⚡ zap

Fast, structured, leveled logging in Go.

Structure

Zap takes an opinionated stance on logging and doesn't provide any printf-style helpers. Rather than logger.Printf("Failed to fetch URL %s (attempt %v), sleeping %s before retry.", url, tryNum, sleepFor), zap encourages the more structured

logger.Info("Failed to fetch URL.",
  zap.String("url", url),
  zap.Int("attempt", tryNum),
  zap.Duration("backoff", sleepFor),
)

This a bit more verbose, but it enables powerful ad-hoc analysis, flexible dashboarding, and accurate message bucketing. In short, it helps you get the most out of tools like ELK, Splunk, and Sentry. All log messages are JSON-serialized, though PRs to support other formats are welcome.

Performance

For applications that log in the hot path, reflection-based serialization and string formatting are prohibitively expensive — they're CPU-intensive and make many small allocations. Put differently, using encoding/json and fmt.Println to log tons of interface{}s makes your application slow.

Zap takes a different approach. It includes a reflection-free, zero-allocation JSON encoder, and it offers a variety of type-safe ways to add structured context to your log messages. It strives to avoid serialization overhead and allocations wherever possible, so collecting rich debug logs doesn't impact normal operations.

As measured by its own benchmarking suite, not only is zap more performant than comparable structured logging libraries — it's also faster than the standard library. Like all benchmarks, take these with a grain of salt.¹

Log a message and 10 fields:

Library	Time	Bytes Allocated	Objects Allocated
⚡ zap	1171 ns/op	705 B/op	2 allocs/op
logrus	8410 ns/op	3560 B/op	67 allocs/op
go-kit	7380 ns/op	3204 B/op	70 allocs/op
log15	20610 ns/op	4207 B/op	90 allocs/op

Log a message using a logger that already has 10 fields of context:

Library	Time	Bytes Allocated	Objects Allocated
⚡ zap	231 ns/op	0 B/op	0 allocs/op
logrus	8035 ns/op	3438 B/op	61 allocs/op
go-kit	6790 ns/op	2486 B/op	48 allocs/op
log15	20709 ns/op	3543 B/op	69 allocs/op

Log a static string, without any context or printf-style formatting:

Library	Time	Bytes Allocated	Objects Allocated
⚡ zap	223 ns/op	0 B/op	0 allocs/op
standard library	562 ns/op	32 B/op	2 allocs/op
logrus	2765 ns/op	1336 B/op	26 allocs/op
go-kit	1092 ns/op	624 B/op	13 allocs/op
log15	5513 ns/op	1351 B/op	23 allocs/op

Development Status: Beta

Ready for adventurous users, but breaking API changes are likely.

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Released under the [MIT License](LICENSE.txt).

¹ In particular, note that we may be benchmarking against slightly older versions of other libraries. Versions are pinned in zap's glide.lock file. ↩