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​

• Read the quick start guide
• Explore sink architecture
• Contribute on GitHub

Why Seq?​

Seq offers powerful search over structured properties, dashboards, and alerts. With CLEF, each log is a JSON document with @t, @mt, @l, and arbitrary properties — exactly what LogModel already produces.

Basic usage​

final seqSink = SinkSeq(
  'https://seq.example.com',
  apiKey: 'your-api-key',
  deviceIdentifier: 'checkout-app',
);

logger.addSink(seqSink);

await logger.log(
  'Order {orderId} placed',
  level: LogLevel.info,
  data: {'orderId': 'ORD-991'},
);

Improvements in 0.1.1 and 0.1.2​

Later releases fixed important production details:

• Content-Type: application/vnd.serilog.clef header on POST
• CLEF reserved fields (@t, @mt, @l, DeviceIdentifier) protected from data overwrites
• SinkSeq.close() to release the internal HTTP pool
• URL validation in release builds
• Trailing slash URL normalization

Testability​

The constructor accepts an injected http.Client — essential for unit tests without network:

SinkSeq(
  'https://seq.test',
  client: mockClient,
);

Learn more​

See the full Seq integration guide in the documentation.