diff --git a/README.md b/README.md
index e9b5520..304163d 100644
--- a/README.md
+++ b/README.md
@@ -35,11 +35,11 @@ This is a BETA grade API.
 
 There are implementations for the following logging libraries:
 
+- **a function**: [funcr](https://github.com/go-logr/logr/funcr)
 - **github.com/google/glog**: [glogr](https://github.com/go-logr/glogr)
 - **k8s.io/klog**: [klogr](https://git.k8s.io/klog/klogr)
 - **go.uber.org/zap**: [zapr](https://github.com/go-logr/zapr)
-- **log** (the Go standard library logger):
-  [stdr](https://github.com/go-logr/stdr)
+- **log** (the Go standard library logger): [stdr](https://github.com/go-logr/stdr)
 - **github.com/sirupsen/logrus**: [logrusr](https://github.com/bombsimon/logrusr)
 - **github.com/wojas/genericr**: [genericr](https://github.com/wojas/genericr) (makes it easy to implement your own backend)
 - **logfmt** (Heroku style [logging](https://www.brandur.org/logfmt)): [logfmtr](https://github.com/iand/logfmtr)
@@ -60,7 +60,7 @@ There are implementations for the following logging libraries:
   Similarly to searchability, if you maintain conventions around your
   keys, it becomes easy to gather all log lines related to a particular
   concept.
- 
+
 - **Structured logs allow better dimensions of filtering**: if you have
   structure to your logs, you've got more precise control over how much
   information is logged -- you might choose in a particular configuration
@@ -169,7 +169,7 @@ Then gradually choose levels in between as you need them, working your way
 down from 10 (for debug and trace style logs) and up from 1 (for chattier
 info-type logs).
 
-## How do I choose my keys
+## How do I choose my keys?
 
 - make your keys human-readable
 - constant keys are generally a good idea
@@ -180,4 +180,19 @@ While key names are mostly unrestricted (and spaces are acceptable),
 it's generally a good idea to stick to printable ascii characters, or at
 least match the general character set of your log lines.
 
+## Why should keys be constant values?
+
+The point of structured logging is to make later log processing easier.  Your
+keys are, effectively, the schema of each log message.  If you use different
+keys across instances of the same log-line, you will make your structured logs
+much harder to use.  `Sprintf()` is for values, not for keys!
+
+## Why is this not a pure interface?
+
+The Logger type is implemented as a struct in order to allow the Go compiler to
+optimize things like high-V Info logs that are not triggered.  Not all of these
+implementations are implemented yet, but this structure was suggested as a way to
+ensure they *can* be implemented.  All of the real work is behind the LogSink
+interface.
+
 [warning-makes-no-sense]: http://dave.cheney.net/2015/11/05/lets-talk-about-logging
diff --git a/benchmark/benchmark_test.go b/benchmark/benchmark_test.go
index e16bd07..0c0d0fa 100644
--- a/benchmark/benchmark_test.go
+++ b/benchmark/benchmark_test.go
@@ -27,17 +27,15 @@ import (
 func noop(prefix, args string) {
 }
 
-func BenchmarkInfoOneArg(b *testing.B) {
-	var log logr.Logger = funcr.New(noop, funcr.Options{})
-
+//go:noinline
+func doInfoOneArg(b *testing.B, log logr.Logger) {
 	for i := 0; i < b.N; i++ {
 		log.Info("this is", "a", "string")
 	}
 }
 
-func BenchmarkInfoSeveralArgs(b *testing.B) {
-	var log logr.Logger = funcr.New(noop, funcr.Options{})
-
+//go:noinline
+func doInfoSeveralArgs(b *testing.B, log logr.Logger) {
 	for i := 0; i < b.N; i++ {
 		log.Info("multi",
 			"bool", true, "string", "str", "int", 42,
@@ -45,9 +43,8 @@ func BenchmarkInfoSeveralArgs(b *testing.B) {
 	}
 }
 
-func BenchmarkV0Info(b *testing.B) {
-	var log logr.Logger = funcr.New(noop, funcr.Options{})
-
+//go:noinline
+func doV0Info(b *testing.B, log logr.Logger) {
 	for i := 0; i < b.N; i++ {
 		log.V(0).Info("multi",
 			"bool", true, "string", "str", "int", 42,
@@ -55,9 +52,8 @@ func BenchmarkV0Info(b *testing.B) {
 	}
 }
 
-func BenchmarkV9Info(b *testing.B) {
-	var log logr.Logger = funcr.New(noop, funcr.Options{})
-
+//go:noinline
+func doV9Info(b *testing.B, log logr.Logger) {
 	for i := 0; i < b.N; i++ {
 		log.V(9).Info("multi",
 			"bool", true, "string", "str", "int", 42,
@@ -65,9 +61,8 @@ func BenchmarkV9Info(b *testing.B) {
 	}
 }
 
-func BenchmarkError(b *testing.B) {
-	var log logr.Logger = funcr.New(noop, funcr.Options{})
-
+//go:noinline
+func doError(b *testing.B, log logr.Logger) {
 	err := fmt.Errorf("error message")
 	for i := 0; i < b.N; i++ {
 		log.Error(err, "multi",
@@ -76,20 +71,88 @@ func BenchmarkError(b *testing.B) {
 	}
 }
 
-func BenchmarkWithValues(b *testing.B) {
-	var log logr.Logger = funcr.New(noop, funcr.Options{})
-
+//go:noinline
+func doWithValues(b *testing.B, log logr.Logger) {
 	for i := 0; i < b.N; i++ {
 		l := log.WithValues("k1", "v1", "k2", "v2")
 		_ = l
 	}
 }
 
-func BenchmarkWithName(b *testing.B) {
-	var log logr.Logger = funcr.New(noop, funcr.Options{})
-
+//go:noinline
+func doWithName(b *testing.B, log logr.Logger) {
 	for i := 0; i < b.N; i++ {
 		l := log.WithName("name")
 		_ = l
 	}
 }
+
+func BenchmarkDiscardInfoOneArg(b *testing.B) {
+	var log logr.Logger = logr.Discard()
+	doInfoOneArg(b, log)
+}
+
+func BenchmarkDiscardInfoSeveralArgs(b *testing.B) {
+	var log logr.Logger = logr.Discard()
+	doInfoSeveralArgs(b, log)
+}
+
+func BenchmarkDiscardV0Info(b *testing.B) {
+	var log logr.Logger = logr.Discard()
+	doV0Info(b, log)
+}
+
+func BenchmarkDiscardV9Info(b *testing.B) {
+	var log logr.Logger = logr.Discard()
+	doV9Info(b, log)
+}
+
+func BenchmarkDiscardError(b *testing.B) {
+	var log logr.Logger = logr.Discard()
+	doError(b, log)
+}
+
+func BenchmarkDiscardWithValues(b *testing.B) {
+	var log logr.Logger = logr.Discard()
+	doWithValues(b, log)
+}
+
+func BenchmarkDiscardWithName(b *testing.B) {
+	var log logr.Logger = logr.Discard()
+	doWithName(b, log)
+}
+
+func BenchmarkFuncrInfoOneArg(b *testing.B) {
+	var log logr.Logger = funcr.New(noop, funcr.Options{})
+	doInfoOneArg(b, log)
+}
+
+func BenchmarkFuncrInfoSeveralArgs(b *testing.B) {
+	var log logr.Logger = funcr.New(noop, funcr.Options{})
+	doInfoSeveralArgs(b, log)
+}
+
+func BenchmarkFuncrV0Info(b *testing.B) {
+	var log logr.Logger = funcr.New(noop, funcr.Options{})
+	doV0Info(b, log)
+}
+
+func BenchmarkFuncrV9Info(b *testing.B) {
+	var log logr.Logger = funcr.New(noop, funcr.Options{})
+	doV9Info(b, log)
+}
+
+func BenchmarkFuncrError(b *testing.B) {
+	var log logr.Logger = funcr.New(noop, funcr.Options{})
+	doError(b, log)
+}
+
+func BenchmarkFuncrWithValues(b *testing.B) {
+	var log logr.Logger = funcr.New(noop, funcr.Options{})
+	doWithValues(b, log)
+}
+
+func BenchmarkFuncrWithName(b *testing.B) {
+	var log logr.Logger = funcr.New(noop, funcr.Options{})
+	doWithName(b, log)
+}
diff --git a/discard.go b/discard.go
index 2bafb13..ec02a78 100644
--- a/discard.go
+++ b/discard.go
@@ -16,36 +16,38 @@ limitations under the License.
 
 package logr
 
-// Discard returns a valid Logger that discards all messages logged to it.
-// It can be used whenever the caller is not interested in the logs.
+// Discard returns a Logger that discards all messages logged to it.  It can be
+// used whenever the caller is not interested in the logs.
 func Discard() Logger {
-	return DiscardLogger{}
+	return Logger{
+		level: 0,
+		sink:  discardLogger{},
+	}
 }
 
-// DiscardLogger is a Logger that discards all messages.
-type DiscardLogger struct{}
+// discardLogger is a LogSink that discards all messages.
+type discardLogger struct{}
 
-func (l DiscardLogger) Enabled() bool {
-	return false
+// Verify that it actually implements the interface
+var _ LogSink = discardLogger{}
+
+func (l discardLogger) Init(RuntimeInfo) {
 }
 
-func (l DiscardLogger) Info(msg string, keysAndValues ...interface{}) {
+func (l discardLogger) Enabled(int) bool {
+	return false
 }
 
-func (l DiscardLogger) Error(err error, msg string, keysAndValues ...interface{}) {
+func (l discardLogger) Info(int, string, ...interface{}) {
 }
 
-func (l DiscardLogger) V(level int) Logger {
-	return l
+func (l discardLogger) Error(error, string, ...interface{}) {
 }
 
-func (l DiscardLogger) WithValues(keysAndValues ...interface{}) Logger {
+func (l discardLogger) WithValues(...interface{}) LogSink {
 	return l
 }
 
-func (l DiscardLogger) WithName(name string) Logger {
+func (l discardLogger) WithName(string) LogSink {
 	return l
 }
-
-// Verify that it actually implements the interface
-var _ Logger = DiscardLogger{}
diff --git a/discard_test.go b/discard_test.go
index e2a01cb..c1a6177 100644
--- a/discard_test.go
+++ b/discard_test.go
@@ -23,7 +23,7 @@ import (
 
 func TestDiscard(t *testing.T) {
 	l := Discard()
-	if _, ok := l.(DiscardLogger); !ok {
+	if _, ok := l.sink.(discardLogger); !ok {
 		t.Error("did not return the expected underlying type")
 	}
 	// Verify that none of the methods panic, there is not more we can test.
diff --git a/examples/tab_logger.go b/examples/tab_logger.go
index 445c6d6..84892ae 100644
--- a/examples/tab_logger.go
+++ b/examples/tab_logger.go
@@ -24,18 +24,24 @@ import (
 	"github.com/go-logr/logr"
 )
 
-// TabLogger is a sample logr.Logger that logs to stderr.
-// It's terribly inefficient, and is *only* a basic example.
-type TabLogger struct {
+// tabLogSink is a sample logr.LogSink that logs to stderr.
+// It's terribly inefficient, and is only a basic example.
+type tabLogSink struct {
 	name      string
 	keyValues map[string]interface{}
+	writer    *tabwriter.Writer
+}
+
+var _ logr.LogSink = tabLogSink{}
 
-	writer *tabwriter.Writer
+func (_ tabLogSink) Init(info logr.RuntimeInfo) {
 }
 
-var _ logr.Logger = &TabLogger{}
+func (_ tabLogSink) Enabled(level int) bool {
+	return true
+}
 
-func (l *TabLogger) Info(msg string, kvs ...interface{}) {
+func (l tabLogSink) Info(level int, msg string, kvs ...interface{}) {
 	fmt.Fprintf(l.writer, "%s\t%s\t", l.name, msg)
 	for k, v := range l.keyValues {
 		fmt.Fprintf(l.writer, "%s: %+v  ", k, v)
@@ -47,28 +53,20 @@ func (l *TabLogger) Info(msg string, kvs ...interface{}) {
 	l.writer.Flush()
 }
 
-func (_ *TabLogger) Enabled() bool {
-	return true
-}
-
-func (l *TabLogger) Error(err error, msg string, kvs ...interface{}) {
+func (l tabLogSink) Error(err error, msg string, kvs ...interface{}) {
 	kvs = append(kvs, "error", err)
-	l.Info(msg, kvs...)
-}
-
-func (l *TabLogger) V(_ int) logr.Logger {
-	return l
+	l.Info(0, msg, kvs...)
 }
 
-func (l *TabLogger) WithName(name string) logr.Logger {
-	return &TabLogger{
+func (l tabLogSink) WithName(name string) logr.LogSink {
+	return tabLogSink{
 		name:      l.name + "." + name,
 		keyValues: l.keyValues,
 		writer:    l.writer,
 	}
 }
 
-func (l *TabLogger) WithValues(kvs ...interface{}) logr.Logger {
+func (l tabLogSink) WithValues(kvs ...interface{}) logr.LogSink {
 	newMap := make(map[string]interface{}, len(l.keyValues)+len(kvs)/2)
 	for k, v := range l.keyValues {
 		newMap[k] = v
@@ -76,7 +74,7 @@ func (l *TabLogger) WithValues(kvs ...interface{}) logr.Logger {
 	for i := 0; i < len(kvs); i += 2 {
 		newMap[kvs[i].(string)] = kvs[i+1]
 	}
-	return &TabLogger{
+	return tabLogSink{
 		name:      l.name,
 		keyValues: newMap,
 		writer:    l.writer,
@@ -84,7 +82,8 @@ func (l *TabLogger) WithValues(kvs ...interface{}) logr.Logger {
 }
 
 func NewTabLogger() logr.Logger {
-	return &TabLogger{
+	sink := tabLogSink{
 		writer: tabwriter.NewWriter(os.Stderr, 40, 8, 2, '\t', 0),
 	}
+	return logr.New(sink)
 }
diff --git a/funcr/funcr.go b/funcr/funcr.go
index 613a66d..4262969 100644
--- a/funcr/funcr.go
+++ b/funcr/funcr.go
@@ -33,14 +33,15 @@ import (
 
 // New returns a logr.Logger which is implemented by a function.
 func New(fn func(prefix, args string), opts Options) logr.Logger {
-	return fnlogger{
-		level:     0,
+	fnl := &fnlogger{
 		prefix:    "",
 		values:    nil,
+		depth:     0,
 		write:     fn,
 		logCaller: opts.LogCaller,
 		verbosity: opts.Verbosity,
 	}
+	return logr.New(fnl)
 }
 
 type Options struct {
@@ -63,9 +64,9 @@ const (
 )
 
 type fnlogger struct {
-	level     int
 	prefix    string
 	values    []interface{}
+	depth     int
 	write     func(prefix, args string)
 	logCaller MessageClass
 	verbosity int
@@ -266,32 +267,32 @@ type callerID struct {
 }
 
 func (l fnlogger) caller() callerID {
-	// +1 for this frame, +1 for logr itself.
-	// FIXME: Maybe logr should offer a clue as to how many frames are
-	// needed here?  Or is it part of the contract to LogSinks?
-	_, file, line, ok := runtime.Caller(framesToCaller() + 2)
+	// +1 for this frame.
+	_, file, line, ok := runtime.Caller(framesToCaller() + l.depth + 1)
 	if !ok {
 		return callerID{"<unknown>", 0}
 	}
 	return callerID{filepath.Base(file), line}
 }
 
-func (l fnlogger) Enabled() bool {
-	return l.level <= l.verbosity
+func (l *fnlogger) Init(info logr.RuntimeInfo) {
+	l.depth += info.CallDepth
 }
 
-func (l fnlogger) Info(msg string, kvList ...interface{}) {
-	if l.Enabled() {
-		args := make([]interface{}, 0, 64) // using a constant here impacts perf
-		if l.logCaller == All || l.logCaller == Info {
-			args = append(args, "caller", l.caller())
-		}
-		args = append(args, "level", l.level, "msg", msg)
-		args = append(args, l.values...)
-		args = append(args, kvList...)
-		argsStr := flatten(args...)
-		l.write(l.prefix, argsStr)
+func (l fnlogger) Enabled(level int) bool {
+	return level <= l.verbosity
+}
+
+func (l fnlogger) Info(level int, msg string, kvList ...interface{}) {
+	args := make([]interface{}, 0, 64) // using a constant here impacts perf
+	if l.logCaller == All || l.logCaller == Info {
+		args = append(args, "caller", l.caller())
 	}
+	args = append(args, "level", level, "msg", msg)
+	args = append(args, l.values...)
+	args = append(args, kvList...)
+	argsStr := flatten(args...)
+	l.write(l.prefix, argsStr)
 }
 
 func (l fnlogger) Error(err error, msg string, kvList ...interface{}) {
@@ -311,27 +312,34 @@ func (l fnlogger) Error(err error, msg string, kvList ...interface{}) {
 	l.write(l.prefix, argsStr)
 }
 
-func (l fnlogger) V(level int) logr.Logger {
-	l.level += level
-	return l
-}
-
 // WithName returns a new Logger with the specified name appended.  funcr
 // uses '/' characters to separate name elements.  Callers should not pass '/'
 // in the provided name string, but this library does not actually enforce that.
-func (l fnlogger) WithName(name string) logr.Logger {
-	if len(l.prefix) > 0 {
-		l.prefix = l.prefix + "/"
+func (l *fnlogger) WithName(name string) logr.LogSink {
+	l2 := &fnlogger{}
+	*l2 = *l
+	if len(l2.prefix) > 0 {
+		l.prefix = l2.prefix + "/"
 	}
-	l.prefix += name
+	l2.prefix += name
 	return l
 }
 
-func (l fnlogger) WithValues(kvList ...interface{}) logr.Logger {
+func (l *fnlogger) WithValues(kvList ...interface{}) logr.LogSink {
+	l2 := &fnlogger{}
+	*l2 = *l
 	// Three slice args forces a copy.
 	n := len(l.values)
-	l.values = append(l.values[:n:n], kvList...)
-	return l
+	l2.values = append(l2.values[:n:n], kvList...)
+	return l2
+}
+
+func (l *fnlogger) WithCallDepth(depth int) logr.LogSink {
+	l2 := &fnlogger{}
+	*l2 = *l
+	l2.depth += depth
+	return l2
 }
 
-var _ logr.Logger = fnlogger{}
+var _ logr.LogSink = &fnlogger{}
+var _ logr.CallDepthLogSink = &fnlogger{}
diff --git a/funcr/funcr_test.go b/funcr/funcr_test.go
index 2971e20..c38e0fc 100644
--- a/funcr/funcr_test.go
+++ b/funcr/funcr_test.go
@@ -1,3 +1,19 @@
+/*
+Copyright 2021 The logr Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
 package funcr
 
 import (
diff --git a/logr.go b/logr.go
index 842428b..9ea027b 100644
--- a/logr.go
+++ b/logr.go
@@ -17,84 +17,110 @@ limitations under the License.
 // This design derives from Dave Cheney's blog:
 //     http://dave.cheney.net/2015/11/05/lets-talk-about-logging
 //
-// This is a BETA grade API.  Until there is a significant 2nd implementation,
-// I don't really know how it will change.
+// This is a BETA grade API.
 
-// Package logr defines abstract interfaces for logging.  Packages can depend on
-// these interfaces and callers can implement logging in whatever way is
-// appropriate.
+// Package logr defines a general-purpose logging API and abstract interfaces
+// to back that API.  Packages in the Go ecosystem can depend on this package,
+// while callers can implement logging with whatever backend is appropriate.
 //
 // Usage
+// -----
 //
-// Logging is done using a Logger.  Loggers can have name prefixes and named
-// values attached, so that all log messages logged with that Logger have some
-// base context associated.
+// Logging is done using a Logger instance.  Logger is a concrete type with
+// methods, which defers the actual logging to a LogSink interface.  The main
+// methods of Logger are Info() and Error().  Arguments to Info() and Error()
+// are key/value pairs rather than printf-style formatted strings, emphasizing
+// "structured logging".
 //
-// The term "key" is used to refer to the name associated with a particular
-// value, to disambiguate it from the general Logger name.
+// With Go's standard log package, we might write:
+//   log.Printf("setting target value %s", targetValue)
 //
-// For instance, suppose we're trying to reconcile the state of an object, and
-// we want to log that we've made some decision.
+// With logr's structured logging, we'd write:
+//   logger.Info("setting target", "value", targetValue)
 //
-// With the traditional log package, we might write:
-//   log.Printf("decided to set field foo to value %q for object %s/%s",
-//       targetValue, object.Namespace, object.Name)
+// Errors are much the same.  Instead of:
+//   log.Printf("failed to open the pod bay door for user %s: %v", user, err)
 //
-// With logr's structured logging, we'd write:
-//   // elsewhere in the file, set up the logger to log with the prefix of
-//   // "reconcilers", and the named value target-type=Foo, for extra context.
-//   log := mainLogger.WithName("reconcilers").WithValues("target-type", "Foo")
+// We'd write:
+//   logger.Error(err, "failed to open the pod bay door", "user", user)
 //
-//   // later on...
-//   log.Info("setting foo on object", "value", targetValue, "object", object)
+// Info() and Error() are very similar, but they are separate methods so that
+// LogSink implementations can choose to do things like attach additional
+// information (such as stack traces) on calls to Error().
+//
+// Verbosity
+// ---------
+//
+// Often we want to log information only when the application in "verbose
+// mode".  To write log-lines that are more verbose, Logger has a V() method.
+// The higher the V-level of a log-line, the less critical it is considered.
+// Log-lines with V-levels that are not enabled (as per the LogSink) will not
+// be written.  Level V(0) is the default, and logger.V(0).Info() has the same
+// meaning as logger.Info().  Negative V-levels have the same meaning as V(0).
+//
+// Where we might have written:
+//   if flVerbose >= 2 {
+//       log.Printf("an unusual thing happened")
+//   }
+//
+// We can write:
+//   logger.V(2).Info("an unusual thing happened")
 //
-// Depending on our logging implementation, we could then make logging decisions
-// based on field values (like only logging such events for objects in a certain
-// namespace), or copy the structured information into a structured log store.
+// Logger Names
+// ------------
 //
-// For logging errors, Logger has a method called Error.  Suppose we wanted to
-// log an error while reconciling.  With the traditional log package, we might
-// write:
-//   log.Errorf("unable to reconcile object %s/%s: %v", object.Namespace, object.Name, err)
+// Logger instances can have name strings so that all messages logged through
+// that instance have additional context.  For example, you might want to add
+// a subsystem name:
 //
-// With logr, we'd instead write:
-//   // assuming the above setup for log
-//   log.Error(err, "unable to reconcile object", "object", object)
+//   logger.WithName("compactor").Info("started", "time", time.Now())
 //
-// This functions similarly to:
-//   log.Info("unable to reconcile object", "error", err, "object", object)
+// The WithName() method returns a new Logger, which can be passed to
+// constructors or other functions for further use.  Repeated use of WithName()
+// will accumulate name "segments".  These name segments will be joined in some
+// way by the LogSink implementation.  It is strongly recommended that name
+// segments contain simple identifiers (letters, digits, and hyphen), and do
+// not contain characters that could muddle the log output or confuse the
+// joining operation (e.g. whitespace, commas, periods, slashes, brackets,
+// quotes, etc).
 //
-// However, it ensures that a standard key for the error value ("error") is used
-// across all error logging.  Furthermore, certain implementations may choose to
-// attach additional information (such as stack traces) on calls to Error, so
-// it's preferred to use Error to log errors.
+// Saved Values
+// ------------
 //
-// Parts of a log line
+// Logger instances can store any number of key/value pairs, which will be
+// logged alongside all messages logged through that instance.  For example,
+// you might want to create a Logger instance per managed object:
 //
-// Each log message from a Logger has four types of context:
-// logger name, log verbosity, log message, and the named values.
+// With the standard log package, we might write:
+//   log.Printf("decided to set field foo to value %q for object %s/%s",
+//       targetValue, object.Namespace, object.Name)
+//
+// With logr's we'd write:
+//   // Elsewhere: set up the logger to log the object name.
+//   obj.logger = mainLogger.WithValues(
+//       "name", obj.name, "namespace", obj.namespace)
+//
+//   // later on...
+//   obj.logger.Info("setting foo", "value", targetValue)
 //
-// The Logger name consists of a series of name "segments" added by successive
-// calls to WithName.  These name segments will be joined in some way by the
-// underlying implementation.  It is strongly recommended that name segments
-// contain simple identifiers (letters, digits, and hyphen), and do not contain
-// characters that could muddle the log output or confuse the joining operation
-// (e.g.  whitespace, commas, periods, slashes, brackets, quotes, etc).
+// Best Practices
+// --------------
 //
-// Log verbosity represents how little a log matters.  Level zero, the default,
-// matters most.  Increasing levels matter less and less.  Try to avoid lots of
-// different verbosity levels, and instead provide useful keys, logger names,
-// and log messages for users to filter on.  It's illegal to pass a log level
-// below zero.
+// Logger has very few hard rules, with the goal that LogSink implementations
+// might have a lot of freedom to differentiate.  There are, however, some
+// things to consider.
 //
 // The log message consists of a constant message attached to the log line.
 // This should generally be a simple description of what's occurring, and should
-// never be a format string.
+// never be a format string.  Variable information can then be attached using
+// named values.
 //
-// Variable information can then be attached using named values (key/value
-// pairs).  Keys are arbitrary strings, while values may be any Go value.
+// Keys are arbitrary strings, but should generally be constant values.  Values
+// may be any Go value, but how the value is formatted is determined by the
+// LogSink implementation.
 //
 // Key Naming Conventions
+// ----------------------
 //
 // Keys are not strictly required to conform to any specification or regex, but
 // it is recommended that they:
@@ -125,6 +151,9 @@ limitations under the License.
 // would be necessary to represent at least message and timestamp as ordinary
 // named values).
 //
+// Break Glass
+// -----------
+//
 // Implementations may choose to give callers access to the underlying
 // logging implementation.  The recommended pattern for this is:
 //   // Underlier exposes access to the underlying logging implementation.
@@ -140,75 +169,135 @@ import (
 	"context"
 )
 
-// TODO: consider adding back in format strings if they're really needed
-// TODO: consider other bits of zap/zapcore functionality like ObjectMarshaller (for arbitrary objects)
-// TODO: consider other bits of glog functionality like Flush, OutputStats
+// New returns a new Logger instance.  This is primarily used by libraries
+// implementing LogSink, rather than end users.
+func New(sink LogSink) Logger {
+	logger := Logger{
+		sink: sink,
+	}
+	if withCallDepth, ok := sink.(CallDepthLogSink); ok {
+		logger.withCallDepth = withCallDepth
+	}
+	sink.Init(runtimeInfo)
+	return logger
+}
 
-// Logger represents the ability to log messages, both errors and not.
-type Logger interface {
-	// Enabled tests whether this Logger is enabled.  For example, commandline
-	// flags might be used to set the logging verbosity and disable some info
-	// logs.
-	Enabled() bool
+// Logger is an interface to an abstract logging implementation.  This is a
+// concrete type for performance reasons, but all the real work is passed on
+// to a LogSink.  Implementations of LogSink should provide their own
+// constructors that return Logger, not LogSink.
+type Logger struct {
+	level         int
+	sink          LogSink
+	withCallDepth CallDepthLogSink
+}
 
-	// Info logs a non-error message with the given key/value pairs as context.
-	//
-	// The msg argument should be used to add some constant description to
-	// the log line.  The key/value pairs can then be used to add additional
-	// variable information.  The key/value pairs should alternate string
-	// keys and arbitrary values.
-	Info(msg string, keysAndValues ...interface{})
-
-	// Error logs an error, with the given message and key/value pairs as context.
-	// It functions similarly to calling Info with the "error" named value, but may
-	// have unique behavior, and should be preferred for logging errors (see the
-	// package documentations for more information).
-	//
-	// The msg field should be used to add context to any underlying error,
-	// while the err field should be used to attach the actual error that
-	// triggered this log line, if present.
-	Error(err error, msg string, keysAndValues ...interface{})
+// Enabled tests whether this Logger is enabled.  For example, commandline
+// flags might be used to set the logging verbosity and disable some info
+// logs.
+func (l Logger) Enabled() bool {
+	return l.sink.Enabled(l.level)
+}
 
-	// V returns an Logger value for a specific verbosity level, relative to
-	// this Logger.  In other words, V values are additive.  V higher verbosity
-	// level means a log message is less important.  It's illegal to pass a log
-	// level less than zero.
-	V(level int) Logger
-
-	// WithValues adds some key-value pairs of context to a logger.
-	// See Info for documentation on how key/value pairs work.
-	WithValues(keysAndValues ...interface{}) Logger
-
-	// WithName adds a new element to the logger's name.
-	// Successive calls with WithName continue to append
-	// suffixes to the logger's name.  It's strongly recommended
-	// that name segments contain only letters, digits, and hyphens
-	// (see the package documentation for more information).
-	WithName(name string) Logger
+// Info logs a non-error message with the given key/value pairs as context.
+//
+// The msg argument should be used to add some constant description to
+// the log line.  The key/value pairs can then be used to add additional
+// variable information.  The key/value pairs must alternate string
+// keys and arbitrary values.
+func (l Logger) Info(msg string, keysAndValues ...interface{}) {
+	if l.Enabled() {
+		l.sink.Info(l.level, msg, keysAndValues...)
+	}
 }
 
-// InfoLogger provides compatibility with code that relies on the v0.1.0
-// interface.
+// Error logs an error, with the given message and key/value pairs as context.
+// It functions similarly to Info, but may have unique behavior, and should be
+// preferred for logging errors (see the package documentations for more
+// information).
 //
-// Deprecated: InfoLogger is an artifact of early versions of this API.  New
-// users should never use it and existing users should use Logger instead. This
-// will be removed in a future release.
-type InfoLogger = Logger
+// The msg argument should be used to add context to any underlying error,
+// while the err argument should be used to attach the actual error that
+// triggered this log line, if present.
+func (l Logger) Error(err error, msg string, keysAndValues ...interface{}) {
+	if l.Enabled() {
+		l.sink.Error(err, msg, keysAndValues...)
+	}
+}
 
+// V returns a new Logger instance for a specific verbosity level, relative to
+// this Logger.  In other words, V-levels are additive.  A higher verbosity
+// level means a log message is less important.  Negative V-levels are treated
+// as 0.
+func (l Logger) V(level int) Logger {
+	if level < 0 {
+		level = 0
+	}
+	l.level += level
+	return l
+}
+
+// WithValues returns a new Logger instance with additional key/value pairs.
+// See Info for documentation on how key/value pairs work.
+func (l Logger) WithValues(keysAndValues ...interface{}) Logger {
+	l.sink = l.sink.WithValues(keysAndValues...)
+	return l
+}
+
+// WithName returns a new Logger instance with the specified name element added
+// to the Logger's name.  Successive calls with WithName append additional
+// suffixes to the Logger's name.  It's strongly recommended that name segments
+// contain only letters, digits, and hyphens (see the package documentation for
+// more information).
+func (l Logger) WithName(name string) Logger {
+	l.sink = l.sink.WithName(name)
+	return l
+}
+
+// WithCallDepth returns a Logger instance that offsets the call stack by the
+// specified number of frames when logging call site information, if possible.
+// This is useful for users who have helper functions between the "real" call
+// site and the actual calls to Logger methods.  If depth is 0 the attribution
+// should be to the direct caller of this function.  If depth is 1 the
+// attribution should skip 1 call frame, and so on.  Successive calls to this
+// are additive.
+//
+// If the underlying log implementation supports a WithCallDepth(int) method,
+// it will be called and the result returned.  If the implementation does not
+// support CallDepthLogSink, the original Logger will be returned.
+func (l Logger) WithCallDepth(depth int) Logger {
+	if l.withCallDepth == nil {
+		return l
+	}
+	l.sink = l.withCallDepth.WithCallDepth(depth)
+	return l
+}
+
+// contextKey is how we find Loggers in a context.Context.
 type contextKey struct{}
 
-// FromContext returns a Logger constructed from ctx or nil if no
-// logger details are found.
-func FromContext(ctx context.Context) Logger {
+// FromContext returns a Logger from ctx or an error if no Logger is found.
+func FromContext(ctx context.Context) (Logger, error) {
 	if v, ok := ctx.Value(contextKey{}).(Logger); ok {
-		return v
+		return v, nil
 	}
 
-	return nil
+	return Logger{}, notFoundError{}
+}
+
+// notFoundError exists to carry an IsNotFound method.
+type notFoundError struct{}
+
+func (notFoundError) Error() string {
+	return "no logr.Logger was present"
 }
 
-// FromContextOrDiscard returns a Logger constructed from ctx or a Logger
-// that discards all messages if no logger details are found.
+func (notFoundError) IsNotFound() bool {
+	return true
+}
+
+// FromContextOrDiscard returns a Logger from ctx.  If no Logger is found, this
+// returns a Logger that discards all log messages.
 func FromContextOrDiscard(ctx context.Context) Logger {
 	if v, ok := ctx.Value(contextKey{}).(Logger); ok {
 		return v
@@ -217,12 +306,59 @@ func FromContextOrDiscard(ctx context.Context) Logger {
 	return Discard()
 }
 
-// NewContext returns a new context derived from ctx that embeds the Logger.
-func NewContext(ctx context.Context, l Logger) context.Context {
-	return context.WithValue(ctx, contextKey{}, l)
+// NewContext returns a new Context, derived from ctx, which carries the
+// provided Logger.
+func NewContext(ctx context.Context, logger Logger) context.Context {
+	return context.WithValue(ctx, contextKey{}, logger)
+}
+
+// RuntimeInfo holds information that the logr "core" library knows which
+// LogSinks might want to know.
+type RuntimeInfo struct {
+	// CallDepth is the number of call frames the logr library adds between the
+	// end-user and the LogSink.  LogSink implementations which choose to print
+	// the original logging site (e.g. file & line) should climb this many
+	// additional frames to find it.
+	CallDepth int
+}
+
+// runtimeInfo is a static global.  It must not be changed at run time.
+var runtimeInfo = RuntimeInfo{
+	CallDepth: 1,
+}
+
+// LogSink represents a logging implementation.  End-users will generally not
+// interact with this type.
+type LogSink interface {
+	// Init receives optional information about the logr library for LogSink
+	// implementations that need it.
+	Init(info RuntimeInfo)
+
+	// Enabled tests whether this LogSink is enabled at the specified V-level.
+	// For example, commandline flags might be used to set the logging
+	// verbosity and disable some info logs.
+	Enabled(level int) bool
+
+	// Info logs a non-error message with the given key/value pairs as context.
+	// The level argument is provided for optional logging.  This method will
+	// only be called when Enabled(level) is true. See Logger.Info for more
+	// details.
+	Info(level int, msg string, keysAndValues ...interface{})
+
+	// Error logs an error, with the given message and key/value pairs as
+	// context.  See Logger.Error for more details.
+	Error(err error, msg string, keysAndValues ...interface{})
+
+	// WithValues returns a new LogSink with additional key/value pairs.  See
+	// Logger.WithValues for more details.
+	WithValues(keysAndValues ...interface{}) LogSink
+
+	// WithName returns a new LogSink with the specified name appended.  See
+	// Logger.WithName for more details.
+	WithName(name string) LogSink
 }
 
-// CallDepthLogger represents a Logger that knows how to climb the call stack
+// CallDepthLogSink represents a Logger that knows how to climb the call stack
 // to identify the original call site and can offset the depth by a specified
 // number of frames.  This is useful for users who have helper functions
 // between the "real" call site and the actual calls to Logger methods.
@@ -232,35 +368,11 @@ func NewContext(ctx context.Context, l Logger) context.Context {
 //
 // This is an optional interface and implementations are not required to
 // support it.
-type CallDepthLogger interface {
-	Logger
-
+type CallDepthLogSink interface {
 	// WithCallDepth returns a Logger that will offset the call stack by the
 	// specified number of frames when logging call site information.  If depth
 	// is 0 the attribution should be to the direct caller of this method.  If
 	// depth is 1 the attribution should skip 1 call frame, and so on.
 	// Successive calls to this are additive.
-	WithCallDepth(depth int) Logger
-}
-
-// WithCallDepth returns a Logger that will offset the call stack by the
-// specified number of frames when logging call site information, if possible.
-// This is useful for users who have helper functions between the "real" call
-// site and the actual calls to Logger methods.  If depth is 0 the attribution
-// should be to the direct caller of this function.  If depth is 1 the
-// attribution should skip 1 call frame, and so on.  Successive calls to this
-// are additive.
-//
-// If the underlying log implementation supports the CallDepthLogger interface,
-// the WithCallDepth method will be called and the result returned.  If the
-// implementation does not support CallDepthLogger, the original Logger will be
-// returned.
-//
-// Callers which care about whether this was supported or not should test for
-// CallDepthLogger support themselves.
-func WithCallDepth(logger Logger, depth int) Logger {
-	if decorator, ok := logger.(CallDepthLogger); ok {
-		return decorator.WithCallDepth(depth)
-	}
-	return logger
+	WithCallDepth(depth int) LogSink
 }
diff --git a/logr_test.go b/logr_test.go
index e88f888..cb79a1b 100644
--- a/logr_test.go
+++ b/logr_test.go
@@ -21,91 +21,91 @@ import (
 	"testing"
 )
 
-// testLogger is a Logger just for testing that does nothing.
-type testLogger struct{}
+// testLogSink is a Logger just for testing that does nothing.
+type testLogSink struct{}
 
-func (l *testLogger) Enabled() bool {
-	return false
+func (l *testLogSink) Init(RuntimeInfo) {
 }
 
-func (l *testLogger) Info(msg string, keysAndValues ...interface{}) {
+func (l *testLogSink) Enabled(int) bool {
+	return false
 }
 
-func (l *testLogger) Error(err error, msg string, keysAndValues ...interface{}) {
+func (l *testLogSink) Info(level int, msg string, keysAndValues ...interface{}) {
 }
 
-func (l *testLogger) V(level int) Logger {
-	return l
+func (l *testLogSink) Error(err error, msg string, keysAndValues ...interface{}) {
 }
 
-func (l *testLogger) WithValues(keysAndValues ...interface{}) Logger {
+func (l *testLogSink) WithValues(keysAndValues ...interface{}) LogSink {
 	return l
 }
 
-func (l *testLogger) WithName(name string) Logger {
+func (l *testLogSink) WithName(name string) LogSink {
 	return l
 }
 
 // Verify that it actually implements the interface
-var _ Logger = &testLogger{}
+var _ LogSink = &testLogSink{}
 
 func TestContext(t *testing.T) {
 	ctx := context.TODO()
 
-	if out := FromContext(ctx); out != nil {
-		t.Errorf("expected nil logger, got %#v", out)
+	if out, err := FromContext(ctx); err == nil {
+		t.Errorf("expected error, got %#v", out)
 	}
-	if out := FromContextOrDiscard(ctx); out == nil {
-		t.Errorf("expected non-nil logger")
-	} else if _, ok := out.(DiscardLogger); !ok {
-		t.Errorf("expected a DiscardLogger, got %#v", out)
+	out := FromContextOrDiscard(ctx)
+	if _, ok := out.sink.(discardLogger); !ok {
+		t.Errorf("expected a discardLogger, got %#v", out)
 	}
 
-	logger := &testLogger{}
+	sink := &testLogSink{}
+	logger := New(sink)
 	lctx := NewContext(ctx, logger)
-	if out := FromContext(lctx); out == nil {
-		t.Errorf("expected non-nil logger")
-	} else if out.(*testLogger) != logger {
-		t.Errorf("expected output to be the same as input: got in=%p, out=%p", logger, out)
+	if out, err := FromContext(lctx); err != nil {
+		t.Errorf("unexpected error: %v", err)
+	} else if p := out.sink.(*testLogSink); p != sink {
+		t.Errorf("expected output to be the same as input: got in=%p, out=%p", sink, p)
 	}
-	if out := FromContextOrDiscard(lctx); out == nil {
-		t.Errorf("expected non-nil logger")
-	} else if out.(*testLogger) != logger {
-		t.Errorf("expected output to be the same as input: got in=%p, out=%p", logger, out)
+	out = FromContextOrDiscard(lctx)
+	if p := out.sink.(*testLogSink); p != sink {
+		t.Errorf("expected output to be the same as input: got in=%p, out=%p", sink, p)
 	}
 }
 
-// testCallDepthLogger is a Logger just for testing that does nothing.
-type testCallDepthLogger struct {
-	*testLogger
+// testCallDepthLogSink is a Logger just for testing that does nothing.
+type testCallDepthLogSink struct {
+	*testLogSink
 	depth int
 }
 
-func (l *testCallDepthLogger) WithCallDepth(depth int) Logger {
-	return &testCallDepthLogger{l.testLogger, l.depth + depth}
+func (l *testCallDepthLogSink) WithCallDepth(depth int) LogSink {
+	return &testCallDepthLogSink{l.testLogSink, l.depth + depth}
 }
 
 // Verify that it actually implements the interface
-var _ CallDepthLogger = &testCallDepthLogger{}
+var _ CallDepthLogSink = &testCallDepthLogSink{}
 
 func TestWithCallDepth(t *testing.T) {
 	// Test an impl that does not support it.
 	t.Run("not supported", func(t *testing.T) {
-		in := &testLogger{}
-		out := WithCallDepth(in, 42)
-		if out.(*testLogger) != in {
-			t.Errorf("expected output to be the same as input: got in=%p, out=%p", in, out)
+		in := &testLogSink{}
+		l := New(in)
+		out := l.WithCallDepth(42)
+		if p := out.sink.(*testLogSink); p != in {
+			t.Errorf("expected output to be the same as input: got in=%p, out=%p", in, p)
 		}
 	})
 
 	// Test an impl that does support it.
 	t.Run("supported", func(t *testing.T) {
-		in := &testCallDepthLogger{&testLogger{}, 0}
-		out := WithCallDepth(in, 42)
-		if out.(*testCallDepthLogger) == in {
+		in := &testCallDepthLogSink{&testLogSink{}, 0}
+		l := New(in)
+		out := l.WithCallDepth(42)
+		if out.sink.(*testCallDepthLogSink) == in {
 			t.Errorf("expected output to be different than input: got in=out=%p", in)
 		}
-		if cdl := out.(*testCallDepthLogger); cdl.depth != 42 {
+		if cdl := out.sink.(*testCallDepthLogSink); cdl.depth != 42 {
 			t.Errorf("expected depth=42, got %d", cdl.depth)
 		}
 	})
diff --git a/testing/test.go b/testing/test.go
index 1222fae..98f7f78 100644
--- a/testing/test.go
+++ b/testing/test.go
@@ -20,36 +20,14 @@ import (
 	"testing"
 
 	"github.com/go-logr/logr"
+	"github.com/go-logr/logr/funcr"
 )
 
-// TestLogger is a logr.Logger that prints through a testing.T object.
-// Only error logs will have any effect.
-type TestLogger struct {
-	T *testing.T
-}
-
-var _ logr.Logger = TestLogger{}
-
-func (_ TestLogger) Info(_ string, _ ...interface{}) {
-	// Do nothing.
-}
-
-func (_ TestLogger) Enabled() bool {
-	return false
-}
-
-func (log TestLogger) Error(err error, msg string, args ...interface{}) {
-	log.T.Logf("%s: %v -- %v", msg, err, args)
-}
-
-func (log TestLogger) V(v int) logr.Logger {
-	return log
-}
-
-func (log TestLogger) WithName(_ string) logr.Logger {
-	return log
-}
-
-func (log TestLogger) WithValues(_ ...interface{}) logr.Logger {
-	return log
+// NewTestLogger returns a logr.Logger that prints through a testing.T object.
+// Info logs are only enabled at V(0).
+func NewTestLogger(t *testing.T) logr.Logger {
+	fn := func(prefix, args string) {
+		t.Logf("%s: %s", prefix, args)
+	}
+	return funcr.New(fn, funcr.Options{})
 }
diff --git a/testing/null.go b/testing/test_test.go
similarity index 63%
rename from testing/null.go
rename to testing/test_test.go
index f5444b1..467518d 100644
--- a/testing/null.go
+++ b/testing/test_test.go
@@ -1,5 +1,5 @@
 /*
-Copyright 2019 The logr Authors.
+Copyright 2021 The logr Authors.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -16,11 +16,15 @@ limitations under the License.
 
 package testing
 
-import "github.com/go-logr/logr"
+import (
+	"fmt"
+	"testing"
+)
 
-// NullLogger is a logr.Logger that does nothing.
-//
-// Deprecated: NullLogger is idenitcal to logr.DiscardLogger.  It is retained
-// for backwards compatibility, but new users should use logr.DiscardLogger
-// instead.
-type NullLogger = logr.DiscardLogger
+func TestTestLogger(t *testing.T) {
+	logger := NewTestLogger(t)
+	logger.Info("info")
+	logger.V(0).Info("V(0).info")
+	logger.V(1).Info("v(1).info")
+	logger.Error(fmt.Errorf("error"), "error")
+}