diff --git a/app/store.py b/app/store.py
new file mode 100644
index 0000000..8a1b1e0
--- /dev/null
+++ b/app/store.py
@@ -0,0 +1,68 @@
+import asyncio
+from uuid import UUID, uuid4
+
+from app.models import CurrentStatus, Monitor, MonitorCreate, MonitorUpdate, utcnow
+
+
+class MonitorStore:
+    """Lock-protected process-local storage returning defensive copies."""
+
+    def __init__(self) -> None:
+        self._lock = asyncio.Lock()
+        self._items: dict[UUID, Monitor] = {}
+        self._revisions: dict[UUID, int] = {}
+
+    async def create(self, data: MonitorCreate) -> Monitor:
+        now = utcnow()
+        item = Monitor(id=uuid4(), name=data.name, url=data.url, created_at=now,
+                       updated_at=now, current_status=CurrentStatus())
+        async with self._lock:
+            self._items[item.id] = item
+            self._revisions[item.id] = 0
+            return item.model_copy(deep=True)
+
+    async def list(self) -> list[Monitor]:
+        async with self._lock:
+            values = sorted(self._items.values(), key=lambda item: item.created_at)
+            return [item.model_copy(deep=True) for item in values]
+
+    async def get(self, monitor_id: UUID) -> Monitor | None:
+        async with self._lock:
+            item = self._items.get(monitor_id)
+            return item.model_copy(deep=True) if item else None
+
+    async def snapshot(self, monitor_id: UUID) -> tuple[Monitor, int] | None:
+        async with self._lock:
+            item = self._items.get(monitor_id)
+            if item is None:
+                return None
+            return item.model_copy(deep=True), self._revisions[monitor_id]
+
+    async def update(self, monitor_id: UUID, data: MonitorUpdate) -> Monitor | None:
+        async with self._lock:
+            item = self._items.get(monitor_id)
+            if item is None:
+                return None
+            changes = data.model_dump(exclude_none=True)
+            changes["updated_at"] = utcnow()
+            updated = item.model_copy(update=changes, deep=True)
+            self._items[monitor_id] = updated
+            self._revisions[monitor_id] += 1
+            return updated.model_copy(deep=True)
+
+    async def delete(self, monitor_id: UUID) -> bool:
+        async with self._lock:
+            existed = self._items.pop(monitor_id, None) is not None
+            self._revisions.pop(monitor_id, None)
+            return existed
+
+    async def record_status(self, monitor_id: UUID, revision: int,
+                            status: CurrentStatus) -> Monitor | None:
+        async with self._lock:
+            item = self._items.get(monitor_id)
+            if item is None or self._revisions[monitor_id] != revision:
+                return None
+            updated = item.model_copy(update={"current_status": status,
+                                              "updated_at": utcnow()}, deep=True)
+            self._items[monitor_id] = updated
+            return updated.model_copy(deep=True)