diff --git a/osbase/event/etcd/etcd_test.go b/osbase/event/etcd/etcd_test.go
new file mode 100644
index 0000000..ee3672f
--- /dev/null
+++ b/osbase/event/etcd/etcd_test.go
@@ -0,0 +1,865 @@
+package etcd
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+	"sync"
+	"testing"
+	"time"
+
+	"go.etcd.io/etcd/api/v3/v3rpc/rpctypes"
+	"go.etcd.io/etcd/client/pkg/v3/testutil"
+	clientv3 "go.etcd.io/etcd/client/v3"
+	"go.etcd.io/etcd/tests/v3/integration"
+	"go.uber.org/zap"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/grpclog"
+
+	"source.monogon.dev/osbase/event"
+	"source.monogon.dev/osbase/logtree"
+)
+
+var (
+	cluster   *integration.ClusterV3
+	endpoints []string
+)
+
+// TestMain brings up a 3 node etcd cluster for tests to use.
+func TestMain(m *testing.M) {
+	// This logtree's data is not output anywhere.
+	lt := logtree.New()
+
+	cfg := integration.ClusterConfig{
+		Size:                 3,
+		GRPCKeepAliveMinTime: time.Millisecond,
+		LoggerBuilder: func(memberName string) *zap.Logger {
+			dn := logtree.DN("etcd." + memberName)
+			return logtree.Zapify(lt.MustLeveledFor(dn), zap.WarnLevel)
+		},
+	}
+	tb, cancel := testutil.NewTestingTBProthesis("curator")
+	defer cancel()
+	flag.Parse()
+	integration.BeforeTestExternal(tb)
+	grpclog.SetLoggerV2(logtree.GRPCify(lt.MustLeveledFor("grpc")))
+	cluster = integration.NewClusterV3(tb, &cfg)
+	endpoints = make([]string, 3)
+	for i := range endpoints {
+		endpoints[i] = cluster.Client(i).Endpoints()[0]
+	}
+
+	v := m.Run()
+	cluster.Terminate(tb)
+	os.Exit(v)
+}
+
+// setRaceWg creates a new WaitGroup and sets the given watcher to wait on this
+// WG after it performs the initial retrieval of a value from etcd, but before
+// it starts the watcher. This is used to test potential race conditions
+// present between these two steps.
+func setRaceWg[T any](w event.Watcher[T]) *sync.WaitGroup {
+	var wg sync.WaitGroup
+	w.(*watcher[T]).testRaceWG = &wg
+	return &wg
+}
+
+// setSetupWg creates a new WaitGroup and sets the given watcher to wait on
+// thie WG after an etcd watch channel is created. This is used in tests to
+// ensure that the watcher is fully created before it is tested.
+func setSetupWg[T any](w event.Watcher[T]) *sync.WaitGroup {
+	var wg sync.WaitGroup
+	w.(*watcher[T]).testSetupWG = &wg
+	return &wg
+}
+
+// testClient is an etcd connection to the test cluster.
+type testClient struct {
+	client     *clientv3.Client
+}
+
+func newTestClient(t *testing.T) *testClient {
+	t.Helper()
+	cli, err := clientv3.New(clientv3.Config{
+		Endpoints:            endpoints,
+		DialTimeout:          1 * time.Second,
+		DialKeepAliveTime:    1 * time.Second,
+		DialKeepAliveTimeout: 1 * time.Second,
+	})
+	if err != nil {
+		t.Fatalf("clientv3.New: %v", err)
+	}
+
+	return &testClient{
+		client:     cli,
+	}
+}
+
+func (d *testClient) close() {
+	d.client.Close()
+}
+
+// setEndpoints configures which endpoints (from {0,1,2}) the testClient is
+// connected to.
+func (d *testClient) setEndpoints(nums ...uint) {
+	var eps []string
+	for _, num := range nums {
+		eps = append(eps, endpoints[num])
+	}
+	d.client.SetEndpoints(eps...)
+}
+
+// put uses the testClient to store key with a given string value in etcd. It
+// contains retry logic that will block until the put is successful.
+func (d *testClient) put(t *testing.T, key, value string) {
+	t.Helper()
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	for {
+		ctxT, ctxC := context.WithTimeout(ctx, 100*time.Millisecond)
+		_, err := d.client.Put(ctxT, key, value)
+		ctxC()
+		if err == nil {
+			return
+		}
+		if errors.Is(err, ctxT.Err()) {
+			log.Printf("Retrying after %v", err)
+			continue
+		}
+		// Retry on etcd unavailability - this will happen in this code as the
+		// etcd cluster repeatedly loses quorum.
+		var eerr rpctypes.EtcdError
+		if errors.As(err, &eerr) && eerr.Code() == codes.Unavailable {
+			log.Printf("Retrying after %v", err)
+			continue
+		}
+		t.Fatalf("Put: %v", err)
+	}
+
+}
+
+// remove uses the testClient to remove the given key from etcd. It contains
+// retry logic that will block until the removal is successful.
+func (d *testClient) remove(t *testing.T, key string) {
+	t.Helper()
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	_, err := d.client.Delete(ctx, key)
+	if err == nil {
+		return
+	}
+	t.Fatalf("Delete: %v", err)
+}
+
+// expect runs a Get on the given Watcher, ensuring the returned value is a
+// given string.
+func expect(t *testing.T, w event.Watcher[StringAt], value string) {
+	t.Helper()
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	got, err := w.Get(ctx)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+
+	if got, want := got.Value, value; got != want {
+		t.Errorf("Wanted value %q, got %q", want, got)
+	}
+}
+
+// expectTimeout ensures that the given watcher blocks on a Get call for at
+// least 100 milliseconds. This is used by tests to attempt to verify that the
+// watcher Get is fully blocked, but can cause false positives (eg. when Get
+// blocks for 101 milliseconds). Thus, this function should be used sparingly
+// and in tests that perform other baseline behaviour checks alongside this
+// test.
+func expectTimeout[T any](t *testing.T, w event.Watcher[T]) {
+	t.Helper()
+	ctx, ctxC := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	got, err := w.Get(ctx)
+	ctxC()
+
+	if !errors.Is(err, ctx.Err()) {
+		t.Fatalf("Expected timeout error, got %v, %v", got, err)
+	}
+}
+
+// wait wraps a watcher into a channel of strings, ensuring that the watcher
+// never errors on Get calls and always returns strings.
+func wait(t *testing.T, w event.Watcher[StringAt]) (chan string, func()) {
+	t.Helper()
+	ctx, ctxC := context.WithCancel(context.Background())
+
+	c := make(chan string)
+
+	go func() {
+		for {
+			got, err := w.Get(ctx)
+			if err != nil && errors.Is(err, ctx.Err()) {
+				return
+			}
+			if err != nil {
+				t.Errorf("Get: %v", err)
+				close(c)
+				return
+			}
+			c <- got.Value
+		}
+	}()
+
+	return c, ctxC
+}
+
+// TestSimple exercises the simplest possible interaction with a watched value.
+func TestSimple(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	k := "test-simple"
+	value := NewValue(tc.client, k, DecoderStringAt)
+	tc.put(t, k, "one")
+
+	watcher := value.Watch()
+	defer watcher.Close()
+	expect(t, watcher, "one")
+
+	tc.put(t, k, "two")
+	expect(t, watcher, "two")
+
+	tc.put(t, k, "three")
+	tc.put(t, k, "four")
+	tc.put(t, k, "five")
+	tc.put(t, k, "six")
+
+	q, cancel := wait(t, watcher)
+	// Test will hang here if the above value does not receive the set "six".
+	log.Printf("a")
+	for el := range q {
+		log.Printf("%q", el)
+		if el == "six" {
+			break
+		}
+	}
+	log.Printf("b")
+	cancel()
+}
+
+// stringAtGet performs a Get from a Watcher, expecting a stringAt and updating
+// the given map with the retrieved value.
+func stringAtGet(ctx context.Context, t *testing.T, w event.Watcher[StringAt], m map[string]string) {
+	t.Helper()
+
+	vr, err := w.Get(ctx)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	m[vr.Key] = vr.Value
+}
+
+// TestSimpleRange exercises the simplest behaviour of a ranged watcher,
+// retrieving updaates via Get in a fully blocking fashion.
+func TestSimpleRange(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	ks := "test-simple-range/"
+	ke := "test-simple-range0"
+	value := NewValue(tc.client, ks, DecoderStringAt, Range(ke))
+	tc.put(t, ks+"a", "one")
+	tc.put(t, ks+"b", "two")
+	tc.put(t, ks+"c", "three")
+	tc.put(t, ks+"b", "four")
+
+	w := value.Watch()
+	defer w.Close()
+
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	res := make(map[string]string)
+	stringAtGet(ctx, t, w, res)
+	stringAtGet(ctx, t, w, res)
+	stringAtGet(ctx, t, w, res)
+
+	tc.put(t, ks+"a", "five")
+	tc.put(t, ks+"e", "six")
+
+	stringAtGet(ctx, t, w, res)
+	stringAtGet(ctx, t, w, res)
+
+	for _, te := range []struct {
+		k, w string
+	}{
+		{ks + "a", "five"},
+		{ks + "b", "four"},
+		{ks + "c", "three"},
+		{ks + "e", "six"},
+	} {
+		if want, got := te.w, res[te.k]; want != got {
+			t.Errorf("res[%q]: wanted %q, got %q", te.k, want, got)
+		}
+	}
+}
+
+// TestCancel ensures that watchers can resume after being canceled.
+func TestCancel(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	k := "test-cancel"
+	value := NewValue(tc.client, k, DecoderStringAt)
+	tc.put(t, k, "one")
+
+	watcher := value.Watch()
+	defer watcher.Close()
+	expect(t, watcher, "one")
+
+	ctx, ctxC := context.WithCancel(context.Background())
+	errs := make(chan error, 1)
+	go func() {
+		_, err := watcher.Get(ctx)
+		errs <- err
+	}()
+	ctxC()
+	if want, got := ctx.Err(), <-errs; !errors.Is(got, want) {
+		t.Fatalf("Wanted err %v, got %v", want, got)
+	}
+
+	// Successfully canceled watch, resuming should continue to work.
+	q, cancel := wait(t, watcher)
+	defer cancel()
+
+	tc.put(t, k, "two")
+	if want, got := "two", <-q; want != got {
+		t.Fatalf("Wanted val %q, got %q", want, got)
+	}
+}
+
+// TestCancelOnGet ensures that a context cancellation on an initial Get (which
+// translates to an etcd Get in a backoff loop) doesn't block.
+func TestCancelOnGet(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	k := "test-cancel-on-get"
+	value := NewValue(tc.client, k, DecoderStringAt)
+	watcher := value.Watch()
+	tc.put(t, k, "one")
+
+	// Cause partition between client endpoint and rest of cluster. Any read/write
+	// operations will now hang.
+	tc.setEndpoints(0)
+	cluster.Members[0].InjectPartition(t, cluster.Members[1], cluster.Members[2])
+	// Let raft timeouts expire so that the leader is aware a partition has occurred
+	// and stops serving data if it is not part of a quorum anymore.
+	//
+	// Otherwise, if Member[0] was the leader, there will be a window of opportunity
+	// during which it will continue to serve read data even though it has been
+	// partitioned off. This is an effect of how etcd handles linearizable reads:
+	// they go through the leader, but do not go through raft.
+	//
+	// The value is the default etcd leader timeout (1s) + some wiggle room.
+	time.Sleep(time.Second + time.Millisecond*100)
+
+	// Perform the initial Get(), which should attempt to retrieve a KV entry from
+	// the etcd service. This should hang. Unfortunately, there's no easy way to do
+	// this without an arbitrary sleep hoping that the client actually gets to the
+	// underlying etcd.Get call. This can cause false positives (eg. false 'pass'
+	// results) in this test.
+	ctx, ctxC := context.WithCancel(context.Background())
+	errs := make(chan error, 1)
+	go func() {
+		_, err := watcher.Get(ctx)
+		errs <- err
+	}()
+	time.Sleep(time.Second)
+
+	// Now that the etcd.Get is hanging, cancel the context.
+	ctxC()
+	// And now unpartition the cluster, resuming reads.
+	cluster.Members[0].RecoverPartition(t, cluster.Members[1], cluster.Members[2])
+
+	// The etcd.Get() call should've returned with a context cancellation.
+	err := <-errs
+	switch {
+	case err == nil:
+		t.Errorf("watcher.Get() returned no error, wanted context error")
+	case errors.Is(err, ctx.Err()):
+		// Okay.
+	default:
+		t.Errorf("watcher.Get() returned %v, wanted context error", err)
+	}
+}
+
+// TestClientReconnect forces a 'reconnection' of an active watcher from a
+// running member to another member, by stopping the original member and
+// explicitly reconnecting the client to other available members.
+//
+// This doe not reflect a situation expected during Metropolis runtime, as we
+// do not expect splits between an etcd client and its connected member
+// (instead, all etcd clients only connect to their local member). However, it
+// is still an important safety test to perform, and it also exercies the
+// equivalent behaviour of an etcd client re-connecting for any other reason.
+func TestClientReconnect(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+	tc.setEndpoints(0)
+
+	k := "test-client-reconnect"
+	value := NewValue(tc.client, k, DecoderStringAt)
+	tc.put(t, k, "one")
+
+	watcher := value.Watch()
+	defer watcher.Close()
+	expect(t, watcher, "one")
+
+	q, cancel := wait(t, watcher)
+	defer cancel()
+
+	cluster.Members[0].Stop(t)
+	defer cluster.Members[0].Restart(t)
+	cluster.WaitLeader(t)
+
+	tc.setEndpoints(1, 2)
+	tc.put(t, k, "two")
+
+	if want, got := "two", <-q; want != got {
+		t.Fatalf("Watcher received incorrect data after client restart, wanted %q, got %q", want, got)
+	}
+}
+
+// TestClientPartition forces a temporary partition of the etcd member while a
+// watcher is running, updates the value from across the partition, and undoes
+// the partition.
+// The partition is expected to be entirely transparent to the watcher.
+func TestClientPartition(t *testing.T) {
+	tcOne := newTestClient(t)
+	defer tcOne.close()
+	tcOne.setEndpoints(0)
+
+	tcRest := newTestClient(t)
+	defer tcRest.close()
+	tcRest.setEndpoints(1, 2)
+
+	k := "test-client-partition"
+	valueOne := NewValue(tcOne.client, k, DecoderStringAt)
+	watcherOne := valueOne.Watch()
+	defer watcherOne.Close()
+	valueRest := NewValue(tcRest.client, k, DecoderStringAt)
+	watcherRest := valueRest.Watch()
+	defer watcherRest.Close()
+
+	tcRest.put(t, k, "a")
+	expect(t, watcherOne, "a")
+	expect(t, watcherRest, "a")
+
+	cluster.Members[0].InjectPartition(t, cluster.Members[1], cluster.Members[2])
+
+	tcRest.put(t, k, "b")
+	expect(t, watcherRest, "b")
+	expectTimeout(t, watcherOne)
+
+	cluster.Members[0].RecoverPartition(t, cluster.Members[1], cluster.Members[2])
+
+	expect(t, watcherOne, "b")
+	tcRest.put(t, k, "c")
+	expect(t, watcherOne, "c")
+	expect(t, watcherRest, "c")
+
+}
+
+// TestEarlyUse exercises the correct behaviour of the value watcher on a value
+// that is not yet set.
+func TestEarlyUse(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	k := "test-early-use"
+
+	value := NewValue(tc.client, k, DecoderStringAt)
+	watcher := value.Watch()
+	defer watcher.Close()
+
+	wg := setSetupWg(watcher)
+	wg.Add(1)
+	q, cancel := wait(t, watcher)
+	defer cancel()
+
+	wg.Done()
+
+	tc.put(t, k, "one")
+
+	if want, got := "one", <-q; want != got {
+		t.Fatalf("Expected %q, got %q", want, got)
+	}
+}
+
+// TestRemove exercises the basic functionality of handling deleted values.
+func TestRemove(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	k := "test-remove"
+	tc.put(t, k, "one")
+
+	value := NewValue(tc.client, k, DecoderStringAt)
+	watcher := value.Watch()
+	defer watcher.Close()
+
+	expect(t, watcher, "one")
+	tc.remove(t, k)
+	expect(t, watcher, "")
+}
+
+// TestRemoveRange exercises the behaviour of a Get on a ranged watcher when a
+// value is removed.
+func TestRemoveRange(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	ks := "test-remove-range/"
+	ke := "test-remove-range0"
+	value := NewValue(tc.client, ks, DecoderStringAt, Range(ke))
+	tc.put(t, ks+"a", "one")
+	tc.put(t, ks+"b", "two")
+	tc.put(t, ks+"c", "three")
+	tc.put(t, ks+"b", "four")
+	tc.remove(t, ks+"c")
+
+	w := value.Watch()
+	defer w.Close()
+
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	res := make(map[string]string)
+	stringAtGet(ctx, t, w, res)
+	stringAtGet(ctx, t, w, res)
+
+	for _, te := range []struct {
+		k, w string
+	}{
+		{ks + "a", "one"},
+		{ks + "b", "four"},
+		{ks + "c", ""},
+	} {
+		if want, got := te.w, res[te.k]; want != got {
+			t.Errorf("res[%q]: wanted %q, got %q", te.k, want, got)
+		}
+	}
+}
+
+// TestEmptyRace forces the watcher to retrieve an empty value from the K/V
+// store at first, and establishing the watch channel after a new value has
+// been stored in the same place.
+func TestEmptyRace(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	k := "test-remove-race"
+	tc.put(t, k, "one")
+	tc.remove(t, k)
+
+	value := NewValue(tc.client, k, DecoderStringAt)
+	watcher := value.Watch()
+	defer watcher.Close()
+
+	wg := setRaceWg(watcher)
+	wg.Add(1)
+	q, cancel := wait(t, watcher)
+	defer cancel()
+
+	tc.put(t, k, "two")
+	wg.Done()
+
+	if want, got := "two", <-q; want != got {
+		t.Fatalf("Watcher received incorrect data after client restart, wanted %q, got %q", want, got)
+	}
+}
+
+type errOrInt struct {
+	val int64
+	err error
+}
+
+// TestDecoder exercises the BytesDecoder functionality of the watcher, by
+// creating a value with a decoder that only accepts string-encoded integers
+// that are divisible by three. The test then proceeds to put a handful of
+// values into etcd, ensuring that undecodable values correctly return an error
+// on Get, but that the watcher continues to work after the error has been
+// returned.
+func TestDecoder(t *testing.T) {
+	decoderDivisibleByThree := func(_, value []byte) (int64, error) {
+		num, err := strconv.ParseInt(string(value), 10, 64)
+		if err != nil {
+			return 0, fmt.Errorf("not a valid number")
+		}
+		if (num % 3) != 0 {
+			return 0, fmt.Errorf("not divisible by 3")
+		}
+		return num, nil
+	}
+
+	tc := newTestClient(t)
+	defer tc.close()
+
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	k := "test-decoder"
+	value := NewValue(tc.client, k, decoderDivisibleByThree)
+	watcher := value.Watch()
+	defer watcher.Close()
+	tc.put(t, k, "3")
+	_, err := watcher.Get(ctx)
+	if err != nil {
+		t.Fatalf("Initial Get: %v", err)
+	}
+
+	// Stream updates into arbitrarily-bounded test channel.
+	queue := make(chan errOrInt, 100)
+	go func() {
+		for {
+			res, err := watcher.Get(ctx)
+			if err != nil && errors.Is(err, ctx.Err()) {
+				return
+			}
+			if err != nil {
+				queue <- errOrInt{
+					err: err,
+				}
+			} else {
+				queue <- errOrInt{
+					val: res,
+				}
+			}
+		}
+	}()
+
+	var wantList []*int64
+	wantError := func(val string) {
+		wantList = append(wantList, nil)
+		tc.put(t, k, val)
+	}
+	wantValue := func(val string, decoded int64) {
+		wantList = append(wantList, &decoded)
+		tc.put(t, k, val)
+	}
+
+	wantError("")
+	wantValue("9", 9)
+	wantError("foo")
+	wantValue("18", 18)
+	wantError("10")
+	wantValue("27", 27)
+	wantValue("36", 36)
+
+	for i, want := range wantList {
+		q := <-queue
+		if want == nil && q.err == nil {
+			t.Fatalf("Case %d: wanted error, got no error and value %d", i, q.val)
+		}
+		if want != nil && (*want) != q.val {
+			t.Fatalf("Case %d: wanted value %d, got error %v and value %d", i, *want, q.err, q.val)
+		}
+	}
+}
+
+// TestBacklog ensures that the watcher can handle a large backlog of changes
+// in etcd that the client didnt' keep up with, and that whatever final state
+// is available to the client when it actually gets to calling Get().
+func TestBacklog(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	k := "test-backlog"
+	value := NewValue(tc.client, k, DecoderStringAt)
+	watcher := value.Watch()
+	defer watcher.Close()
+
+	tc.put(t, k, "initial")
+	expect(t, watcher, "initial")
+
+	for i := 0; i < 1000; i++ {
+		tc.put(t, k, fmt.Sprintf("val-%d", i))
+	}
+
+	ctx, ctxC := context.WithTimeout(context.Background(), time.Second)
+	defer ctxC()
+	for {
+		valB, err := watcher.Get(ctx)
+		if err != nil {
+			t.Fatalf("Get() returned error before expected final value: %v", err)
+		}
+		if valB.Value == "val-999" {
+			break
+		}
+	}
+}
+
+// TestBacklogRange ensures that the ranged etcd watcher can handle a large
+// backlog of changes in etcd that the client didn't keep up with.
+func TestBacklogRange(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+
+	ks := "test-backlog-range/"
+	ke := "test-backlog-range0"
+	value := NewValue(tc.client, ks, DecoderStringAt, Range(ke))
+	w := value.Watch()
+	defer w.Close()
+
+	for i := 0; i < 100; i++ {
+		if i%2 == 0 {
+			tc.put(t, ks+"a", fmt.Sprintf("val-%d", i))
+		} else {
+			tc.put(t, ks+"b", fmt.Sprintf("val-%d", i))
+		}
+	}
+
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	res := make(map[string]string)
+	stringAtGet(ctx, t, w, res)
+	stringAtGet(ctx, t, w, res)
+
+	for _, te := range []struct {
+		k, w string
+	}{
+		{ks + "a", "val-98"},
+		{ks + "b", "val-99"},
+	} {
+		if want, got := te.w, res[te.k]; want != got {
+			t.Errorf("res[%q]: wanted %q, got %q", te.k, want, got)
+		}
+	}
+}
+
+// TestBacklogOnly exercises the BacklogOnly option for non-ranged watchers,
+// which effectively makes any Get operation non-blocking (but also showcases
+// that unless a Get without BacklogOnly is issues, no new data will appear by
+// itself in the watcher - which is an undocumented implementation detail of the
+// option).
+func TestBacklogOnly(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	k := "test-backlog-only"
+	tc.put(t, k, "initial")
+
+	value := NewValue(tc.client, k, DecoderStringAt)
+	watcher := value.Watch()
+	defer watcher.Close()
+
+	d, err := watcher.Get(ctx, event.BacklogOnly[StringAt]())
+	if err != nil {
+		t.Fatalf("First Get failed: %v", err)
+	}
+	if want, got := "initial", d.Value; want != got {
+		t.Fatalf("First Get: wanted value %q, got %q", want, got)
+	}
+
+	// As expected, next call to Get with BacklogOnly fails - there truly is no new
+	// updates to emit.
+	_, err = watcher.Get(ctx, event.BacklogOnly[StringAt]())
+	if want, got := event.ErrBacklogDone, err; !errors.Is(got, want) {
+		t.Fatalf("Second Get: wanted %v, got %v", want, got)
+	}
+
+	// Implementation detail: even though there is a new value ('second'),
+	// BacklogOnly will still return ErrBacklogDone.
+	tc.put(t, k, "second")
+	_, err = watcher.Get(ctx, event.BacklogOnly[StringAt]())
+	if want, got := event.ErrBacklogDone, err; !errors.Is(got, want) {
+		t.Fatalf("Third Get: wanted %v, got %v", want, got)
+	}
+
+	// ... However, a Get  without BacklogOnly will return the new value.
+	d, err = watcher.Get(ctx)
+	if err != nil {
+		t.Fatalf("Fourth Get failed: %v", err)
+	}
+	if want, got := "second", d.Value; want != got {
+		t.Fatalf("Fourth Get: wanted value %q, got %q", want, got)
+	}
+}
+
+// TestBacklogOnlyRange exercises the BacklogOnly option for ranged watchers,
+// showcasing how it expected to be used for keeping up with the external state
+// of a range by synchronizing to a local map.
+func TestBacklogOnlyRange(t *testing.T) {
+	tc := newTestClient(t)
+	defer tc.close()
+	ctx, ctxC := context.WithCancel(context.Background())
+	defer ctxC()
+
+	ks := "test-backlog-only-range/"
+	ke := "test-backlog-only-range0"
+
+	for i := 0; i < 100; i++ {
+		if i%2 == 0 {
+			tc.put(t, ks+"a", fmt.Sprintf("val-%d", i))
+		} else {
+			tc.put(t, ks+"b", fmt.Sprintf("val-%d", i))
+		}
+	}
+
+	value := NewValue(tc.client, ks, DecoderStringAt, Range(ke))
+	w := value.Watch()
+	defer w.Close()
+
+	// Collect results into a map from key to value.
+	res := make(map[string]string)
+
+	// Run first Get - this is the barrier defining what's part of the backlog.
+	g, err := w.Get(ctx, event.BacklogOnly[StringAt]())
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	res[g.Key] = g.Value
+
+	// These won't be part of the backlog.
+	tc.put(t, ks+"a", "val-100")
+	tc.put(t, ks+"b", "val-101")
+
+	// Retrieve the rest of the backlog until ErrBacklogDone is returned.
+	nUpdates := 1
+	for {
+		g, err := w.Get(ctx, event.BacklogOnly[StringAt]())
+		if errors.Is(err, event.ErrBacklogDone) {
+			break
+		}
+		if err != nil {
+			t.Fatalf("Get: %v", err)
+		}
+		nUpdates += 1
+		res[g.Key] = g.Value
+	}
+
+	// The backlog should've been compacted to just two entries at their newest
+	// state.
+	if want, got := 2, nUpdates; want != got {
+		t.Fatalf("wanted backlog in %d updates, got it in %d", want, got)
+	}
+
+	for _, te := range []struct {
+		k, w string
+	}{
+		{ks + "a", "val-98"},
+		{ks + "b", "val-99"},
+	} {
+		if want, got := te.w, res[te.k]; want != got {
+			t.Errorf("res[%q]: wanted %q, got %q", te.k, want, got)
+		}
+	}
+}