JSON 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.

This documentation explains how AppSignal supports log messages formatted in JSON.


If you specify JSON as your log format, AppSignal will parse incoming log message as key-value pairs. Each key-value pair will be stored as an attribute on the log line, allowing you to filter and search for specific log lines based on their attributes.


The following example shows how AppSignal parses a log message in the JSON format.

{ "timestamp": "2022-06-02T04:17:25.783Z", "group": "organisations", "severity": "warn", "hostname": "frontend1", "message": "This is a test message", "org": "appsignal", "step": 1, "seen_terms": true, "entries": 10.01, "user": { "id": 1, "remote_id": "123" } }


The above JSON will be parsed with the message "this is a test message". You can either use the msg or message key to override the message.

If no msg or message key is available, the entire log line will be used as the message.


The log attributes from the platform or integration (such as hostname, timestamp and group set by e.g. the Heroku platform) can be overriden with the values from the JSON message.

Attriubute KeyType

The severity can contain one of the following values: unknown, trace, debug, info, notice, warn, error, critical, alert and fatal.

This is an excellent way to control the severity of your log lines from within application, regardless of the (usually defaulted info) severity set by the platform or integration.


Any other key/value pair is added as a searchable attributes:

Attriubute KeyTypeValue

Formatting Rules

Keys must be alphanumeric strings. Array values are not supported.

Key behavior:

  • 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.
  • Keys must be alphanumeric.
  • Keys cannot contain spaces, but may contain fullstops (.), underscores (_) and hyphens (-).
  • Keys may not contain more than 50 characters

Value behavior:

  • Values must not be longer than 255 characters
  • No more than 25 attributes per log message.
  • Values may contain nested Objects, but not Arrrays.
  • Values may be of the following types:
    • String
    • Integer (e.g. 10)
    • Double (e.g. 10.01)
    • Boolean (true/false)
    • Object (e.g. { "foo": "bar" })

For nested objects, the attributes will be flattened, separated by a . character.

Key value pairs that do not follow these rules are appended to the message of a log, unless a message or msg key is provided.