Skip to main content

Introducing structured_logger for Flutter

· One min read
Altamir
Altamir
Maintainer

Logging in Flutter apps is often a mix of print, debugPrint, and ad-hoc string concatenation. It works day to day, but makes search, alerts, and correlation harder as the app grows.

The problem with unstructured logs​

Imagine debugging a production issue with this line:

User 42 failed to checkout after 3 retries

Who is user 42? What was the error? Which attempt failed? Without separate fields, every observability tool needs fragile free-text parsing.

The structured approach​

With structured_logger, you separate the message template from properties:

await logger.log(
  'User {userId} failed checkout after {retries} retries: {error}',
  level: LogLevel.error,
  data: {
    'userId': 42,
    'retries': 3,
    'error': 'payment_timeout',
  },
);

The template stays stable for grouping; data becomes indexable in backends like Seq.

Sinks: one event, many destinations​

The package's strength is the sink model. The same event can go to:

  • Terminal (SimpleLineSink) — quick reading during development
  • DevTools (DefaultSink) — inspection in the Dart developer log
  • Seq (SinkSeq) — centralization in production
final logger = StructureLogger();
logger.addSink(SimpleLineSink());
logger.addSink(SinkSeq('https://seq.example.com', apiKey: apiKey));

Next steps​