Fast, structured, leveled logging in Go

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Blazing fast, structured, leveled logging in Go.


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.

For compatibility with the standard library and bark, zap provides the zwrap.Standardize and zbark.Barkify wrappers. Both are slower than the core zap logger, but faster than the libraries they replace.


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.Fprintf 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 1241 ns/op 705 B/op 2 allocs/op
logrus 9713 ns/op 5275 B/op 78 allocs/op
go-kit 11632 ns/op 3204 B/op 70 allocs/op
log15 23077 ns/op 4783 B/op 91 allocs/op

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

Library Time Bytes Allocated Objects Allocated
zap 238 ns/op 0 B/op 0 allocs/op
logrus 7946 ns/op 3438 B/op 61 allocs/op
go-kit 6445 ns/op 2486 B/op 48 allocs/op
log15 21728 ns/op 4120 B/op 70 allocs/op

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

Library Time Bytes Allocated Objects Allocated
zap 219 ns/op 0 B/op 0 allocs/op
standard library 607 ns/op 32 B/op 2 allocs/op
logrus 3324 ns/op 1336 B/op 26 allocs/op
go-kit 1008 ns/op 624 B/op 13 allocs/op
log15 5769 ns/op 1351 B/op 23 allocs/op

Development Status: Beta

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

Released under the MIT License.

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.