diff --git a/cal/memory.go b/cal/memory.go
new file mode 100644
index 0000000..3cbab2f
--- /dev/null
+++ b/cal/memory.go
@@ -0,0 +1,60 @@
+package main
+
+import (
+	"errors"
+	"sync"
+
+	"go-mod.ewintr.nl/planner/item"
+)
+
+type Memory struct {
+	events map[string]item.Event
+	mutex  sync.RWMutex
+}
+
+func NewMemory() *Memory {
+	return &Memory{
+		events: make(map[string]item.Event),
+	}
+}
+
+func (r *Memory) Find(id string) (item.Event, error) {
+	r.mutex.RLock()
+	defer r.mutex.RUnlock()
+
+	event, exists := r.events[id]
+	if !exists {
+		return item.Event{}, errors.New("event not found")
+	}
+	return event, nil
+}
+
+func (r *Memory) FindAll() ([]item.Event, error) {
+	r.mutex.RLock()
+	defer r.mutex.RUnlock()
+
+	events := make([]item.Event, 0, len(r.events))
+	for _, event := range r.events {
+		events = append(events, event)
+	}
+	return events, nil
+}
+
+func (r *Memory) Store(e item.Event) error {
+	r.mutex.Lock()
+	defer r.mutex.Unlock()
+
+	r.events[e.ID] = e
+	return nil
+}
+
+func (r *Memory) Delete(id string) error {
+	r.mutex.Lock()
+	defer r.mutex.Unlock()
+
+	if _, exists := r.events[id]; !exists {
+		return errors.New("event not found")
+	}
+	delete(r.events, id)
+	return nil
+}
diff --git a/sync/service/memory.go b/sync/service/memory.go
index c421ee5..cbcaf8b 100644
--- a/sync/service/memory.go
+++ b/sync/service/memory.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"slices"
+	"sync"
 	"time"
 
 	"go-mod.ewintr.nl/planner/item"
@@ -9,6 +10,7 @@ import (
 
 type Memory struct {
 	items map[string]item.Item
+	mutex sync.RWMutex
 }
 
 func NewMemory() *Memory {
@@ -18,12 +20,18 @@ func NewMemory() *Memory {
 }
 
 func (m *Memory) Update(item item.Item) error {
+	m.mutex.Lock()
+	defer m.mutex.Unlock()
+
 	m.items[item.ID] = item
 
 	return nil
 }
 
 func (m *Memory) Updated(kinds []item.Kind, timestamp time.Time) ([]item.Item, error) {
+	m.mutex.RLock()
+	defer m.mutex.RUnlock()
+
 	result := make([]item.Item, 0)
 
 	for _, i := range m.items {