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BenchmarkSampler_CheckWithHook was randomly failing esp. under load because under load, Zap sampler can end up dropping less than the expected amount (esp. with Hooks). Moreover, we were making a single comparison after all the benchmarks had run, which causes the time dilation effect to aggregate over all the tests, making it more likely to fail.

This changes the benchmark to compare the dropped and sampled ratio in between test runs and test the dropped/sampled ratio. The change was verified by running the benchmark 500 times on a MacBook Pro i7 model as well as devpod (96 core Linux) and confirming no failures happened.

Fix #996
Internal Ref: GO-858

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Jul 5, 2021
Jun 16, 2021

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 862 ns/op +0% 5 allocs/op
zap (sugared) 1250 ns/op +45% 11 allocs/op
zerolog 4021 ns/op +366% 76 allocs/op
go-kit 4542 ns/op +427% 105 allocs/op
apex/log 26785 ns/op +3007% 115 allocs/op
logrus 29501 ns/op +3322% 125 allocs/op
log15 29906 ns/op +3369% 122 allocs/op

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

Package Time Time % to zap Objects Allocated
zap 126 ns/op +0% 0 allocs/op
zap (sugared) 187 ns/op +48% 2 allocs/op
zerolog 88 ns/op -30% 0 allocs/op
go-kit 5087 ns/op +3937% 103 allocs/op
log15 18548 ns/op +14621% 73 allocs/op
apex/log 26012 ns/op +20544% 104 allocs/op
logrus 27236 ns/op +21516% 113 allocs/op

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

Package Time Time % to zap Objects Allocated
zap 118 ns/op +0% 0 allocs/op
zap (sugared) 191 ns/op +62% 2 allocs/op
zerolog 93 ns/op -21% 0 allocs/op
go-kit 280 ns/op +137% 11 allocs/op
standard library 499 ns/op +323% 2 allocs/op
apex/log 1990 ns/op +1586% 10 allocs/op
logrus 3129 ns/op +2552% 24 allocs/op
log15 3887 ns/op +3194% 23 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.