Instructor Demo: Docker Compose

In this demo, we'll illustrate:

Starting an app defined in a docker compose file
Inter-service communication using DNS resolution of service names

Exploring the Compose File

Please download the DockerCoins app from Github and change directory to ~/orchestration-workshop/dockercoins.

[centos@node-0 ~]$ git clone -b ee2.1 \
    https://github.com/docker-training/orchestration-workshop.git
[centos@node-0 ~]$ cd ~/orchestration-workshop/dockercoins

Let's take a quick look at our Compose file for Dockercoins:

version: "3.1"

services:
rng:
    image: training/dockercoins_rng:1.0
    networks:
    - dockercoins
    ports:
    - "8001:80"

hasher:
    image: training/dockercoins_hasher:1.0
    networks:
    - dockercoins
    ports:
    - "8002:80"

webui:
    image: training/dockercoins_webui:1.0
    networks:
    - dockercoins
    ports:
    - "8000:80"

redis:
    image: redis
    networks:
    - dockercoins

worker:
    image: training/dockercoins_worker:1.0
    networks:
    - dockercoins

networks:
    dockercoins:

This Compose file contains 5 services, along with a bridge network.

When we start the app, we will see the service images getting downloaded one at a time:
```
[centos@node-0 dockercoins]$ docker-compose up -d
```
After starting, the images required for this app have been downloaded:
```
[centos@node-0 dockercoins]$ docker image ls | grep "dockercoins"
```

Make sure the services are up and running, as is the dedicated network:

[centos@node-0 dockercoins]$ docker-compose ps
[centos@node-0 dockercoins]$ docker network ls

If everyting is up, visit your app at <node-0 public IP>:8000 to see Dockercoins in action.

Communicating Between Containers

In this section, we'll demonstrate that containers created as part of a service in a Compose file are able to communicate with containers belonging to other services using just their service names. Let's start by listing our DockerCoins containers:
```
[centos@node-0 dockercoins]$ docker container ls | grep 'dockercoins'
```

Now, connect into one container; let's pick webui:

[centos@node-0 dockercoins]$ docker container exec -it <Container ID> bash

From within the container, ping rng by name:

[root@<Container ID>]# ping rng

Logs should be outputted resembling this:

PING rng (172.18.0.5) 56(84) bytes of data.
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=1 ttl=64 time=0.108 ms
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=2 ttl=64 time=0.049 ms
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=3 ttl=64 time=0.073 ms
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=4 ttl=64 time=0.067 ms
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=5 ttl=64 time=0.057 ms
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=6 ttl=64 time=0.074 ms
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=7 ttl=64 time=0.052 ms
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=8 ttl=64 time=0.057 ms
64 bytes from dockercoins_rng_1.dockercoins_dockercoins (172.18.0.5): icmp_seq=9 ttl=64 time=0.080 ms

Use CTRL+C to terminate the ping. DNS lookup for the services in DockerCoins works because they are all attached to the user-defined dockercoins network.

After exiting this container, let's navigate to the worker folder and take a look at a section of worker.py:

[centos@node-0 dockercoins]$ cd worker
[centos@node-0 worker]$ cat worker.py

import logging
import os
from redis import Redis
import requests
import time

DEBUG = os.environ.get("DEBUG", "").lower().startswith("y")

log = logging.getLogger(__name__)
if DEBUG:
    logging.basicConfig(level=logging.DEBUG)
else:
    logging.basicConfig(level=logging.INFO)
    logging.getLogger("requests").setLevel(logging.WARNING)

redis = Redis("redis")

def get_random_bytes():
    r = requests.get("http://rng/32")
    return r.content

def hash_bytes(data):
    r = requests.post("http://hasher/",
                    data=data,
                    headers={"Content-Type": "application/octet-stream"})
    hex_hash = r.text
    return hex_hash

As we can see in the last two stanzas, we can direct traffic to a service via a DNS name that exactly matches the service name defined in the docker compose file.

Shut down Dockercoins and clean up its resources:

[centos@node-0 dockercoins]$ docker-compose down

Conclusion

In this exercise, we stood up an application using Docker Compose. The most important new idea here is the notion of Docker Services, which are collections of identically configured containers. Docker Service names are resolvable by DNS, so that we can write application logic designed to communicate service to service; all service discovery and load balancing between your application's services is abstracted away and handled by Docker.