ip consul):8500" \
--engine-label="com.docker.network.driver.overlay.bind_interface=eth0" \
--engine-label="com.docker.network.driver.overlay.neighbor_ip=$(docker-machine ip swarm-0)" \
swarm-1
docker $(docker-machine config swarm-1) run -d \
--restart="always" \
--net="bridge" \
swarm:latest join \
--addr "$(docker-machine ip swarm-1):2376" \
"token://$SWARM_TOKEN"
You can create more Swarm nodes if you want - it’s best to give them sensible names (swarm-2, swarm-3, etc).
Finally, point Docker at your swarm:
export DOCKER_HOST=tcp://"$(docker-machine ip swarm-0):3376" export DOCKER_TLS_VERIFY=1 export DOCKER_CERT_PATH="$HOME/.docker/machine/machines/swarm-0"
Now that you’ve got a swarm up and running, you can create containers on it just like a single Docker instance:
$ docker run busybox echo hello world hello world
If you run docker ps -a, you can see what node that container was started on by looking at its name (here it’s swarm-3):
$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 41f59749737b busybox "echo hello world" 15 seconds ago Exited (0) 13 seconds ago swarm-3/trusting_leakey
As you start more containers, they’ll be placed on different nodes across the cluster, thanks to Swarm’s default “spread” scheduling strategy.
Every container started on this swarm will use the “overlay:multihost” network by default, meaning they can all intercommunicate. Each container gets an IP address on that network, and an /etc/hosts file which will be updated on-the-fly with every other container’s IP address and name. That means that if you have a running container named ‘foo’, other containers can access it at the hostname ‘foo’.
Let’s verify that multi-host networking is functioning. Start a long-running container:
$ docker run -d --name long-running busybox top <container id>
If you start a new container and inspect its /etc/hosts file, you’ll see the long-running container in there:
$ docker run busybox cat /etc/hosts ... 172.21.0.6 long-running
Verify that connectivity works between containers:
$ docker run busybox ping long-running PING long-running (172.21.0.6): 56 data bytes 64 bytes from 172.21.0.6: seq=0 ttl=64 time=7.975 ms 64 bytes from 172.21.0.6: seq=1 ttl=64 time=1.378 ms 64 bytes from 172.21.0.6: seq=2 ttl=64 time=1.348 ms ^C --- long-running ping statistics --- 3 packets transmitted, 3 packets received, 0% packet loss round-trip min/avg/max = 1.140/2.099/7.975 ms
Here’s an example of a simple Python + Redis app using multi-host networking on a swarm.
Create a directory for the app:
$ mkdir composetest $ cd composetest
Inside this directory, create 2 files.
First, create app.py - a simple web app that uses the Flask framework and increments a value in Redis:
from flask import Flask
from redis import Redis
import os
app = Flask(__name__)
redis = Redis(host='composetest_redis_1', port=6379)
@app.route('/')
def hello():
redis.incr('hits')
return 'Hello World! I have been seen %s times.' % redis.get('hits')
if __name__ == "__main__":
app.run(host="0.0.0.0", debug=True)
Note that we’re connecting to a host called composetest_redis_1 - this is the name of the Redis container that Compose will start.
Second, create a Dockerfile for the app container:
FROM python:2.7 RUN pip install flask redis ADD . /code WORKDIR /code CMD ["python", "app.py"]
Build the Docker image and push it to the Hub (you’ll need a Hub account). Replace <username> with your Docker Hub username:
$ docker build -t <username>/counter . $ docker push <username>/counter
Next, create a docker-compose.yml, which defines the configuration for the web and redis containers. Once again, replace <username> with your Hub username:
web: image: <username>/counter ports: - "80:5000" redis: image: redis
Now start the app:
$ docker-compose up -d Pulling web (username/counter:latest)... swarm-0: Pulling username/counter:latest... : downloaded swarm-2: Pulling username/counter:latest... : downloaded swarm-1: Pulling username/counter:latest... : downloaded swarm-3: Pulling username/counter:latest... : downloaded swarm-4: Pulling username/counter:latest... : downloaded Creating composetest_web_1... Pulling redis (redis:latest)... swarm-2: Pulling redis:latest... : downloaded swarm-1: Pulling redis:latest... : downloaded swarm-3: Pulling redis:latest... : downloaded swarm-4: Pulling redis:latest... : downloaded swarm-0: Pulling redis:latest... : downloaded Creating composetest_redis_1...
Swarm has created containers for both web and redis, and placed them on different nodes, which you can check with docker ps:
$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 92faad2135c9 redis "/entrypoint.sh redi 43 seconds ago Up 42 seconds swarm-2/composetest_redis_1 adb809e5cdac username/counter "/bin/sh -c 'python 55 seconds ago Up 54 seconds 45.67.8.9:80->5000/tcp swarm-1/composetest_web_1
You can also see that the web container has exposed port 80 on its swarm node. If you curl that IP, you’ll get a response from the container:
$ curl http://45.67.8.9 Hello World! I have been seen 1 times.
If you hit it repeatedly, the counter will increment, demonstrating that the web and redis container are communicating:
$ curl http://45.67.8.9 Hello World! I have been seen 2 times. $ curl http://45.67.8.9 Hello World! I have been seen 3 times. $ curl http://45.67.8.9 Hello World! I have been seen 4 times.