JSON Logging

Structured logging is essential for modern observability stacks. Logly-Zig supports JSON output out of the box.

Enabling JSON

You can enable JSON output globally or for specific sinks.

Global JSON

var config = logly.Config.default();
config.json = true;
logger.configure(config);

Per-Sink JSON

_ = try logger.add(.{  // Short alias for addSink()
    .path = "logs/app.json",
    .json = true,
});

JSON Structure

The JSON output contains the following fields:

{
  "timestamp": 1710930645000,
  "level": "INFO",
  "module": "main",
  "message": "Application started",
  "service": "my-service",
  "region": "us-east-1",
  "trace_id": "trace-12345",
  "span_id": "span-6789",
  "user_id": "12345",
  "request_id": "abc-123"
}

• timestamp: Unix timestamp in milliseconds
• level: Log level string (uses custom level name if set)
• service: Service name (if distributed.service_name is set)
• trace_id: Distributed Trace ID (if present)
• span_id: Distributed Span ID (if present)
• module: Module name (if enabled)
• function: Function name (if enabled)
• file: Filename (if enabled)
• line: Line number (if enabled)
• message: The log message
• Context variables are added as top-level fields

Custom Levels in JSON

Custom levels display their actual names in JSON output:

try logger.addCustomLevel("audit", 35, "35");
try logger.custom("audit", "Security event", @src());

Output:

{
  "timestamp": 1710930645000,
  "level": "AUDIT",
  "message": "Security event"
}

Pretty Printing

For development or debugging, you can enable pretty printing to format the JSON with indentation.

config.pretty_json = true;