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Logfmt Log Formatting

Your log data must not contain any personal data, such as names, email addresses, etc. It is your responsibility to ensure this data is filtered out before being sent to AppSignal, and when identifying a person is necessary that your application uses alternative forms of identification such as a user ID, hash, or pseudonym.

Logfmt is a log format that is easy for humans to read and write, and is supported by AppSignal. It was initially used as the format for the Heroku Logdrains, and is now used by many other logging providers.

Most of our platforms and endpoints support the Logfmt message format, which we'll describe in better details below

Format

Logfmt is a key-value format, where each key-value pair is separated by a space. The key and value are separated by an equals sign (=). The key-value pairs are appended to the message at the end of the log line.

Any key-value pairs that do not follow the Logfmt format are appended to the message of the log line.

Examples

The following examples show how AppSignal parses logs in the Logfmt format:

Message with attributes

shell
this is foo=bar duration=10 a value="with spaces" message

This will be parsed to a log line with the message "this is a message", and the following attributes: { "foo": "bar", "duration": 10, "value": "with spaces"}

Message with override

shell
this is foo=bar a duration=10 message message="I am a message"

This will be parsed to a log line with the message "I am a message", and the following attributes: { "foo": "bar", "duration": 10 }.

The rest of the original message is ignored, in favor of the message key.

Attributes only

shell
foo=bar duration=100

This will be parsed to a log line with the message "foo=bar duration=100", and the following attributes: { "foo": "bar", "duration": 10 }

Formatting Rules

  • Keys must be alphanumeric.
  • Keys cannot contain spaces, but may contain fullstops (.), underscores (_) and hyphens (-).
  • Keys may not contain more than 50 characters
  • Values must start with an alphanumeric character, and may not be longer than 100 characters
  • No more than 25 attributes per log message.

Key value pairs that do not follow these rules are appended to the message of a log.

Key behavior:

  • If a key does not have a value, it will be appended to the message field.
  • If a key-value pair has a key with the name of msg or message the value of this key will be parsed as the log message.
  • If no message/msg key is available, the entire log line will be considered the message.