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# Frequently Asked Questions

## Design

### Why spend so much effort on logger performance?

Of course, most applications won't notice the impact of a slow logger: they
already take tens or hundreds of milliseconds for each operation, so an extra
millisecond doesn't matter.

On the other hand, why *not* make structured logging fast? The `SugaredLogger`
isn't any harder to use than other logging packages, and the `Logger` makes
structured logging possible in performance-sensitive contexts. Across a fleet
of Go microservices, making each application even slightly more efficient adds
up quickly.

### Why aren't `Logger` and `SugaredLogger` interfaces?

Unlike the familiar `io.Writer` and `http.Handler`, `Logger` and
`SugaredLogger` interfaces would include *many* methods. As [Rob Pike points
out][go-proverbs], "The bigger the interface, the weaker the abstraction."
Interfaces are also rigid — *any* change requires releasing a new major
version, since it breaks all third-party implementations.

Making the `Logger` and `SugaredLogger` concrete types doesn't sacrifice much
abstraction, and it lets us add methods without introducing breaking changes.
Your applications should define and depend upon an interface that includes
just the methods you use.

### Why are some of my logs missing?

Logs are dropped intentionally by zap when sampling is enabled. The production
configuration (as returned by `NewProductionConfig()` enables sampling which will
cause repeated logs within a second to be sampled. See more details on why sampling
is enabled in [Why sample application logs](https://github.com/uber-go/zap/blob/master/FAQ.md#why-sample-application-logs).

### Why sample application logs?

Applications often experience runs of errors, either because of a bug or
because of a misbehaving user. Logging errors is usually a good idea, but it
can easily make this bad situation worse: not only is your application coping
with a flood of errors, it's also spending extra CPU cycles and I/O logging
those errors. Since writes are typically serialized, logging limits throughput
when you need it most.

Sampling fixes this problem by dropping repetitive log entries. Under normal
conditions, your application writes out every entry. When similar entries are
logged hundreds or thousands of times each second, though, zap begins dropping
duplicates to preserve throughput.

### Why do the structured logging APIs take a message in addition to fields?

Subjectively, we find it helpful to accompany structured context with a brief
description. This isn't critical during development, but it makes debugging
and operating unfamiliar systems much easier.

More concretely, zap's sampling algorithm uses the message to identify
duplicate entries. In our experience, this is a practical middle ground
between random sampling (which often drops the exact entry that you need while
debugging) and hashing the complete entry (which is prohibitively expensive).

### Why include package-global loggers?

Since so many other logging packages include a global logger, many
applications aren't designed to accept loggers as explicit parameters.
Changing function signatures is often a breaking change, so zap includes
global loggers to simplify migration.

Avoid them where possible.

### Why include dedicated Panic and Fatal log levels?

In general, application code should handle errors gracefully instead of using
`panic` or `os.Exit`. However, every rule has exceptions, and it's common to
crash when an error is truly unrecoverable. To avoid losing any information
— especially the reason for the crash — the logger must flush any
buffered entries before the process exits.

Zap makes this easy by offering `Panic` and `Fatal` logging methods that
automatically flush before exiting. Of course, this doesn't guarantee that
logs will never be lost, but it eliminates a common error.

See the discussion in uber-go/zap#207 for more details.

### What's `DPanic`?

`DPanic` stands for "panic in development." In development, it logs at
`PanicLevel`; otherwise, it logs at `ErrorLevel`. `DPanic` makes it easier to
catch errors that are theoretically possible, but shouldn't actually happen,
*without* crashing in production.

If you've ever written code like this, you need `DPanic`:

```go
if err != nil {
  panic(fmt.Sprintf("shouldn't ever get here: %v", err))
}
```

## Installation

### What does the error `expects import "go.uber.org/zap"` mean?

Either zap was installed incorrectly or you're referencing the wrong package
name in your code.

Zap's source code happens to be hosted on GitHub, but the [import
path][import-path] is `go.uber.org/zap`. This gives us, the project
maintainers, the freedom to move the source code if necessary. However, it
means that you need to take a little care when installing and using the
package.

If you follow two simple rules, everything should work: install zap with `go
get -u go.uber.org/zap`, and always import it in your code with `import
"go.uber.org/zap"`. Your code shouldn't contain *any* references to
`github.com/uber-go/zap`.

## Usage

### Does zap support log rotation?

Zap doesn't natively support rotating log files, since we prefer to leave this
to an external program like `logrotate`.

However, it's easy to integrate a log rotation package like
[`gopkg.in/natefinch/lumberjack.v2`][lumberjack] as a `zapcore.WriteSyncer`.

```go
// lumberjack.Logger is already safe for concurrent use, so we don't need to
// lock it.
w := zapcore.AddSync(&lumberjack.Logger{
  Filename:   "/var/log/myapp/foo.log",
  MaxSize:    500, // megabytes
  MaxBackups: 3,
  MaxAge:     28, // days
})
core := zapcore.NewCore(
  zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
  w,
  zap.InfoLevel,
)
logger := zap.New(core)
```

## Extensions

We'd love to support every logging need within zap itself, but we're only
familiar with a handful of log ingestion systems, flag-parsing packages, and
the like. Rather than merging code that we can't effectively debug and
support, we'd rather grow an ecosystem of zap extensions.

We're aware of the following extensions, but haven't used them ourselves:

| Package | Integration |
| --- | --- |
| `github.com/tchap/zapext` | Sentry, syslog |
| `github.com/fgrosse/zaptest` | Ginkgo |
| `github.com/blendle/zapdriver` | Stackdriver |
| `github.com/moul/zapgorm` | Gorm |
| `github.com/moul/zapfilter` | Advanced filtering rules |

[go-proverbs]: https://go-proverbs.github.io/
[import-path]: https://golang.org/cmd/go/#hdr-Remote_import_paths
[lumberjack]: https://godoc.org/gopkg.in/natefinch/lumberjack.v2