slogdriver

slog based logging library optimized for Cloud Logging. This packaged inspired by Zapdriver and zerodriver.

What is this package?

This package provides simple structured logger optimized for Cloud Logging based on slog.

Requirements

Go 1.21 or above

Usage

Initialize a logger.

logger := slogdriver.New(os.Stdout, slogdriver.HandlerOptions{})

This logger is *slog.Logger. Then, write by using slog API.

logger.Info("Hello World!", slog.String("key", "value"))

GCP specific fields

If your log follows LogEntry format, you can query logs or create metrics alert easier and efficiently on GCP Cloud Logging console.

Severity

You can use slog.Level(Debug, Info, Warn, Error). And this library prepare all severities for Cloud Logging.

• DEFAULT
• DEBUG
• INFO
• NOTICE
• WARNING
• ERROR
• CRITICAL
• ALERT
• EMERGENCY

logger := slogdriver.New(os.Stdout, slogdriver.HandlerOptions{
  Level: slogdriver.LevelDefault,
})
logger.Log(context.Background(), slogdriver.LevelEmergency, "emergency msg")

HTTP request

To log HTTP related metrics and information, you can create slog.Attr with the following function.

var req *http.Request
var res *http.Response
logger.Info("Http Request finished", slogdriver.MakeHTTPAttr(req, res))

The following fields needs to be set manually:

• ServerIP
• Latency
• CacheLookup
• CacheHit
• CacheValidatedWithOriginServer
• CacheFillBytes

Using these feature, you can log HTTP related information as follows,

p := slogdriver.MakeHTTPPayload(req, res)
p.Latency = time.Since(start)
logger.Info("http finished", slogdriver.MakeHTTPAttrFromHTTPPayload(p))
// Or, you can create attr manually
logger.Info("http finished", slog.Any(slogdriver.HTTPKey, p))

Trace context

import "go.opentelemetry.io/otel"

// Set projectId or, you can set environment GOOGLE_CLOUD_PROJECT
logger := slogdriver.New(os.Stdout, slogdriver.HandlerOptions{ProjectID: "YOUR_PROJECT_ID"})

ctx, span := otel.Tracer(traceName).Start(context.Background(), "span")
defer span.End()

logger.InfoContext(ctx, "Hello World")
// got:
// {"severity":"INFO","message":"Hello World","logging.googleapis.com/trace":"projects/YOUR_PROJECT_ID/traces/00000000000000000000000000000000","logging.googleapis.com/spanId":"0000000000000000","logging.googleapis.com/trace_sampled":true}

If you use go.opentelemetry.io/otel, it is able to set traceId with request context like the below:

import (
	gcppropagator "github.com/GoogleCloudPlatform/opentelemetry-operations-go/propagator"
	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
	"go.opentelemetry.io/otel/propagation"
)

otel.SetTextMapPropagator(
    propagation.NewCompositeTextMapPropagator(
        gcppropagator.CloudTraceOneWayPropagator{},
        propagation.TraceContext{},
    ),
)
handler := NewServeMux()
handler.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
	logger.InfoContext(r.Context(), "Hello World") // This log should include trace information.
})
handler := otelhttp.NewHandler(handler, traceName)

You can see example for Cloud Run.

Labels

You can add any "labels" to your log as following:

logger.Info("", slog.Group(slogdriver.LabelKey, slog.String("label", "hoge")))

You can set common label like the follow:

logger = logger.With(slog.Group(slogdriver.LabelKey, slog.String("commonLabel", "hoge")))
logger.Info("Hello World", slog.Group(slogdriver.LabelKey, slog.String("label1", "fuga")))
// got:
// {"severity":"INFO","message":"Hello World","logging.googleapis.com/labels":{"commonLabel":"hoge","label1":"fuga"}}
logger.Warn("Hello World", slog.Group(slogdriver.LabelKey, slog.String("label2", "fuga")))
// got:
// {"severity":"WARNING","message":"Hello World","logging.googleapis.com/labels":{"commonLabel":"hoge","label2":"fuga"}}

Source Location

logger := slogdriver.New(os.Stdout, slogdriver.HandlerOptions{AddSource: true})
logger.Info("Hello World")
// {"severity":"INFO","message":"Hello World","logging.googleapis.com/sourceLocation":{"file":"/path/to/source.go","line":"12","function":"yourFunction"}}

TODO

• severity
• message
• httpRequest
• time, timestamp
• insertId
• labels
• operation
• sourceLocation
• spanId
• trace
• traceSampled

docs

https://cloud.google.com/logging/docs/structured-logging#special-payload-fields