strc

A simple tracing library. When OpenTelemetry is a bit too much. Features:

• Simple code instrumentation.
• Serialization into log/slog (structured logging library).
• Handler with multiple sub-handlers for further processing.
• Simple exporting slog handler for callback-based exporters.
• Adding "trace_id" root field to all logs for easy correlation between logs and traces.

Instrumentation

Add the following code to function you want to trace:

span, ctx := strc.Start(ctx, "span name")
defer span.End()

Optionally, additional "save points" can be inserted:

span.Event("an event")

Result

All results are stored in log/slog records. Each span creates one record with group named span with the following data:

• span.name: span name
• span.id: span ID
• span.parent: parent ID or 0000000 when there is no parent
• span.trace: trace ID
• span.event: event name (only on event)
• span.at: duration within a span (only on event)
• span.duration: trace duration (only when span ends)
• span.time: log time (can be enabled in exporter)

Spans end up in log sink too, for better readability, the following fields are added to the root namespace:

• msg: message in the form span XXX started or event XXX
• trace_id - trace ID (disable by setting strc.TraceIDFieldKey to empty string)
• build_id - build Git sha (disable by setting strc.BuildIDFieldKey to empty string)

Propagation

A simple HTTP header-based propagation API is available. Note this is not meant to be used directly, there is HTTP middleware and client wrapper available:

// create new trace id
id := strc.NewTraceID()

// create a new context with trace id value
ctx := strc.WithContext(context.Background(), id)

// fetch the id from the context
id := strc.TraceIDFromContext(ctx)

// add the id from context to a HTTP request
strc.AddTraceIDHeader(ctx, request)

// fetch the id from a request
id := TraceIDFromRequest(request)

Middleware

A standard library HTTP middleware is provided for trace propagation

middleware := strc.NewMiddleware(logger)
mux := http.NewServeMux()
mux.Handle("/", middleware(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
	// ...
})))

The library does not provide native Echo middleware, but a standard middleware can be easily used from Echo:

middleware := strc.NewMiddleware(logger)
e.Use(echo.WrapMiddleware(middleware))

The strc.Middeware can be optionally configured to log detailed debug information like request or reply HTTP headers or even full body. This is turned off by default, see strc.MiddlewareConfig for more info.

HTTP client

A TracingDoer type can be used to decorate HTTP clients adding necessary propagation automatically as long as tracing information is in the request context:

r, _ := http.NewRequestWithContext(ctx, http.MethodGet, "https://home.zapletalovi.com/", nil)
doer := strc.NewTracingDoer(http.DefaultClient)
doer.Do(r)

The TracingDoer can be optionally configured to log detailed debug information like request or reply HTTP headers or even full body. This is turned off by default, see strc.TracingDoerConfig for more info.

Example headers generated or parsed by HTTP client & middleware code:

X-Strc-Trace-ID: LOlIxiHprrrvHqD
X-Strc-Span-ID: VIPEcES.yuufaHI

Full example

package main

import (
	"context"
	"log/slog"

	"github.com/osbuild/logging/pkg/strc"
)

func subProcess(ctx context.Context) {
	span, ctx := strc.Start(ctx, "subProcess")
	defer span.End()

	span.Event("an event")
}

func process(ctx context.Context) {
	span, ctx := strc.Start(ctx, "process")
	defer span.End()

	subProcess(ctx)
}

func main() {
	span, ctx := strc.Start(context.Background(), "main")
	defer span.End()

	process(ctx)
}

Note the above example will print nothing as tracing is disabled by default, two things must be done. First, a slog destination logger must be set:

strc.SetLogger(slog.Default())

Second, the destination logger must have debug level handing enabled. The default logging level can be increased if needed:

strc.Level = slog.LevelInfo

Run the example with the following command:

go run github.com/osbuild/logging/internal/example_print/

Which prints something like (removed time and log level for readability):

span main started span.name=main span.id=pEnFDti span.parent=0000000 span.trace=SVyjVloJYogpNPq span.source=main.go:28
span process started span.name=process span.id=fRfWksO span.parent=pEnFDti span.trace=SVyjVloJYogpNPq span.source=main.go:18
span subProcess started span.name=subProcess span.id=gSouhiv span.parent=fRfWksO span.trace=SVyjVloJYogpNPq span.source=main.go:11
span subProcess event an event span.name=subProcess span.id=gSouhiv span.parent=fRfWksO span.trace=SVyjVloJYogpNPq span.event="an event" span.at=21.644µs span.source=main.go:14
span subProcess finished in 47.355µs span.name=subProcess span.id=gSouhiv span.parent=fRfWksO span.trace=SVyjVloJYogpNPq span.dur=47.355µs span.source=main.go:15
span process finished in 94.405µs span.name=process span.id=fRfWksO span.parent=pEnFDti span.trace=SVyjVloJYogpNPq span.dur=94.405µs span.source=main.go:22
span main finished in 285.246µs span.name=main span.id=pEnFDti span.parent=0000000 span.trace=SVyjVloJYogpNPq span.dur=285.246µs span.source=main.go:32

Exporting data

While the main goal of this library is just instrumenting and sending data into slog, a simple function callback exporter handler is provided by the package for quick collecting or exporting capabilities as well as multi-handler for chaining handlers which is useful to keep sending standard logging data to logging systems. It uses slog.Attr type as the data carrier:

package main

import (
	"context"
	"log/slog"
	"os"

	"github.com/osbuild/logging/pkg/strc"
)

func exportFunc(ctx context.Context, attrs []slog.Attr) {
	for _, attr := range attrs {
		println("exporting trace data", attr.Key, attr.Value.String())
	}
}

func main() {
	textHandler := slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{Level: slog.LevelDebug})
	exportHandler := strc.NewExportHandler(exportFunc)
	multiHandler := strc.NewMultiHandler(textHandler, exportHandler)

	logger := slog.New(multiHandler)
	slog.SetDefault(logger)
	strc.SetLogger(logger)

	span, _ := strc.Start(context.Background(), "main")
	defer span.End()
}

There is additional NewMultiHandlerCustom which allows adding custom attributes from context via a callback function. This is useful when additional correlation id (e.g. background job UUID) needs to be added to every single regular log record. The multi-handler creates the following new keys in the root element:

• trace_id - trace ID (disable by setting strc.TraceIDFieldKey to empty string)
• build_id - build Git sha (disable by setting strc.BuildIDFieldKey to empty string)

Run the example with the following command:

go run github.com/osbuild/logging/internal/example_export/

For the best performance, we a dedicated exporting handler should be written customized to the output format. For more info, see writing an slog handler.