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Promtail

Promtail is an agent which ships the contents of local logs to a private Loki instance or Grafana Cloud. It is usually deployed to every machine that has applications needed to be monitored.

It primarily:

  1. Discovers targets
  2. Attaches labels to log streams
  3. Pushes them to the Loki instance.

Currently, Promtail can tail logs from two sources: local log files and the systemd journal (on AMD64 machines only).

Log File Discovery

Before Promtail can ship any data from log files to Loki, it needs to find out information about its environment. Specifically, this means discovering applications emitting log lines to files that need to be monitored.

Promtail borrows the same service discovery mechanism from Prometheus, although it currently only supports static and kubernetes service discovery. This limitation is due to the fact that promtail is deployed as a daemon to every local machine and, as such, does not discover label from other machines. kubernetes service discovery fetches required labels from the Kubernetes API server while static usually covers all other use cases.

Just like Prometheus, promtail is configured using a scrape_configs stanza. relabel_configs allows for fine-grained control of what to ingest, what to drop, and the final metadata to attach to the log line. Refer to the docs for configuring Promtail for more details.

Labeling and Parsing

During service discovery, metadata is determined (pod name, filename, etc.) that may be attached to the log line as a label for easier identification when querying logs in Loki. Through relabel_configs, discovered labels can be mutated into the desired form.

To allow more sophisticated filtering afterwards, Promtail allows to set labels not only from service discovery, but also based on the contents of each log line. The pipeline_stages can be used to add or update labels, correct the timestamp, or re-write log lines entirely. Refer to the documentation for pipelines for more details.

Shipping

Once Promtail has a set of targets (i.e., things to read from, like files) and all labels are set correctly, it will start tailing (continuously reading) the logs from targets. Once enough data is read into memory or after a configurable timeout, it is flushed as a single batch to Loki.

As Promtail reads data from sources (files and systemd journal, if configured), it will track the last offset it read in a positions file. By default, the positions file is stored at /var/log/positions.yaml. The positions file helps Promtail continue reading from where it left off in the case of the Promtail instance restarting.

API

Promtail features an embedded web server exposing a web console at / and the following API endpoints:

GET /ready

This endpoint returns 200 when Promtail is up and running, and there's at least one working target.

GET /metrics

This endpoint returns Promtail metrics for Prometheus. See "Operations > Observability" to get a list of exported metrics.

Promtail web server config

The web server exposed by Promtail can be configured in the Promtail .yaml config file:

server:
  http_listen_address: 127.0.0.1
  http_listen_port: 9080
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