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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 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 |
+
+[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