Structured logging extensions for Golang using samber/slog-**** packages — multi-handler pipelines (slog-multi), log sampling (slog-sampling), attribute formatting (slog-formatter), HTTP middleware (slog-fiber, slog-gin, slog-chi, slog-echo), and backend routing (slog-datadog, slog-sentry, slog-loki

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](https://github.com/samber/slog multi) — handler composition [github.com/samber/slog sampling](https://github.com/samber/slog sampling) — throughput control [github.com/samber/slog formatter](https://github.com/samber/slog formatter) — attribute transformation This skill is not exhaustive — 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 ), preferred 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. 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 Library Purpose Key constructors slog multi Handler composition Fanout , Router , FirstMatch , Failover , Pool , Pipe slog sampling Throughput control UniformSamplingOption , ThresholdSamplingOption , AbsoluteSamplingOption , CustomSamplingOption slog formatter Attribute transforms PIIFormatter , ErrorFormatter , FormatByType[T] , FormatByKey , FlattenFormatterMiddleware slog multi — Handler Composition Six composition patterns, each for a different routing need: Pattern Behavior Latency impact Fanout(handlers...) Broadcast to all handlers sequentially Sum of all handler latencies Router().Add(h, predicate).Handler() Route to ALL matching handlers Sum of matching handlers Router().Add(...).FirstMatch().Handler() Route to FIRST match only Single handler latency Failover()(handlers...) Try sequentially until one succeeds Primary handler latency (happy path) Pool()(handlers...) Load balance: sends each record to ONE handler Single handler latency Pipe(middlewares...).Handler(sink) Middleware chain before sink Middleware overhead + sink Built in predicates: LevelIs , LevelIsNot , MessageIs , MessageIsNot , MessageContains , MessageNotContains , AttrValueIs , AttrKindIs . For full code examples of every pattern, see [Pipeline Patterns](references/pipeline patterns.md). slog sampling — Throughput Control Strategy Behavior Best for Uniform Drop fixed % of all records Dev/staging noise reduction Threshold Log first N per interval, then sample at rate R Production — preserves initial visibility Absolute Cap at N records per interval globally Hard cost control Custom User function returns sample rate per record Level 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. Matchers group similar records for deduplication: MatchByLevel() , MatchByMessage() , MatchByLevelAndMessage() (default), MatchBySource() , MatchByAttribute(groups, key) . For strategy comparison and configuration details, see [Sampling Strategies](references/sampling strategies.md). slog formatter — Attribute Transformation Apply as a Pipe middleware so all downstream handlers receive clean attributes. 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 . For framework specific setup, see [HTTP Middlewares](references/http middlewares.md). Backend Sinks All follow the Option{}.NewXxxHandler() constructor pattern. Category Packages Cloud slog datadog , slog sentry , slog loki , slog graylog Messaging slog kafka , slog fluentd , slog logstash , slog nats Notification slog slack , slog telegram , slog webhook Storage slog parquet Bridges slog 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](references/backend handlers.md). Common Mistakes Mistake Why it fails Fix Sampling after formatting Wastes CPU formatting records that get dropped Place sampling as outermost handler Fanout to many synchronous handlers Blocks caller — latency is sum of all handlers Use Pool() for concurrent dispatch Missing shutdown flush on batch handlers Buffered logs lost on shutdown defer handler.Stop(ctx) (Datadog), defer lokiClient.Stop() (Loki), defer writer.Close() (Kafka) Router without default/catch all handler Unmatched records silently dropped Add a handler with no predicate as catch all AttrFromContext without HTTP middleware Context has no request attributes to extract Install slog gin / echo / fiber / chi middleware first Using Pipe with no middleware No op wrapper adding per record overhead Remove 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](https://github.com/samber/slog multi/issues), [slog sampling/issues](https://github.com/samber/slog sampling/issues)).

golang-samber-slog