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This is just a small nit from #1281 - I think the expected and got
fields of the test case structs are flipped.

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zap GoDoc Build Status Coverage Status

Blazing fast, structured, leveled logging in Go.


go get -u

Note that zap only supports the two most recent minor versions of Go.

Quick Start

In contexts where performance is nice, but not critical, use the SugaredLogger. It's 4-10x faster than other structured logging packages and includes both structured and printf-style APIs.

logger, _ := zap.NewProduction()
defer logger.Sync() // flushes buffer, if any
sugar := logger.Sugar()
sugar.Infow("failed to fetch URL",
  // Structured context as loosely typed key-value pairs.
  "url", url,
  "attempt", 3,
  "backoff", time.Second,
sugar.Infof("Failed to fetch URL: %s", url)

When performance and type safety are critical, use the Logger. It's even faster than the SugaredLogger and allocates far less, but it only supports structured logging.

logger, _ := zap.NewProduction()
defer logger.Sync()
logger.Info("failed to fetch URL",
  // Structured context as strongly typed Field values.
  zap.String("url", url),
  zap.Int("attempt", 3),
  zap.Duration("backoff", time.Second),

See the documentation and FAQ for more details.


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 the base Logger strives to avoid serialization overhead and allocations wherever possible. By building the high-level SugaredLogger on that foundation, zap lets users choose when they need to count every allocation and when they'd prefer a more familiar, loosely typed API.

As measured by its own benchmarking suite, not only is zap more performant than comparable structured logging packages — 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:

Package Time Time % to zap Objects Allocated
zap 1744 ns/op +0% 5 allocs/op
zap (sugared) 2483 ns/op +42% 10 allocs/op
zerolog 918 ns/op -47% 1 allocs/op
go-kit 5590 ns/op +221% 57 allocs/op
slog 5640 ns/op +223% 40 allocs/op
apex/log 21184 ns/op +1115% 63 allocs/op
logrus 24338 ns/op +1296% 79 allocs/op
log15 26054 ns/op +1394% 74 allocs/op

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

Package Time Time % to zap Objects Allocated
zap 193 ns/op +0% 0 allocs/op
zap (sugared) 227 ns/op +18% 1 allocs/op
zerolog 81 ns/op -58% 0 allocs/op
slog 322 ns/op +67% 0 allocs/op
go-kit 5377 ns/op +2686% 56 allocs/op
apex/log 19518 ns/op +10013% 53 allocs/op
log15 19812 ns/op +10165% 70 allocs/op
logrus 21997 ns/op +11297% 68 allocs/op

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

Package Time Time % to zap Objects Allocated
zap 165 ns/op +0% 0 allocs/op
zap (sugared) 212 ns/op +28% 1 allocs/op
zerolog 95 ns/op -42% 0 allocs/op
slog 296 ns/op +79% 0 allocs/op
go-kit 415 ns/op +152% 9 allocs/op
standard library 422 ns/op +156% 2 allocs/op
apex/log 1601 ns/op +870% 5 allocs/op
logrus 3017 ns/op +1728% 23 allocs/op
log15 3469 ns/op +2002% 20 allocs/op

Development Status: Stable

All APIs are finalized, and no breaking changes will be made in the 1.x series of releases. Users of semver-aware dependency management systems should pin zap to ^1.


We encourage and support an active, healthy community of contributors — including you! Details are in the contribution guide and the code of conduct. The zap maintainers keep an eye on issues and pull requests, but you can also report any negative conduct to That email list is a private, safe space; even the zap maintainers don't have access, so don't hesitate to hold us to a high standard.

Released under the MIT License.

1 In particular, keep in mind that we may be benchmarking against slightly older versions of other packages. Versions are pinned in the benchmarks/go.mod file.