golang-samber-slog

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, slog-syslog, slog-logstash, slog-graylog...). Apply when using or adopting slog, or when the codebase already imports any github.com/samber/slog-* package.

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From: samber/cc-skills-golang

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Skill instructions

Rendered from the published SKILL.md, 181 lines.

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 — 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.

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

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

// 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

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.

// 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.

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.

// Gin with filters — skip health checks
router.Use(sloggin.NewWithConfig(logger, sloggin.Config{
    DefaultLevel:     slog.LevelInfo,
    ClientErrorLevel: slog.LevelWarn,
    ServerErrorLevel: slog.LevelError,
    WithRequestBody:  true,

Show the whole skill (61 more lines)

Filters: []sloggin.Filter{
    sloggin.IgnorePath("/health", "/metrics"),
},

}))


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)).

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Source repository samber/cc-skills-golang
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