slog-multi
Chain of slog.Handler (pipeline, fanout...)
About slog-multi
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# slog-multi: Advanced Handler Composition for Go's Structured Logging (pipelining, fanout, routing, failover...)
[](https://github.com/samber/slog-multi/releases)
[](https://pkg.go.dev/github.com/samber/slog-multi)
[](https://goreportcard.com/report/github.com/samber/slog-multi)
[](https://codecov.io/gh/samber/slog-multi)
[](https://github.com/samber/slog-multi/graphs/contributors)
[](./LICENSE)
**slog-multi** provides advanced composition patterns for Go's structured logging (`slog`). It enables you to build sophisticated logging workflows by combining multiple handlers with different strategies for distribution, routing, transformation, and error handling.
## 🎯 Features
- **🔄 Fanout**: Distribute logs to multiple handlers in parallel
- **🛣️ Router**: Conditionally route logs based on custom criteria
- **🎯 First Match**: Route logs to the first matching handler only
- **🔄 Failover**: High-availability logging with automatic fallback
- **⚖️ Load Balancing**: Distribute load across multiple handlers
- **🔗 Pipeline**: Transform and filter logs with middleware chains
- **🛡️ Error Recovery**: Graceful handling of logging failures
Middlewares:
- **⚡ Inline Handlers**: Quick implementation of custom handlers
- **🔧 Inline Middleware**: Rapid development of transformation logic
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**See also:**
- [slog-multi](https://github.com/samber/slog-multi): `slog.Handler` chaining, fanout, routing, failover, load balancing...
- [slog-formatter](https://github.com/samber/slog-formatter): `slog` attribute formatting
- [slog-sampling](https://github.com/samber/slog-sampling): `slog` sampling policy
- [slog-mock](https://github.com/samber/slog-mock): `slog.Handler` for test purposes
**HTTP middlewares:**
- [slog-gin](https://github.com/samber/slog-gin): Gin middleware for `slog` logger
- [slog-echo](https://github.com/samber/slog-echo): Echo middleware for `slog` logger
- [slog-fiber](https://github.com/samber/slog-fiber): Fiber middleware for `slog` logger
- [slog-chi](https://github.com/samber/slog-chi): Chi middleware for `slog` logger
- [slog-http](https://github.com/samber/slog-http): `net/http` middleware for `slog` logger
**Loggers:**
- [slog-zap](https://github.com/samber/slog-zap): A `slog` handler for `Zap`
- [slog-zerolog](https://github.com/samber/slog-zerolog): A `slog` handler for `Zerolog`
- [slog-logrus](https://github.com/samber/slog-logrus): A `slog` handler for `Logrus`
**Log sinks:**
- [slog-datadog](https://github.com/samber/slog-datadog): A `slog` handler for `Datadog`
- [slog-betterstack](https://github.com/samber/slog-betterstack): A `slog` handler for `Betterstack`
- [slog-rollbar](https://github.com/samber/slog-rollbar): A `slog` handler for `Rollbar`
- [slog-loki](https://github.com/samber/slog-loki): A `slog` handler for `Loki`
- [slog-sentry](https://github.com/samber/slog-sentry): A `slog` handler for `Sentry`
- [slog-syslog](https://github.com/samber/slog-syslog): A `slog` handler for `Syslog`
- [slog-logstash](https://github.com/samber/slog-logstash): A `slog` handler for `Logstash`
- [slog-fluentd](https://github.com/samber/slog-fluentd): A `slog` handler for `Fluentd`
- [slog-graylog](https://github.com/samber/slog-graylog): A `slog` handler for `Graylog`
- [slog-quickwit](https://github.com/samber/slog-quickwit): A `slog` handler for `Quickwit`
- [slog-slack](https://github.com/samber/slog-slack): A `slog` handler for `Slack`
- [slog-telegram](https://github.com/samber/slog-telegram): A `slog` handler for `Telegram`
- [slog-mattermost](https://github.com/samber/slog-mattermost): A `slog` handler for `Mattermost`
- [slog-microsoft-teams](https://github.com/samber/slog-microsoft-teams): A `slog` handler for `Microsoft Teams`
- [slog-webhook](https://github.com/samber/slog-webhook): A `slog` handler for `Webhook`
- [slog-kafka](https://github.com/samber/slog-kafka): A `slog` handler for `Kafka`
- [slog-nats](https://github.com/samber/slog-nats): A `slog` handler for `NATS`
- [slog-parquet](https://github.com/samber/slog-parquet): A `slog` handler for `Parquet` + `Object Storage`
- [slog-channel](https://github.com/samber/slog-channel): A `slog` handler for Go channels
## 🚀 Installation
```sh
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 (!)
>
> Excessive logging —with multiple processing steps and destinations— can introduce significant overhead, which is generally undesirable in performance-critical paths. Logging is always expensive, and sometimes, metrics or a sampling strategy are cheaper. The library itself does not generate extra load.
## 💡 Usage
GoDoc: [https://pkg.go.dev/github.com/samber/slog-multi](https://pkg.go.dev/github.com/samber/slog-multi)
### Broadcast: `slogmulti.Fanout()`
Distribute logs to multiple `slog.Handler` in parallel for maximum throughput and redundancy.
```go
import (
"net"
slogmulti "github.com/samber/slog-multi"
"log/slog"
"os"
"time"
)
func main() {
logstash, _ := net.Dial("tcp", "logstash.acme:4242") // use github.com/netbrain/goautosocket for auto-reconnect
datadogHandler := slogdatadog.NewDatadogHandler(slogdatadog.Option{
APIKey: "your-api-key",
Service: "my-service",
})
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
datadogHandler,
// ...
),
)
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:
```json
{
"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` based on custom criteria like log level, attributes, or business logic.
```go
import (
"context"
slogmulti "github.com/samber/slog-multi"
slogslack "github.com/samber/slog-slack"
"log/slog"
"os"
)
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()
consoleHandler := slog.NewTextHandler(os.Stderr, nil)
logger := slog.New(
slogmulti.Router().
Add(slackChannelUS, recordMatchRegion("us")).
Add(slackChannelEU, recordMatchRegion("eu")).
Add(slackChannelAPAC, recordMatchRegion("apac")).
Add(consoleHandler, slogmulti.LevelIs(slog.LevelInfo, slog.LevelDebug)).
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
}
}
```
**Use Cases:**
- Environment-specific logging (dev vs prod)
- Level-based routing (errors to Slack, info to console)
- Business logic routing (user actions vs system events)
### First Match Routing: `Router().FirstMatch()`
Route logs to the **first matching handler only**, unlike regular routing which sends to all matching handlers. Perfect for priority-based routing where you want exactly one handler to receive each log.
```go
import (
slogmulti "github.com/samber/slog-multi"
slogslack "github.com/samber/slog-slack"
"log/slog"
)
func main() {
queryChannel := slogslack.Option{Level: slog.LevelDebug, WebhookURL: "xxx", Channel: "db-queries"}.NewSlackHandler()
requestChannel := slogslack.Option{Level: slog.LevelError, WebhookURL: "xxx", Channel: "service-requests"}.NewSlackHandler()
influxdbChannel := slogslack.Option{Level: slog.LevelInfo, WebhookURL: "xxx", Channel: "influxdb-metrics"}.NewSlackHandler()
fallbackChannel := slogslack.Option{Level: slog.LevelError, WebhookURL: "xxx", Channel: "logs"}.NewSlackHandler()
logger := slog.New(
slogmulti.Router().
Add(queryChannel, slogmulti.AttrKindIs("query", slog.KindString, "args", slog.KindAny)).
Add(requestChannel, slogmulti.AttrKindIs("method", slog.KindString, "body", slog.KindAny)).
Add(influxdbChannel, slogmulti.AttrValueIs("scope", "influx")).
Add(fallbackChannel). // Catch-all for everything else
FirstMatch(). // ← Enable first-match routing
Handler(),
)
// Goes to queryChannel only (stops at first match)
logger.Debug("Executing SQL query", "query", "SELECT * FROM users WHERE id = ?", "args", []int{1})
// Goes to requestChannel only (stops at first match)
logger.Error("Incoming request failed", "method", "POST", "body", "{'name':'test'}")
// Goes to fallbackChannel (no other handlers matched)
logger.Error("An unexpected error occurred")
}
```
#### Built-in Predicates
**Level predicates:**
- `LevelIs(levels ...slog.Level)` - Match specific log levels
- `LevelIsNot(levels ...slog.Level)` - Exclude specific log levels
**Message predicates:**
- `MessageIs(msg string)` - Exact message match
- `MessageIsNot(msg string)` - Message doesn't match
- `MessageContains(part string)` - Message contains substring
- `MessageNotContains(part string)` - Message doesn't contain substring
**Attribute predicates:**
- `AttrValueIs(key, value, ...)` - Check attributes have exact values
- `AttrKindIs(key, kind, ...)` - Check attributes have specific types
### Failover: `slogmulti.Failover()`
Ensure logging reliability by trying multiple handlers in order until one succeeds. Perfect for high-availability scenarios.
```go
import (
"net"
slogmulti "github.com/samber/slog-multi"
"log/slog"
"os"
"time"
)
func main() {
// Create connections to multiple log servers
// 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), // Primary
slog.HandlerOptions{}.NewJSONHandler(logstash2, nil), // Secondary
slog.HandlerOptions{}.NewJSONHandler(logstash3, nil), // Tertiary
),
)
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")
}
```
**Use Cases:**
- High-availability logging infrastructure
- Disaster recovery scenarios
- Multi-region deployments
### Load balancing: `slogmulti.Pool()`
Distribute logging load across multiple handlers using round-robin with randomization to increase throughput and provide redundancy.
```go
import (
"net"
slogmulti "github.com/samber/slog-multi"
"log/slog"
"os"
"time"
)
func main() {
// Create multiple log servers
// 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 for each log
slog.HandlerOptions{}.NewJSONHandler(logstash1, nil),
slog.HandlerOptions{}.NewJSONHandler(logstash2, nil),
slog.HandlerOptions{}.NewJSONHandler(logstash3, nil),
),
)
// High-volume logging
for i := 0; i < 1000; i++ {
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")
}
}
```
**Use Cases:**
- High-throughput logging scenarios
- Distributed logging infrastructure
- Performance optimization
### Recover errors: `slogmulti.RecoverHandlerError()`
Gracefully handle logging failures without crashing the application. Catches both panics and errors from handlers.
```go
import (
"context"
slogformatter "github.com/samber/slog-formatter"
slogmulti "github.com/samber/slog-multi"
"log/slog"
"os"
)
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="********"
```
### Pipelining: `slogmulti.Pipe()`
Transform and filter logs using middleware chains. Perfect for data privacy, formatting, and cross-cutting concerns.
```go
import (
"context"
slogmulti "github.com/samber/slog-multi"
"log/slog"
"os"
"time"
)
func main() {
// First middleware: format Go `error` type into an structured object {error: "*myCustomErrorType", message: "could not reach https://a.b/c"}
errorFormattingMiddleware := slogmulti.NewHandleInlineMiddleware(func(ctx context.Context, record slog.Record, next func(context.Context, slog.Record) error) error {
record.Attrs(func(attr slog.Attr) bool {
if attr.Key == "error" && attr.Value.Kind() == slog.KindAny {
if err, ok := attr.Value.Any().(error); ok {
record.AddAttrs(
slog.String("error_type", "error"),
slog.String("error_message", err.Error()),
)
}
}
return true
})
return next(ctx, record)
})
// Second middleware: remove PII
gdprMiddleware := slogmulti.NewHandleInlineMiddleware(func(ctx context.Context, record slog.Record, next func(context.Context, slog.Record) error) error {
record.Attrs(func(attr slog.Attr) bool {
if attr.Key == "email" || attr.Key == "phone" || attr.Key == "created_at" {
record.AddAttrs(slog.String(attr.Key, "*********"))
}
return true
})
return next(ctx, record)
})
// 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:
```json
{
"time":"2023-04-10T14:00:0.000000+00:00",
"level":"ERROR",
"msg":"A message",
"user":{
"email":"*******",
"phone":"*******",
"created_at":"*******"
},
"environment":"dev",
"foo":"bar",
"error":{
"type":"*myCustomErrorType",
"message":"an error"
}
}
```
**Use Cases:**
- Data privacy and GDPR compliance
- Error formatting and standardization
- Log enrichment and transformation
- Performance monitoring and metrics
## 🔧 Advanced Patterns
### Custom middleware
Middleware must match the following prototype:
```go
type Middleware func(slog.Handler) slog.Handler
```
The example above uses:
- a custom middleware, [see here](./examples/pipe/gdpr.go)
- an inline middleware, [see here](./examples/pipe/errors.go)
> **Note**: `WithAttrs` and `WithGroup` methods of custom middleware must return a new instance, not `this`.
#### Inline handler
Inline handlers provide shortcuts to implement `slog.Handler` without creating full struct implementations.
```go
mdw := slogmulti.NewHandleInlineHandler(
// simulate "Handle()" method
func(ctx context.Context, groups []string, attrs []slog.Attr, record slog.Record) error {
// Custom logic here
// [...]
return nil
},
)
```
```go
mdw := slogmulti.NewInlineHandler(
// simulate "Enabled()" method
func(ctx context.Context, groups []string, attrs []slog.Attr, level slog.Level) bool {
// Custom logic here
// [...]
return true
},
// simulate "Handle()" methodFrequently Asked Questions
What is slog-multi?
slog-multi is a Logging library for the Go programming language. Chain of slog.Handler (pipeline, fanout...)
How do I install slog-multi?
Install slog-multi with the Go module system using `go get samber/slog-multi`. Check the repository for the current installation instructions.
What category does slog-multi belong to?
slog-multi is listed under Logging, specifically Logging.