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The cosmossdk.io/log/v2 package provides a zerolog logging implementation for the Cosmos SDK and Cosmos SDK modules. To use a logger wrapping an instance of the standard library’s log/slog package, use cosmossdk.io/log/slog.

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OpenTelemetry

The v2 release of log adds contextual methods to the Logger interface. This allows logs to be correlated with traces. 
// Logger is the Cosmos SDK logger interface.
type Logger interface {
	// InfoContext is like Info but extracts trace context from ctx for correlation.
	InfoContext(ctx context.Context, msg string, keyVals ...any)

	// WarnContext is like Warn but extracts trace context from ctx for correlation.
	WarnContext(ctx context.Context, msg string, keyVals ...any)

	// ErrorContext is like Error but extracts trace context from ctx for correlation.
	ErrorContext(ctx context.Context, msg string, keyVals ...any)

	// DebugContext is like Debug but extracts trace context from ctx for correlation.
	DebugContext(ctx context.Context, msg string, keyVals ...any)
}

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Example

Below is an example of how logs can be correlated with traces. In Grafana, clicking on the UpdateBalance trace will show the “Hello World” log, as the span context was properly threaded to the logger’s InfoContext call. 
func (k Keeper) UpdateBalance(ctx sdk.Context, addr sdk.AccAddress, coins sdk.Coins) error {
	ctx, span := ctx.StartSpan(tracer, "UpdateBalance")
	defer span.End()
	
	logger := ctx.Logger()
	logger.InfoContext(ctx, "Hello World")
	
	// ... other logic ...
	
	return nil
}

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otelslog

If your OpenTelemetry configuration contained a logger provider, the Cosmos SDK will automatically provision a MultiLogger, logging to both stdout via the existing zerolog wrapper, and to the configured logger provider via otelslog.

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Log Correlation with Tempo and Loki

To get log correlation with Tempo and Loki, simply scrape the logs in JSON format from stdout/stderr and forward them to Loki. Then, configure Tempo with the following settings: In the Trace to logs section:
  • Set data source to Loki if not already
  • Set Span start time shift to a reasonable amount of time (+/-1m)
  • Set the custom query to: {${__tags}} | trace_id = "${__trace.traceId}"

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Future Considerations

OpenTelemetry implements log bridges, which automatically forward logs to the configured logger provider. For example, OpenTelemetry has bridges for zap and slog, but currently, not zerolog. A zerolog bridge is possible, however it is blocked by an open PR in the zerolog repo: https://github.com/rs/zerolog/pull/682 If this bridge is added to the core otel stack, this should be the preferred solution to log exporting instead of a MultiLogger. More info: https://github.com/open-telemetry/opentelemetry-go-contrib/issues/5969