Persona: You are a Go logging architect. You design log pipelines where every record flows through the right handlers — sampling drops noise early, formatters strip PII before records leave the process, and routers send errors to Sentry while info goes to Loki.

samber/slog-**** — Structured Logging Pipeline for Go#

20+ composable slog.Handler packages for Go 1.21+. Three core pipeline libraries plus HTTP middlewares and backend sinks that all implement the standard slog.Handler interface.

Official resources:

• github.com/samber/slog-multi — handler composition
• github.com/samber/slog-sampling — throughput control
• github.com/samber/slog-formatter — attribute transformation

This skill is not exhaustive. Please refer to library documentation and code examples for more information. For Go package docs, symbols, versions, importers, and known vulnerabilities, → See samber/cc-skills-golang@golang-pkg-go-dev skill ( godig) — prefer it over Context7 for Go package facts. To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See samber/cc-skills-golang@golang-gopls skill ( gopls). Context7 remains a fallback for docs not indexed on pkg.go.dev.

The Pipeline Model#

Every samber/slog pipeline follows a canonical ordering. Records flow left to right — place sampling first to drop early and avoid wasting CPU on records that never reach a sink.

text

record → [Sampling] → [Pipe: trace/PII] → [Router] → [Sinks]

Order matters: sampling before formatting saves CPU. Formatting before routing ensures all sinks receive clean attributes. Reversing this wastes work on records that get dropped.

Core Libraries#

LibraryPurposeKey constructorsslog-multiHandler compositionFanout, Router, FirstMatch, Failover, Pool, Pipe``slog-samplingThroughput controlUniformSamplingOption, ThresholdSamplingOption, AbsoluteSamplingOption, CustomSamplingOption``slog-formatterAttribute transformsPIIFormatter, ErrorFormatter, FormatByType[T], FormatByKey, FlattenFormatterMiddleware

slog-multi — Handler Composition#

Six composition patterns, each for a different routing need:

PatternBehaviorLatency impactFanout(handlers...)Broadcast to all handlers sequentiallySum of all handler latenciesRouter().Add(h, predicate).Handler()Route to ALL matching handlersSum of matching handlersRouter().Add(...).FirstMatch().Handler()Route to FIRST match onlySingle handler latencyFailover()(handlers...)Try sequentially until one succeedsPrimary handler latency (happy path)Pool()(handlers...)Load-balance: sends each record to ONE handlerSingle handler latencyPipe(middlewares...).Handler(sink)Middleware chain before sinkMiddleware overhead + sinkgo

// Route errors to Sentry, all logs to stdout
logger := slog.New(
    slogmulti.Router().
        Add(sentryHandler, slogmulti.LevelIs(slog.LevelError)).
        Add(slog.NewJSONHandler(os.Stdout, nil)).
        Handler(),
)

Built-in predicates: LevelIs, LevelIsNot, MessageIs, MessageIsNot, MessageContains, MessageNotContains, AttrValueIs, AttrKindIs.

For full code examples of every pattern, see Pipeline Patterns.

slog-sampling — Throughput Control#

StrategyBehaviorBest forUniformDrop fixed % of all recordsDev/staging noise reductionThresholdLog first N per interval, then sample at rate RProduction — preserves initial visibilityAbsoluteCap at N records per interval globallyHard cost controlCustomUser function returns sample rate per recordLevel-aware or time-aware rules

Sampling MUST be the outermost handler in the pipeline — placing it after formatting wastes CPU on records that get dropped.

go

// Threshold: log first 10 per 5s, then 10% — errors always pass through via Router
logger := slog.New(
    slogmulti.
        Pipe(slogsampling.ThresholdSamplingOption{
            Tick: 5 * time.Second, Threshold: 10, Rate: 0.1,
        }.NewMiddleware()).
        Handler(innerHandler),
)

Matchers group similar records for deduplication: MatchByLevel(), MatchByMessage(), MatchByLevelAndMessage() (default), MatchBySource(), MatchByAttribute(groups, key).

For strategy comparison and configuration details, see Sampling Strategies.

slog-formatter — Attribute Transformation#

Apply as a Pipe middleware so all downstream handlers receive clean attributes.

go

logger := slog.New(
    slogmulti.Pipe(slogformatter.NewFormatterMiddleware(
        slogformatter.PIIFormatter("user"),          // mask PII fields
        slogformatter.ErrorFormatter("error"),       // structured error info
        slogformatter.IPAddressFormatter("client"),  // mask IP addresses
    )).Handler(slog.NewJSONHandler(os.Stdout, nil)),
)

Key formatters: PIIFormatter, ErrorFormatter, TimeFormatter, UnixTimestampFormatter, IPAddressFormatter, HTTPRequestFormatter, HTTPResponseFormatter. Generic formatters: FormatByType[T], FormatByKey, FormatByKind, FormatByGroup, FormatByGroupKey. Flatten nested attributes with FlattenFormatterMiddleware.

HTTP Middlewares#

Consistent pattern across frameworks: router.Use(slogXXX.New(logger)).

Available: slog-gin, slog-echo, slog-fiber, slog-chi, slog-http (net/http).

All share a Config struct with: DefaultLevel, ClientErrorLevel, ServerErrorLevel, WithRequestBody, WithResponseBody, WithUserAgent, WithRequestID, WithTraceID, WithSpanID, Filters.

go

// Gin with filters — skip health checks
router.Use(sloggin.NewWithConfig(logger, sloggin.Config{
    DefaultLevel:     slog.LevelInfo,
    ClientErrorLevel: slog.LevelWarn,
    ServerErrorLevel: slog.LevelError,
    WithRequestBody:  true,
    Filters: []sloggin.Filter{
        sloggin.IgnorePath("/health", "/metrics"),
    },
}))

For framework-specific setup, see HTTP Middlewares.

Backend Sinks#

All follow the Option{}.NewXxxHandler() constructor pattern.

CategoryPackagesCloudslog-datadog, slog-sentry, slog-loki, slog-graylogMessagingslog-kafka, slog-fluentd, slog-logstash, slog-natsNotificationslog-slack, slog-telegram, slog-webhookStorageslog-parquetBridgesslog-zap, slog-zerolog, slog-logrus

Batch handlers require graceful shutdown — slog-datadog, slog-loki, slog-kafka, and slog-parquet buffer records internally. Flush on shutdown (e.g., handler.Stop(ctx) for Datadog, lokiClient.Stop() for Loki, writer.Close() for Kafka) or buffered logs are lost.

For configuration examples and shutdown patterns, see Backend Handlers.

Common Mistakes#

MistakeWhy it failsFixSampling after formattingWastes CPU formatting records that get droppedPlace sampling as outermost handlerFanout to many synchronous handlersBlocks caller — latency is sum of all handlersUse Pool() for concurrent dispatchMissing shutdown flush on batch handlersBuffered logs lost on shutdowndefer handler.Stop(ctx) (Datadog), defer lokiClient.Stop() (Loki), defer writer.Close() (Kafka)Router without default/catch-all handlerUnmatched records silently droppedAdd a handler with no predicate as catch-allAttrFromContext without HTTP middlewareContext has no request attributes to extractInstall slog-gin/ echo/ fiber/ chi middleware firstUsing Pipe with no middlewareNo-op wrapper adding per-record overheadRemove Pipe() if no middleware needed

Performance Warnings#

• Fanout latency = sum of all handler latencies (sequential). With 5 handlers at 10ms each, every log call costs 50ms. Use Pool() to reduce to max(latencies)
• Pipe middleware adds per-record function call overhead — keep chains short (2-4 middlewares)
• slog-formatter processes attributes sequentially — many formatters compound. For hot-path attribute formatting, prefer implementing slog.LogValuer on your types instead
• Benchmark your pipeline with go test -bench before production deployment

Diagnose: measure per-record allocation and latency of your pipeline and identify which handler in the chain allocates most.

Best Practices#

1. Sample first, format second, route last — this canonical ordering minimizes wasted work and ensures all sinks see clean data
2. Use Pipe for cross-cutting concerns — trace ID injection and PII scrubbing belong in middleware, not per-handler logic
3. Test pipelines with slogmulti.NewHandleInlineHandler — assert on records reaching each stage without real sinks
4. Use AttrFromContext to propagate request-scoped attributes from HTTP middleware to all handlers
5. Prefer Router over Fanout when handlers need different record subsets — Router evaluates predicates and skips non-matching handlers

Cross-References#

• → See samber/cc-skills-golang@golang-observability skill for slog fundamentals (levels, context, handler setup, migration)
• → See samber/cc-skills-golang@golang-error-handling skill for the log-or-return rule
• → See samber/cc-skills-golang@golang-security skill for PII handling in logs
• → See samber/cc-skills-golang@golang-samber-oops skill for structured error context with samber/oops

If you encounter a bug or unexpected behavior in any samber/slog-* package, open an issue at the relevant repository (e.g., slog-multi/issues, slog-sampling/issues).