diff --git a/metropolis/pkg/event/memory_value.go b/metropolis/pkg/event/memory_value.go
new file mode 100644
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+++ b/metropolis/pkg/event/memory_value.go
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+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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 event
+
+import (
+	"context"
+	"fmt"
+	"sync"
+)
+
+var (
+	// Type assert that *MemoryValue implements Value. We do this artificially, as
+	// there currently is no code path that needs this to be strictly true. However,
+	// users of this library might want to rely on the Value type instead of
+	// particular Value implementations.
+	_ Value = &MemoryValue{}
+)
+
+// MemoryValue implements a Value stored in memory. It is safe to construct an
+// empty object of this type. However, this must not be copied.
+type MemoryValue struct {
+	// mu guards the inner, innerSet and watchers fields.
+	mu sync.RWMutex
+	// inner is the latest data Set on the MemoryValue. It is used to provide
+	// the newest version of the Set data to new Watchers.
+	inner interface{}
+	// innerSet is true when inner has been Set at least once. It is used to
+	// differentiate between a nil and unset value.
+	innerSet bool
+	// watchers is the list of watchers that should be updated when new data is
+	// Set. It will grow on every .Watch() and shrink any time a Watcher is
+	// determined to have been closed.
+	watchers []*MemoryWatcher
+
+	// Sync, if set to true, blocks all .Set() calls on the MemoryValue until
+	// all Watchers derived from it actively .Get() the new value. This can be
+	// used to ensure Watchers always receive a full log of all Set() calls.
+	//
+	// This must not be changed after the first .Set/.Watch call.
+	//
+	// This is an experimental API and subject to change. It might be migrated
+	// to per-Watcher settings defined within the main Value/Watcher
+	// interfaces.
+	Sync bool
+}
+
+// Set updates the Value to the given data. It is safe to call this from
+// multiple goroutines, including concurrently.
+//
+// For more information about guarantees, see Value.Set.
+func (m *MemoryValue) Set(val interface{}) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Update the data that is provided on first Get() to Watchers.
+	m.inner = val
+	m.innerSet = true
+
+	// Go through all watchers, updating them on the new value and filtering out
+	// all closed watchers.
+	newWatchers := make([]*MemoryWatcher, 0, len(m.watchers))
+	for _, w := range m.watchers {
+		w := w
+		if w.closed() {
+			continue
+		}
+		w.update(m.Sync, val)
+		newWatchers = append(newWatchers, w)
+	}
+	m.watchers = newWatchers
+}
+
+// MemoryWatcher implements the Watcher interface for watchers returned by
+// MemoryValue.
+type MemoryWatcher struct {
+	// activeReqC is a channel used to request an active submission channel
+	// from a pending Get function, if any.
+	activeReqC chan chan interface{}
+	// deadletterSubmitC is a channel used to communicate a value that
+	// attempted to be submitted via activeReqC. This will be received by the
+	// deadletter worker of this Watcher and passed on to the next .Get call
+	// that occurs.
+	deadletterSubmitC chan interface{}
+
+	// getSem is a channel-based semaphore (which is of size 1, and thus in
+	// fact a mutex) that is used to ensure that only a single .Get() call is
+	// active. It is implemented as a channel to permit concurrent .Get() calls
+	// to error out instead of blocking.
+	getSem chan struct{}
+	// close is a channel that is closed when this Watcher is itself Closed.
+	close chan struct{}
+}
+
+// Watch retrieves a Watcher that keeps track on the version of the data
+// contained within the Value that was last seen by a consumer.
+//
+// For more information about guarantees, see Value.Watch.
+func (m *MemoryValue) Watch() Watcher {
+	waiter := &MemoryWatcher{
+		activeReqC:        make(chan chan interface{}),
+		deadletterSubmitC: make(chan interface{}),
+		close:             make(chan struct{}),
+		getSem:            make(chan struct{}, 1),
+	}
+	// Start the deadletter worker as a goroutine. It will be stopped when the
+	// Watcher is Closed() (as signaled by the close channel).
+	go waiter.deadletterWorker()
+
+	// Append this watcher to the MemoryValue.
+	m.mu.Lock()
+	m.watchers = append(m.watchers, waiter)
+	// If the MemoryValue already has some value set, communicate that to the
+	// first Get call by going through the deadletter worker.
+	if m.innerSet {
+		waiter.deadletterSubmitC <- m.inner
+	}
+	m.mu.Unlock()
+
+	return waiter
+}
+
+// deadletterWorker runs the 'deadletter worker', as goroutine that contains
+// any data that has been Set on the Value that is being watched that was
+// unable to be delivered directly to a pending .Get call.
+//
+// It watches the deadletterSubmitC channel for updated data, and overrides
+// previously received data. Then, when a .Get() begins to pend (and respond to
+// activeReqC receives), the deadletter worker will deliver that value.
+func (m *MemoryWatcher) deadletterWorker() {
+	// Current value, and flag to mark it as set (vs. nil).
+	var cur interface{}
+	var set bool
+
+	for {
+		if !set {
+			// If no value is yet available, only attempt to receive one from the
+			// submit channel, as there's nothing to submit to pending .Get() calls
+			// yet.
+			val, ok := <-m.deadletterSubmitC
+			if !ok {
+				// If the channel has been closed (by Close()), exit.
+				return
+			}
+			cur = val
+			set = true
+		} else {
+			// If a value is available, update the inner state. Otherwise, if a
+			// .Get() is pending, submit our current state and unset it.
+			select {
+			case val, ok := <-m.deadletterSubmitC:
+				if !ok {
+					// If the channel has been closed (by Close()), exit.
+					return
+				}
+				cur = val
+			case c := <-m.activeReqC:
+				// Potential race: a .Get() might've been active, but might've
+				// quit by the time we're here (and will not receive on the
+				// responded channel). Handle this gracefully by just returning
+				// to the main loop if that's the case.
+				select {
+				case c <- cur:
+					set = false
+				default:
+				}
+			}
+		}
+	}
+}
+
+// closed returns whether this watcher has been closed.
+func (m *MemoryWatcher) closed() bool {
+	select {
+	case _, ok := <-m.close:
+		if !ok {
+			return true
+		}
+	default:
+	}
+	return false
+}
+
+// update is the high level update-this-watcher function called by MemoryValue.
+func (m *MemoryWatcher) update(sync bool, val interface{}) {
+	// If synchronous delivery was requested, block until a watcher .Gets it.
+	if sync {
+		c := <-m.activeReqC
+		c <- val
+		return
+	}
+
+	// Otherwise, deliver asynchronously. This means either delivering directly
+	// to a pending .Get if one exists, or submitting to the deadletter worker
+	// otherwise.
+	select {
+	case c := <-m.activeReqC:
+		// Potential race: a .Get() might've been active, but might've  quit by
+		// the time we're here (and will not receive on the responded channel).
+		// Handle this gracefully by falling back to the deadletter worker.
+		select {
+		case c <- val:
+		default:
+			m.deadletterSubmitC <- val
+		}
+	default:
+		m.deadletterSubmitC <- val
+	}
+}
+
+func (m *MemoryWatcher) Close() error {
+	close(m.deadletterSubmitC)
+	close(m.close)
+	return nil
+}
+
+// Get blocks until a Value's data is available. See Watcher.Get for guarantees
+// and more information.
+func (m *MemoryWatcher) Get(ctx context.Context) (interface{}, error) {
+	// Make sure we're the only active .Get call.
+	select {
+	case m.getSem <- struct{}{}:
+	default:
+		return nil, fmt.Errorf("cannot Get() concurrently on a single waiter")
+	}
+	defer func() {
+		<-m.getSem
+	}()
+
+	c := make(chan interface{})
+
+	// Start responding on activeReqC. This signals to .update and to the
+	// deadletter worker that we're ready to accept data updates.
+
+	// There is a potential for a race condition here that hasn't been observed
+	// in tests but might happen:
+	//   1) Value.Watch returns a Watcher 'w'.
+	//   2) w.Set(0) is called, no .Get() is pending, so 0 is submitted to the
+	//      deadletter worker.
+	//   3) w.Get() is called, and activeReqC begins to be served.
+	//   4) Simultaneously:
+	//     a) w.Set(1) is called, attempting to submit via activeReqC
+	//     b) the deadletter worker attempts to submit via activeReqC
+	//
+	// This could theoretically cause .Get() to first return 1, and then 0, if
+	// the Set activeReqC read and subsequent channel write is served before
+	// the deadletter workers' read/write is.
+	// As noted, however, this has not been observed in practice, even though
+	// TestConcurrency explicitly attempts to trigger this condition. More
+	// research needs to be done to attempt to trigger this (or to lawyer the
+	// Go channel spec to see if this has some guarantees that resolve this
+	// either way), or a preemptive fix can be attempted by adding monotonic
+	// counters associated with each .Set() value, ensuring an older value does
+	// not replace a newer value.
+	//
+	// TODO(q3k): investigate this.
+	for {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		case m.activeReqC <- c:
+		case val := <-c:
+			return val, nil
+		}
+	}
+}
+