slog/meld provides a simple slog.Handler designed to recursively merge and de-duplicate log attributes, ensuring clean, concise, and informative log entries.

go get -u snqk.dev/slog/meld

• Attribute Merging: Combines attributes with the same key, while preserving context.
• Recursive Group Merging: Handles nested slog.Group attributes, ensuring proper merging at all levels.
• De-duplication: Eliminates duplicate keys within groups, preventing cluttered logs.
• Order Preservation: Maintains the original order of attributes as they were defined, even after merging or replacing.
• Lightweight: A pure go library without any dependencies.
• Simplified Bootstrap: Doesn't require any configuration options.

• Thread Safety: Handler immutability ensures merge operations do not conflict with each other.
• Greedy Merge: Attributes are merged ahead-of-time vs when logging, where possible. IE when calling Logger.With() or Logger.WithGroup().

For most implementations, the following wrapper would suffice.

package main

import (
	"log/slog"

	"snqk.dev/slog/meld"
)

func init() {
	slog.SetDefault(slog.New(meld.NewHandler(slog.Default().Handler())))
}

Play: https://go.dev/play/p/7r6D-reA4Pd

package main

import (
	"log/slog"
	"os"

	"github.com/veqryn/slog-dedup"
	"snqk.dev/slog/meld"
)

func main() {
	hello(slog.New(slog.NewJSONHandler(os.Stdout, nil)))
	hello(slog.New(slogdedup.NewOverwriteHandler(slog.NewJSONHandler(os.Stdout, nil), nil)))
	hello(slog.New(meld.NewHandler(slog.NewJSONHandler(os.Stdout, nil))))
}

func hello(log *slog.Logger) {
	log = log.With(slog.Group("alice", slog.String("foo", "initial_attr"), slog.String("bar", "old_attr")))
	// do stuff...
	log = log.With(slog.Group("alice", slog.String("foo", "overwritten_attr"), slog.String("qux", "new_attr")))
	// do stuff...
	log = log.WithGroup("bob")
	// do stuff...
	log.Info("hello_world", "foo", "inline_attr")
}

Newer attributes are overlay on top of the older tree, and merged together.

{
  "time": "2009-11-10T23:00:00Z",
  "level": "INFO",
  "msg": "hello_world",
  "alice": {
    "foo": "overwritten_attr",
    "bar": "old_attr",
    "qux": "new_attr"
  },
  "bob": {
    "foo": "inline_attr"
  }
}

This approach means we only ever append &/ update older values, never deleting any attributes. This makes slog/meld partially immutable, where the keys cannot be changed, but their values are mutable.

Alternatively, consider implementing slog.LogValuer for dynamically rendering attributes for different situations.

The fully immutable nature of log/slog means we can't modify previously added attributes (keys and values), only add more of the same as their duplicates.

{
  "time": "2009-11-10T23:00:00Z",
  "level": "INFO",
  "msg": "hello_world",
  "alice": {
    "foo": "initial_attr",
    "bar": "old_attr"
  },
  "alice": {
    "foo": "overwritten_attr",
    "qux": "new_attr"
  },
  "bob": {
    "foo": "inline_attr"
  }
}

While this is legal JSON, parsers like jq will process it by overwriting earlier values with later ones. Doing so results in the same output as from veqryn/slog-dedup.NewOverwriteHandler.

Here, {"bar": "old_attr"} is missing due to its parent group being completely overwritten, as deduplication alone isn't meant to handle attribute merging.

{
  "time": "2009-11-10T23:00:00Z",
  "level": "INFO",
  "msg": "hello_world",
  "alice": {
    "foo": "overwritten_attr",
    "qux": "new_attr"
  },
  "bob": {
    "foo": "inline_attr"
  }
}

Benchmarks were also performed against the scenario above, comparing log/slog, slog/meld, and veqryn/slog-dedup.NewOverwriteHandler.

$ go test -bench=. -benchtime 4s -benchmem -cpu 1,2,4,8,16 -run notest
goos: linux
goarch: amd64
pkg: snqk.dev/slog/meld
cpu: Intel(R) Core(TM) i7-5960X CPU @ 3.00GHz
BenchmarkDefaultLogger           1306116              3653 ns/op            1272 B/op         25 allocs/op
BenchmarkDefaultLogger-2         2257365              2152 ns/op            1272 B/op         25 allocs/op
BenchmarkDefaultLogger-4         4360447              1102 ns/op            1272 B/op         25 allocs/op
BenchmarkDefaultLogger-8         4590700              1073 ns/op            1274 B/op         25 allocs/op
BenchmarkDefaultLogger-16        3770030              1267 ns/op            1278 B/op         25 allocs/op
BenchmarkMeldLogger               830655              5680 ns/op            2392 B/op         62 allocs/op
BenchmarkMeldLogger-2            1460762              3385 ns/op            2392 B/op         62 allocs/op
BenchmarkMeldLogger-4            2653039              1793 ns/op            2393 B/op         62 allocs/op
BenchmarkMeldLogger-8            2548966              1844 ns/op            2397 B/op         62 allocs/op
BenchmarkMeldLogger-16           2263243              2224 ns/op            2404 B/op         62 allocs/op
BenchmarkOverwriteLogger          301058             18183 ns/op           13134 B/op         65 allocs/op
BenchmarkOverwriteLogger-2        483166              9033 ns/op           14032 B/op         65 allocs/op
BenchmarkOverwriteLogger-4        557324              9961 ns/op           15775 B/op         65 allocs/op
BenchmarkOverwriteLogger-8        422300             10334 ns/op           20254 B/op         65 allocs/op
BenchmarkOverwriteLogger-16       395548             12089 ns/op           28342 B/op         65 allocs/op
PASS
ok      snqk.dev/slog/meld      96.097s