Datadog offers native Docker container monitoring, either by running the Agent on the host or running in a sidecar container. Which is the best way to run it? It ultimately depends on the tooling you have in place to manage the Agent's configuration. If you want to go Docker all the way, you can run the Agent as a sidecar and control its configuration with custom Dockerfiles.

Let's see what it looks like.

To build a meaningful setup, we start from the example that Docker put together to illustrate Compose. A simple python web application that connects to Redis to store the number of hits.

Here is the docker-compose.yml that powers the whole setup.

version: "3"
services:
  web:
    build: web
    command: python app.py
    ports:
     - "5000:5000"
    volumes:
     - ./web:/code # modified here to take into account the new app path
    links:
     - redis
    environment:
     - DATADOG_HOST=datadog # used by the web app to initialize the Datadog library
  redis:
    image: redis
  # agent section
  datadog:
    build: datadog
    links:
     - redis # ensures that redis is a host that the container can find
     - web # ensures that the web app can send metrics
    environment:
     - DD_API_KEY=__your_datadog_api_key_here__
    volumes:
     - /var/run/docker.sock:/var/run/docker.sock
     - /proc/:/host/proc/:ro
     - /sys/fs/cgroup:/host/sys/fs/cgroup:ro

Because the Agent needs to monitor redis it needs:

1. the proper redisdb.yaml in the container's /etc/datadog-agent/conf.d
2. to find the redis node.

The Agent's Dockerfile takes care of #1.

FROM datadog/agent:latest
ADD conf.d/redisdb.yaml /etc/datadog-agent/conf.d/redisdb.yaml

And the Compose yaml files creates the link to redis with:

  links:
    - redis

How to test this?

1. Install Docker Compose
2. Clone this repository
3. Update your DD_API_KEY in docker-compose.yml
4. Run all containers with docker-compose up
5. Verify in Datadog that your container picks up the docker and redis metrics