loki-docker-sd

Use Loki with Docker without any configuration! (mostly)

A service discovery mechanism for Grafana Loki (actually Promtail) that discovers targets (log files) directly from a running Docker daemon

Why

Docker capabilities of upstream Loki may be insufficient, as they require changing the Docker daemon configuration (/etc/docker/daemon.json) or setting --log-opts on every container, which may not be desired or even impossible (e.g. in provided / managed environments):

• Loki Docker logging driver requires installing a Docker plugin on every machine the Docker daemon runs on, also --log-driver=loki must be passed to every docker run. Very tedious
• Others suggest using --log-opt tag=... to "hook in" container information and pass that out using relabel rules. Again, requires setting something on every container (or in /daemon.json), which is tedious.

The Kubernetes capability of Loki however do not face this problem, as kubernetes_sd hands Promtail the container ID, which can be used to grab the correct logs from /var/log/docker/containers, while also retaining lots of __meta_ labels for identifying logs.

But Loki knows no docker_sd so we are out of luck? No!

Docker knows file_sd and the we can docker ls containers to get their ID. And it gets even better, docker inspect even tells us LogPath – the absolute path to the JSON file the containers logs are in.

loki-docker-sd leverages that to write targets to a JSON file suitable for file_sd and sets __path__ to LogPath so that Promtail can easily tail our containers logs.

Running

There is a docker container at shorez/loki-docker-sd. Requires access to /var/run/docker.sock:

$ docker run -v /var/run/docker.sock:/var/run/docker.sock:ro shorez/loki-docker-sd

For a complete example, see docker-compose.yml

Configuration

You said without configuration. LIAR

Yeah. lol. No Docker configuration at least and that's what counts.

A bit of Promtail configuration is still required (but that's not an issue). See example-config.yml for that

Meta labels

loki-docker-sd provides the following labels during relabeling:

• __meta_docker_container_id: Docker container ID
• __meta_docker_container_name: Docker container name
• __meta_docker_container_status: Docker container status (running, etc.)
• __meta_docker_container_label_<label>: Docker container labels (. and - are substituted by _)
• __path__: Path on disk to the log file