Structured logging in Go with log/slog

Go announced log/slog in 1.21, bringing structured logging into the standard library. This isn’t a tour of the slog package. It’s a simple guide on how I chose to log in my Go applications.

Leveled Logging

When choosing to log a message, always provide a level. Here’s my guidance for consistent logging style across microservices:

CRITICAL: Any error forcing a shutdown to prevent data loss.

ERROR: Unexpected or fatal errors for the current operation, but not the service itself. Requires immediate attention in monitoring.

WARN: A potential problem that isn’t crippling to the service.

INFO: General operational information. Helps understand what the application is doing.

DEBUG: Detailed information for developers and SREs. Can be highly verbose.

Structured Log Format

The simplest base format I’ve successfully used:

{
  "time": "2024-03-23T01:52:50.409805437Z",
  "level": "INFO",
  "msg": "hello",
  "source": {
    "function": "LogHello",
    "file": "hello.go",
    "line": 14
  }
}

• time: UTC timestamp. Available by default; choose your format.
• level: Defaults to ERROR in this package.
• msg: Lowercase freeform string, no interpolation.
• source: Function and line. I prefer the shortened form of both fields.

Separate Attributes From Log Messages

In the past I formatted attributes directly into the message string:

{ "msg": "hello foo=bar" }

With log/slog, I keep all attributes separate from the message. This makes reading logs easier and makes Grafana monitoring significantly more valuable, especially with RequestID tracing.

{
  "time": "2024-03-23T01:52:50.409805437Z",
  "level": "INFO",
  "msg": "hello",
  "source": { "function": "LogHello", "file": "hello.go", "line": 14 },
  "foo": "bar"
}

Context Logger

If you integrate with tools like OpenTelemetry and export data to Grafana, always log with context. This allows span/trace data to propagate across context boundaries for distributed tracing.

slog.InfoContext(ctx, "hello", "foo", "bar")

The context carries the trace and span IDs, so every log line is automatically associated with the active trace in Grafana. No extra work per log call.