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Message templates

The message parameter of StructureLogger.log() is a message template: text with {name} placeholders filled from data.

Syntax​

await logger.log(
  'Payment {status} for order {orderId} — amount {amount}',
  level: LogLevel.info,
  data: {
    'status': 'approved',
    'orderId': 'ORD-991',
    'amount': 149.90,
  },
);
  • The template is stored in LogModel.mt (@mt in CLEF).
  • Properties live in LogModel.data, separate from the rendered message.
  • Tools like Seq index data for search and dashboards.

Interpolation in SimpleLineSink​

SimpleLineSink uses the regex \{(.*?)\} to replace each placeholder with the corresponding value in data:

// Template: "Hello {name}"
// data: {name: "Ana"}
// Result: "Hello Ana"

Values are converted with .toString(). Missing keys produce an empty substitution.

Best practices​

  1. Use descriptive names — {userId} instead of {id} when ambiguous.
  2. Keep templates stable — easier grouping and alerts in structured backends.
  3. Put variable data in data — avoid manual string concatenation in the template.
  4. Avoid sensitive data — passwords and tokens should not appear in data if logs are exported.

CLEF compatibility​

The model follows Serilog/CLEF conventions:

{
  "@t": "2026-06-25T10:00:00.000",
  "@mt": "Payment {status} for order {orderId}",
  "@l": "info",
  "status": "approved",
  "orderId": "ORD-991"
}

In SinkSeq, data properties are merged into the CLEF event while @t, @mt, @l, and DeviceIdentifier are reserved and cannot be accidentally overwritten by keys in data.