Open source RSS

Troubleshooting Promtail

This document describes known failure modes of Promtail on edge cases and the adopted trade-offs.

Dry running

Promtail can be configured to print log stream entries instead of sending them to Loki. This can be used in combination with piping data to debug or troubleshoot Promtail log parsing.

In dry run mode, Promtail still support reading from a positions file however no update will be made to the targeted file, this is to ensure you can easily retry the same set of lines.

To start Promtail in dry run mode use the flag --dry-run as shown in the example below:

cat my.log | promtail --stdin --dry-run --client.url

Inspecting a config file

Promtail can validate and syntatically check your config file for a valid configuration. This can be used to check for errors and inconsistency in your config file and help prevent deploying invalid configurations.

In check syntax mode, promtail will just validate the config file and then exit:

promtail -config.file=myConfigFile.yaml -check-syntax

Inspecting pipeline stages

Promtail can output all changes to log entries as each pipeline stage is executed. Each log entry contains four fields:

  • line
  • timestamp
  • labels
  • extracted fields

Enable the inspection output using the --inspect command-line option. The --inspect option can be used in combination with --stdin and --dry-run.

cat my.log | promtail --stdin --dry-run --inspect --client.url


The output uses color to highlight changes. Additions are in green, modifications in yellow, and removals in red.

If no changes are applied during a stage, that is usually an indication of a misconfiguration or undesired behavior.

The --inspect flag should not be used in production, as the calculation of changes between pipeline stages negatively impacts Promtail’s performance.

Pipe data to Promtail

Promtail supports piping data for sending logs to Loki (via the flag --stdin). This is a very useful way to troubleshooting your configuration. Once you have Promtail installed you can for instance use the following command to send logs to a local Loki instance:

cat my.log | promtail --stdin  --client.url

You can also add additional labels from command line using:

cat my.log | promtail --stdin  --client.url --client.external-labels=k1=v1,k2=v2

This will add labels k1 and k2 with respective values v1 and v2.

In pipe mode Promtail also support file configuration using --config.file, however do note that positions config is not used and only the first scrape config is used.

static_configs: can be used to provide static labels, although the targets property is ignored.

If you don’t provide any scrape_config: a default one is used which will automatically adds the following default labels: {job="stdin",hostname="<detected_hostname>"}.

For example you could use this config below to parse and add the label level on all your piped logs:

  - url: http://localhost:3100/loki/api/v1/push

- job_name: system
  - regex:
      expression: '(level|lvl|severity)=(?P<level>\\w+)'
  - labels:
  - labels:
      job: my-stdin-logs
cat my.log | promtail --config.file promtail.yaml

A tailed file is truncated while Promtail is not running

Given the following order of events:

  1. Promtail is tailing /app.log
  2. Promtail current position for /app.log is 100 (byte offset)
  3. Promtail is stopped
  4. /app.log is truncated and new logs are appended to it
  5. Promtail is restarted

When Promtail is restarted, it reads the previous position (100) from the positions file. Two scenarios are then possible:

  • /app.log size is less than the position before truncating
  • /app.log size is greater than or equal to the position before truncating

If the /app.log file size is less than the previous position, then the file is detected as truncated and logs will be tailed starting from position 0. Otherwise, if the /app.log file size is greater than or equal to the previous position, Promtail can’t detect it was truncated while not running and will continue tailing the file from position 100.

Generally speaking, Promtail uses only the path to the file as key in the positions file. Whenever Promtail is started, for each file path referenced in the positions file, Promtail will read the file from the beginning if the file size is less than the offset stored in the position file, otherwise it will continue from the offset, regardless the file has been truncated or rolled multiple times while Promtail was not running.

Loki is unavailable

For each tailing file, Promtail reads a line, process it through the configured pipeline_stages and push the log entry to Loki. Log entries are batched together before getting pushed to Loki, based on the max batch duration client.batch-wait and size client.batch-size-bytes, whichever comes first.

In case of any error while sending a log entries batch, Promtail adopts a “retry then discard” strategy:

  • Promtail retries to send log entry to the ingester up to max_retries times
  • If all retries fail, Promtail discards the batch of log entries (which will be lost) and proceeds with the next one

You can configure the max_retries and the delay between two retries via the backoff_config in the Promtail config file:

      min_period: 100ms
      max_period: 10s
      max_retries: 10

The following table shows an example of the total delay applied by the backoff algorithm with min_period: 100ms and max_period: 10s:

RetryMin delayMax delayTotal min delayTotal max delay

Log entries pushed after a Promtail crash / panic / abruptly termination

When Promtail shuts down gracefully, it saves the last read offsets in the positions file, so that on a subsequent restart it will continue tailing logs without duplicates neither losses.

In the event of a crash or abruptly termination, Promtail can’t save the last read offsets in the positions file. When restarted, Promtail will read the positions file saved at the last sync period and will continue tailing the files from there. This means that if new log entries have been read and pushed to the ingester between the last sync period and the crash, these log entries will be sent again to the ingester on Promtail restart.

If Loki is not configured to accept out-of-order writes, Loki will reject all log lines received in what it perceives is out of order. If Promtail happens to crash, it may re-send log lines that were sent prior to the crash. The default behavior of Promtail is to assign a timestamp to logs at the time it read the entry from the tailed file. This would result in duplicate log lines being sent to Loki; to avoid this issue, if your tailed file has a timestamp embedded in the log lines, a timestamp stage should be added to your pipeline.