Fluentd output plugin to ship logs to a Loki server.

This plugin offers two line formats and uses protobuf to send compressed data to Loki.

Key features:

  • extra_labels - labels to be added to every line of a logfile, useful for designating environments
  • label_keys - customizable list of keys for stream labels


$ gem install fluent-plugin-grafana-loki


In your Fluentd configuration, use @type loki. Additional configuration is optional, default values would look like this:

<match **>
  @type loki
  url "https://logs-us-west1.grafana.net"
  username "#{ENV['LOKI_USERNAME']}"
  password "#{ENV['LOKI_PASSWORD']}"
  extra_labels {"env":"dev"}
  flush_interval 10s
  flush_at_shutdown true
  buffer_chunk_limit 1m

Docker Image

There is a Docker image grafana/fluent-plugin-grafana-loki:master which contains default configuration files to git log information a host's /var/log dir, and from the host's Journald. To use it, you can set the LOKI_URL, LOKI_USERNAME, and LOKI_PASSWORD environment variables (you can leave the USERNAME and PASSWORD blank if they're not used.)

A Docker Swarm Compose configuration that will work looks like:

    image: grafana/fluent-plugin-grafana-loki:master
      - "fluentd"
      - "-v"
      - "-p"
      - "/fluentd/plugins"
      LOKI_URL: http://loki:3100
      mode: global
      - source: loki_config
        target: /fluentd/etc/loki/loki.conf
      - loki
      - host_logs:/var/log
      # Needed for journald log ingestion:
      - /etc/machine-id:/etc/machine-id
      - /dev/log:/dev/log
      - /var/run/systemd/journal/:/var/run/systemd/journal/
         tag: infra.monitoring



The url of the Loki server to send logs to. When sending data the publish path (/api/prom/push) will automatically be appended. By default the url is set to https://logs-us-west1.grafana.net, the url of the Grafana Labs preview (hosted Loki)[https://grafana.com/loki] service.

Proxy Support

Starting with version 0.8.0, this gem uses excon, which supports proxy with environment variables - https://github.com/excon/excon#proxy-support

username / password

Specify a username and password if the Loki server requires authentication. If using the GrafanaLab's hosted Loki, the username needs to be set to your instanceId and the password should be a Grafana.com api key.


Loki is a multi-tenant log storage platform and all requests sent must include a tenant. For some installations the tenant will be set automatically by an authenticating proxy. Otherwise you can define a tenant to be passed through. The tenant can be any string value.

output format

Loki is intended to index and group log streams using only a small set of labels. It is not intended for full-text indexing. When sending logs to Loki the majority of log message will be sent as a single log "line".

There are 3 configurations settings to control the output format.

  • extra_labels: (default: nil) set of labels to include with every Loki stream. eg {"env":"dev", "datacenter": "dc1"}
  • label_keys: (default: "job,instance") comma separated list of keys to use as stream labels. All other keys will be placed into the log line
  • line_format: format to use when flattening the record to a log line. Valid values are "json" or "key_value". If set to "json" the log line sent to Loki will be the fluentd record (excluding any keys extracted out as labels) dumped as json. If set to "key_value", the log line will be each item in the record concatenated together (separated by a single space) in the format <key>=<value>.
  • drop_single_key: if set to true and after extracting label_keys a record only has a single key remaining, the log line sent to Loki will just be the value of the record key.

Buffer options

fluentd-plugin-loki extends Fluentd's builtin Output plugin and use compat_parameters plugin helper. It adds the following options:

buffer_type memory
flush_interval 10s
retry_limit 17
retry_wait 1.0
num_threads 1


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in fluent-plugin-grafana-loki.gemspec, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

To create the gem: gem build fluent-plugin-grafana-loki.gemspec

Useful additions: gem install rubocop

  • Copyright(c) 2018- Grafana Labs
  • License
    • Apache License, Version 2.0