slog: Handler chaining, fanout, routing, failover, load balancing...

Design workflows of slog handlers:

• Fanout: distribute log.Record to multiple slog.Handler in parallel
• Pipe: rewrite log.Record on the fly (eg: for privacy reasons)
• Router: forward log.Record to all matching slog.Handler
• Failover: forward log.Record to the first available slog.Handler
• Pool: increase log bandwidth by sending log.Record to a pool of slog.Handler
• RecoverHandlerError: catch panics and errors from handlers

Here is a simple workflow with both pipeline and fanout:

Middlewares:

• Inline handler: a shortcut to implement slog.Handler
• Inline middleware: a shortcut to implement slogmulti.Middleware

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See also:

• slog-multi: slog.Handler chaining, fanout, routing, failover, load balancing...
• slog-formatter: slog attribute formatting
• slog-sampling: slog sampling policy
• slog-mock: slog.Handler for test purposes

HTTP middlewares:

• slog-gin: Gin middleware for slog logger
• slog-echo: Echo middleware for slog logger
• slog-fiber: Fiber middleware for slog logger
• slog-chi: Chi middleware for slog logger
• slog-http: net/http middleware for slog logger

Loggers:

• slog-zap: A slog handler for Zap
• slog-zerolog: A slog handler for Zerolog
• slog-logrus: A slog handler for Logrus

Log sinks:

• slog-datadog: A slog handler for Datadog
• slog-betterstack: A slog handler for Betterstack
• slog-rollbar: A slog handler for Rollbar
• slog-loki: A slog handler for Loki
• slog-sentry: A slog handler for Sentry
• slog-syslog: A slog handler for Syslog
• slog-logstash: A slog handler for Logstash
• slog-fluentd: A slog handler for Fluentd
• slog-graylog: A slog handler for Graylog
• slog-quickwit: A slog handler for Quickwit
• slog-slack: A slog handler for Slack
• slog-telegram: A slog handler for Telegram
• slog-mattermost: A slog handler for Mattermost
• slog-microsoft-teams: A slog handler for Microsoft Teams
• slog-webhook: A slog handler for Webhook
• slog-kafka: A slog handler for Kafka
• slog-nats: A slog handler for NATS
• slog-parquet: A slog handler for Parquet + Object Storage
• slog-channel: A slog handler for Go channels

🚀 Install

go get github.com/samber/slog-multi

Compatibility: go >= 1.21

No breaking changes will be made to exported APIs before v2.0.0.

Warning

Use this library carefully, log processing can be very costly (!)

💡 Usage

GoDoc: https://pkg.go.dev/github.com/samber/slog-multi

Broadcast: slogmulti.Fanout()

Distribute logs to multiple slog.Handler in parallel.

import (
    slogmulti "github.com/samber/slog-multi"
    "log/slog"
)

func main() {
    logstash, _ := net.Dial("tcp", "logstash.acme:4242")    // use github.com/netbrain/goautosocket for auto-reconnect
    stderr := os.Stderr

    logger := slog.New(
        slogmulti.Fanout(
            slog.NewJSONHandler(logstash, &slog.HandlerOptions{}),  // pass to first handler: logstash over tcp
            slog.NewTextHandler(stderr, &slog.HandlerOptions{}),    // then to second handler: stderr
            // ...
        ),
    )

    logger.
        With(
            slog.Group("user",
                slog.String("id", "user-123"),
                slog.Time("created_at", time.Now()),
            ),
        ).
        With("environment", "dev").
        With("error", fmt.Errorf("an error")).
        Error("A message")
}

Stderr output:

time=2023-04-10T14:00:0.000000+00:00 level=ERROR msg="A message" user.id=user-123 user.created_at=2023-04-10T14:00:0.000000+00:00 environment=dev error="an error"

Netcat output:

{
	"time":"2023-04-10T14:00:0.000000+00:00",
	"level":"ERROR",
	"msg":"A message",
	"user":{
		"id":"user-123",
		"created_at":"2023-04-10T14:00:0.000000+00:00"
	},
	"environment":"dev",
	"error":"an error"
}

Routing: slogmulti.Router()

Distribute logs to all matching slog.Handler in parallel.

import (
    slogmulti "github.com/samber/slog-multi"
    slogslack "github.com/samber/slog-slack"
    "log/slog"
)

func main() {
    slackChannelUS := slogslack.Option{Level: slog.LevelError, WebhookURL: "xxx", Channel: "supervision-us"}.NewSlackHandler()
    slackChannelEU := slogslack.Option{Level: slog.LevelError, WebhookURL: "xxx", Channel: "supervision-eu"}.NewSlackHandler()
    slackChannelAPAC := slogslack.Option{Level: slog.LevelError, WebhookURL: "xxx", Channel: "supervision-apac"}.NewSlackHandler()

    logger := slog.New(
        slogmulti.Router().
            Add(slackChannelUS, recordMatchRegion("us")).
            Add(slackChannelEU, recordMatchRegion("eu")).
            Add(slackChannelAPAC, recordMatchRegion("apac")).
            Handler(),
    )

    logger.
        With("region", "us").
        With("pool", "us-east-1").
        Error("Server desynchronized")
}

func recordMatchRegion(region string) func(ctx context.Context, r slog.Record) bool {
    return func(ctx context.Context, r slog.Record) bool {
        ok := false

        r.Attrs(func(attr slog.Attr) bool {
            if attr.Key == "region" && attr.Value.Kind() == slog.KindString && attr.Value.String() == region {
                ok = true
                return false
            }

            return true
        })

        return ok
    }
}

Failover: slogmulti.Failover()

List multiple targets for a slog.Record instead of retrying on the same unavailable log management system.

import (
    "net"
    slogmulti "github.com/samber/slog-multi"
    "log/slog"
)


func main() {
    // ncat -l 1000 -k
    // ncat -l 1001 -k
    // ncat -l 1002 -k

    // list AZs
    // use github.com/netbrain/goautosocket for auto-reconnect
    logstash1, _ := net.Dial("tcp", "logstash.eu-west-3a.internal:1000")
    logstash2, _ := net.Dial("tcp", "logstash.eu-west-3b.internal:1000")
    logstash3, _ := net.Dial("tcp", "logstash.eu-west-3c.internal:1000")

    logger := slog.New(
        slogmulti.Failover()(
            slog.HandlerOptions{}.NewJSONHandler(logstash1, nil),    // send to this instance first
            slog.HandlerOptions{}.NewJSONHandler(logstash2, nil),    // then this instance in case of failure
            slog.HandlerOptions{}.NewJSONHandler(logstash3, nil),    // and finally this instance in case of double failure
        ),
    )

    logger.
        With(
            slog.Group("user",
                slog.String("id", "user-123"),
                slog.Time("created_at", time.Now()),
            ),
        ).
        With("environment", "dev").
        With("error", fmt.Errorf("an error")).
        Error("A message")
}

Load balancing: slogmulti.Pool()

Increase log bandwidth by sending log.Record to a pool of slog.Handler.

import (
    "net"
    slogmulti "github.com/samber/slog-multi"
    "log/slog"
)

func main() {
    // ncat -l 1000 -k
    // ncat -l 1001 -k
    // ncat -l 1002 -k

    // list AZs
    // use github.com/netbrain/goautosocket for auto-reconnect
    logstash1, _ := net.Dial("tcp", "logstash.eu-west-3a.internal:1000")
    logstash2, _ := net.Dial("tcp", "logstash.eu-west-3b.internal:1000")
    logstash3, _ := net.Dial("tcp", "logstash.eu-west-3c.internal:1000")

    logger := slog.New(
        slogmulti.Pool()(
            // a random handler will be picked
            slog.HandlerOptions{}.NewJSONHandler(logstash1, nil),
            slog.HandlerOptions{}.NewJSONHandler(logstash2, nil),
            slog.HandlerOptions{}.NewJSONHandler(logstash3, nil),
        ),
    )

    logger.
        With(
            slog.Group("user",
                slog.String("id", "user-123"),
                slog.Time("created_at", time.Now()),
            ),
        ).
        With("environment", "dev").
        With("error", fmt.Errorf("an error")).
        Error("A message")
}

Recover errors: slog.RecoverHandlerError()

Returns a slog.Handler that recovers from panics or error of the chain of handlers.

import (
	slogformatter "github.com/samber/slog-formatter"
	slogmulti "github.com/samber/slog-multi"
	"log/slog"
)

recovery := slogmulti.RecoverHandlerError(
    func(ctx context.Context, record slog.Record, err error) {
        // will be called only if subsequent handlers fail or return an error
        log.Println(err.Error())
    },
)
sink := NewSinkHandler(...)

logger := slog.New(
    slogmulti.
        Pipe(recovery).
        Handler(sink),
)

err := fmt.Errorf("an error")
logger.Error("a message",
    slog.Any("very_private_data", "abcd"),
    slog.Any("user", user),
    slog.Any("err", err))

// outputs:
// time=2023-04-10T14:00:0.000000+00:00 level=ERROR msg="a message" error.message="an error" error.type="*errors.errorString" user="John doe" very_private_data="********"

Chaining: slogmulti.Pipe()

Rewrite log.Record on the fly (eg: for privacy reason).

func main() {
    // first middleware: format go `error` type into an object {error: "*myCustomErrorType", message: "could not reach https://a.b/c"}
    errorFormattingMiddleware := slogmulti.NewHandleInlineMiddleware(errorFormattingMiddleware)

    // second middleware: remove PII
    gdprMiddleware := NewGDPRMiddleware()

    // final handler
    sink := slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{})

    logger := slog.New(
        slogmulti.
            Pipe(errorFormattingMiddleware).
            Pipe(gdprMiddleware).
            // ...
            Handler(sink),
    )

    logger.
        With(
            slog.Group("user",
                slog.String("id", "user-123"),
                slog.String("email", "user-123"),
                slog.Time("created_at", time.Now()),
            ),
        ).
        With("environment", "dev").
        Error("A message",
            slog.String("foo", "bar"),
            slog.Any("error", fmt.Errorf("an error")),
        )
}

Stderr output:

{
    "time":"2023-04-10T14:00:0.000000+00:00",
    "level":"ERROR",
    "msg":"A message",
    "user":{
        "id":"*******",
        "email":"*******",
        "created_at":"*******"
    },
    "environment":"dev",
    "foo":"bar",
    "error":{
        "type":"*myCustomErrorType",
        "message":"an error"
    }
}

Custom middleware

Middleware must match the following prototype:

type Middleware func(slog.Handler) slog.Handler

The example above uses:

• a custom middleware, see here
• an inline middleware, see here

Note: WithAttrs and WithGroup methods of custom middleware must return a new instance, instead of this.

Inline handler

An "inline handler" (aka. lambda), is a shortcut to implement slog.Handler, that hooks a single method and proxies others.

mdw := slogmulti.NewHandleInlineHandler(
    // simulate "Handle()"
    func(ctx context.Context, groups []string, attrs []slog.Attr, record slog.Record) error {
        // [...]
        return nil
    },
)

mdw := slogmulti.NewInlineHandler(
    // simulate "Enabled()"
    func(ctx context.Context, groups []string, attrs []slog.Attr, level slog.Level) bool {
        // [...]
        return true
    },
    // simulate "Handle()"
    func(ctx context.Context, groups []string, attrs []slog.Attr, record slog.Record) error {
        // [...]
        return nil
    },
)

Inline middleware

An "inline middleware" (aka. lambda), is a shortcut to implement middleware, that hooks a single method and proxies others.

// hook `logger.Enabled` method
mdw := slogmulti.NewEnabledInlineMiddleware(func(ctx context.Context, level slog.Level, next func(context.Context, slog.Level) bool) bool{
    // [...]
    return next(ctx, level)
})

// hook `logger.Handle` method
mdw := slogmulti.NewHandleInlineMiddleware(func(ctx context.Context, record slog.Record, next func(context.Context, slog.Record) error) error {
    // [...]
    return next(ctx, record)
})

// hook `logger.WithAttrs` method
mdw := slogmulti.NewWithAttrsInlineMiddleware(func(attrs []slog.Attr, next func([]slog.Attr) slog.Handler) slog.Handler{
    // [...]
    return next(attrs)
})

// hook `logger.WithGroup` method
mdw := slogmulti.NewWithGroupInlineMiddleware(func(name string, next func(string) slog.Handler) slog.Handler{
    // [...]
    return next(name)
})

A super inline middleware that hooks all methods.

Warning: you would rather implement your own middleware.

mdw := slogmulti.NewInlineMiddleware(
    func(ctx context.Context, level slog.Level, next func(context.Context, slog.Level) bool) bool{
        // [...]
        return next(ctx, level)
    },
    func(ctx context.Context, record slog.Record, next func(context.Context, slog.Record) error) error{
        // [...]
        return next(ctx, record)
    },
    func(attrs []slog.Attr, next func([]slog.Attr) slog.Handler) slog.Handler{
        // [...]
        return next(attrs)
    },
    func(name string, next func(string) slog.Handler) slog.Handler{
        // [...]
        return next(name)
    },
)

🤝 Contributing

• Ping me on twitter @samuelberthe (DMs, mentions, whatever :))
• Fork the project
• Fix open issues or request new features

Don't hesitate ;)

# Install some dev dependencies
make tools

# Run tests
make test
# or
make watch-test

👤 Contributors

💫 Show your support

Give a ⭐️ if this project helped you!

📝 License

Copyright © 2023 Samuel Berthe.

This project is MIT licensed.