Logfmts

Logfmt is a lightweight, human- and machine-readable log format used to emit structured log lines as a sequence of key=value pairs. It originated in the Go and Unix logging ecosystems as a simple alternative to JSON for structured logs, emphasizing readability and streaming-friendly parsing.

Format details: Each log line consists of space-separated key=value pairs. Values containing spaces or special characters are typically quoted, and quotes within values may be escaped. Keys are plain identifiers (for example time, level, msg, source, and event). The exact set of keys is application-defined, but a common pattern includes a timestamp (time or ts) and a severity level.

Tooling and interoperability: Logfmt is supported by a range of libraries across languages, including Go, Python, and Java. It is commonly used with log shippers and aggregation systems that process line-oriented logs because it can be parsed incrementally in streaming pipelines.

Advantages and limitations: The format is compact, easy to read by humans, and straightforward to parse without a full JSON parser. It is well suited for adding structured fields to log lines in high-throughput environments. Its main limitation is a flat, non-nested structure, which makes representing complex data harder than with JSON or XML, and relying on quoting/escaping rules can introduce parsing edge cases.

Example: 2024-06-01T12:00:00Z level=info msg="service started" component=auth user=alice

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