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Loki Docker Logging Driver

Overview

Docker logging driver plugins extends Docker's logging capabilities. You can use Loki Docker logging driver plugin to send Docker container logs directly to your Loki instance or Grafana Cloud.

Docker plugins are not yet supported on Windows; see Docker's logging driver plugin documentation

If you have any questions or issues using the Docker plugin feel free to open an issue in this repository.

Plugin Installation

You need to install the plugin on each Docker host with container from which you want to collect logs.

You can install the plugin from our Docker hub repository by running on the Docker host the following command:

docker plugin install  grafana/loki-docker-driver:latest --alias loki --grant-all-permissions

To check the status of installed plugins, use the docker plugin ls command. Plugins that start successfully are listed as enabled in the output:

docker plugin ls
ID                  NAME         DESCRIPTION           ENABLED
ac720b8fcfdb        loki         Loki Logging Driver   true

You can now configure the plugin.

Plugin Configuration

The Docker daemon on each Docker host has a default logging driver; each container on the Docker host uses the default driver, unless you configure it to use a different logging driver.

Configure the logging driver for a container

When you start a container, you can configure it to use a different logging driver than the Docker daemon’s default, using the --log-driver flag. If the logging driver has configurable options, you can set them using one or more instances of the --log-opt <NAME>=<VALUE> flag. Even if the container uses the default logging driver, it can use different configurable options.

The following command configure the container grafana to start with the loki drivers which will send logs to logs-us-west1.grafana.net Loki instance, using a batch size of 400 entries and will retry maximum 5 times if it fails.

docker run --log-driver=loki \
    --log-opt loki-url="https://<user_id>:<password>@logs-us-west1.grafana.net/api/prom/push" \
    --log-opt loki-retries=5 \
    --log-opt loki-batch-size=400 \
    grafana/grafana

Note: The Loki logging driver still uses the json-log driver in combination with sending logs to Loki, this is mainly useful to keep the docker logs command working. You can adjust file size and rotation using the respective log option max-size and max-file.

Configure the default logging driver

To configure the Docker daemon to default to Loki logging driver, set the value of log-driver to loki logging driver in the daemon.json file, which is located in /etc/docker/. The following example explicitly sets the default logging driver to Loki:

{
    "debug" : true,
    "log-driver": "loki"
}

The logging driver has configurable options, you can set them in the daemon.json file as a JSON array with the key log-opts. The following example sets the Loki push endpoint and batch size of the logging driver:

{
    "debug" : true,
    "log-driver": "loki",
    "log-opts": {
        "loki-url": "https://<user_id>:<password>@logs-us-west1.grafana.net/api/prom/push",
        "loki-batch-size": "400"
    }
}

Note: log-opt configuration options in the daemon.json configuration file must be provided as strings. Boolean and numeric values (such as the value for loki-batch-size in the example above) must therefore be enclosed in quotes (").

Restart the Docker daemon and it will be configured with Loki logging driver, all containers from that host will send logs to Loki instance.

Configure the logging driver for a Swarm service or Compose

You can also configure the logging driver for a swarm service directly in your compose file, this also work for a docker-compose deployment:

version: "3.7"
services:
  logger:
    image: grafana/grafana
    logging:
      driver: loki
      options:
        loki-url: "https://<user_id>:<password>@logs-us-west1.grafana.net/api/prom/push"

You can then deploy your stack using:

docker stack deploy my_stack_name --compose-file docker-compose.yaml

Once deployed the Grafana service will be sending logs automatically to Loki.

Note: stack name and service name for each swarm service and project name and service name for each compose service are automatically discovered and sent as Loki labels, this way you can filter by them in Grafana.

Labels

Loki can received a set of labels along with log line. These labels are used to index log entries and query back logs using LogQL stream selector.

By default the Docker driver will add the filename where the log is written, the host where the log has been generated as well as the container_name. Additionally swarm_stack and swarm_service are added for Docker Swarm deployments.

You can add more labels by using loki-external-labels,loki-pipeline-stage-file,labels,env and env-regex options as described below.

log-opt options

To specify additional logging driver options, you can use the --log-opt NAME=VALUE flag.

Option Required? Default Value Description
loki-url Yes Loki HTTP push endpoint.
loki-external-labels No container_name={{.Name}} Additional label value pair separated by , to send with logs. The value is expanded with the Docker tag template format. (eg: container_name={{.ID}}.{{.Name}},cluster=prod)
loki-timeout No 10s The timeout to use when sending logs to the Loki instance. Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
loki-batch-wait No 1s The amount of time to wait before sending a log batch complete or not. Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
loki-batch-size No 102400 The maximum size of a log batch to send.
loki-min-backoff No 100ms The minimum amount of time to wait before retrying a batch. Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
loki-max-backoff No 10s The maximum amount of time to wait before retrying a batch. Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
loki-retries No 10 The maximum amount of retries for a log batch.
loki-pipeline-stage-file No The location of a pipeline stage configuration file (example). Pipeline stages allows to parse log lines to extract more labels. see documentation
loki-tls-ca-file No Set the path to a custom certificate authority.
loki-tls-cert-file No Set the path to a client certificate file.
loki-tls-key-file No Set the path to a client key.
loki-tls-server-name No Name used to validate the server certificate.
loki-tls-insecure-skip-verify No false Allow to skip tls verification.
loki-proxy-url No Proxy URL use to connect to Loki.
max-size No -1 The maximum size of the log before it is rolled. A positive integer plus a modifier representing the unit of measure (k, m, or g). Defaults to -1 (unlimited). This is used by json-log required to keep the docker log command working.
max-file No 1 The maximum number of log files that can be present. If rolling the logs creates excess files, the oldest file is removed. Only effective when max-size is also set. A positive integer. Defaults to 1.
labels No Comma-separated list of keys of labels, which should be included in message, if these labels are specified for container.
env No Comma-separated list of keys of environment variables to be included in message if they specified for a container.
env-regex No A regular expression to match logging-related environment variables. Used for advanced log label options. If there is collision between the label and env keys, the value of the env takes precedence. Both options add additional fields to the labels of a logging message.

Uninstall the plugin

To cleanly disable and remove the plugin, run:

docker plugin disable loki
docker plugin rm loki

Upgrade the plugin

To upgrade the plugin to the last version, run:

docker plugin disable loki
docker plugin upgrade loki grafana/loki-docker-driver:master
docker plugin enable loki

Troubleshooting

Plugin logs can be found as docker daemon log. To enable debug mode refer to the Docker daemon documentation: https://docs.docker.com/config/daemon/

Stdout of a plugin is redirected to Docker logs. Such entries have a plugin= suffix.

To find out the plugin ID of Loki, use the command below and look for Loki plugin entry.

docker plugin ls

Depending on your system, location of Docker daemon logging may vary. Refer to Docker documentation for Docker daemon log location for your specific platform. (see)