logfmt

Logfmt is a simple, human-readable log format designed for structured logging. It represents each log event as a single line containing a set of key=value pairs, separated by whitespace. The format emphasizes readability and easy parsing in streaming pipelines, making it popular in microservice architectures and infrastructure tooling. Each line stands alone and typically records fields such as time, level, and message, though any arbitrary key can be included.

Syntax: Fields are pairs of key and value joined by an equals sign. Values can be unquoted when they do not contain spaces or special characters; when a value contains spaces or quotes, it is enclosed in double quotes, with backslash escaping supported inside quotes. The same line may include multiple fields; there is no imposed order or schema, and missing fields are allowed.

Usage and tooling: Logfmt is language-agnostic with libraries in several languages, including Go (for example, github.com/go-logfmt/logfmt) and bindings for Python, Java, and JavaScript. It is well-suited to parsing with standard text-processing tools and to piping through log-analysis systems. Common conventions use keys such as time or ts for timestamps, level for severity, msg or message for the human-readable content, and service or host as optional metadata.

Relation and suitability: It is often contrasted with JSON for simplicity and compactness, and with plain text logs for readability. Logfmt's flexible, schema-free nature makes it a pragmatic choice when logs originate from heterogeneous sources and need to be indexed or filtered efficiently.