GelF

GELF, or Graylog Extended Log Format, is a log format designed to enable centralized, structured logging. It is an open format created by Graylog for sending log messages to Graylog servers and other systems that understand GELF. The format is JSON-based and structured to carry both essential metadata and application data in a way that is easy to parse and search.

A GELF message typically includes required fields such as version, host, and short_message. Optional fields include full_message, timestamp, level, and facility. In addition, any number of custom fields may be included, provided they are prefixed with an underscore, for example _user_id or _transaction_id. The timestamp is a UNIX epoch value with fractional seconds allowed, and level follows the syslog severity scale.

GELF messages can be carried over multiple transports, most commonly UDP, TCP, or HTTP. UDP is efficient for high-volume, low-latency scenarios, while TCP and HTTP inputs are preferred when reliability and firewall traversal are priorities. GELF also supports compression and message chunking to accommodate large payloads: messages may be compressed to reduce bandwidth and large messages can be split into chunks and reassembled by the receiver.

Adoption of GELF is widespread in modern logging stacks, particularly where Graylog is used, and it has influenced other log collectors to support structured, JSON-based logging. The format emphasizes machine readability and structured data to facilitate filtering, searching, and visualization in dashboards and analytics tools.