# Experimental: Compose, Swarm and Multi-Host Networking
The [experimental build of Docker](https://github.com/docker/docker/tree/master/experimental) 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.
## Prerequisites
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](https://github.com/docker/docker/tree/master/experimental#install-docker-experimental).
- To install the experimental Docker build on a Mac, run these commands:
You’ll also need a [Docker Hub](https://hub.docker.com/account/signup/) account and a [Digital Ocean](https://www.digitalocean.com/) account.
## Set up a swarm with multi-host networking
Set the `DIGITALOCEAN_ACCESS_TOKEN` environment variable to a valid Digital Ocean API token, which you can generate in the [API panel](https://cloud.digitalocean.com/settings/applications).
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
<containerid>
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
## Run a Compose application
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:
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:
