diff --git a/app.js b/app.js
index d211058..8c92d60 100644
--- a/app.js
+++ b/app.js
@@ -1,26 +1,30 @@
 // Tiny in-memory notes API. handleRequest is transport-agnostic so the tests
 // can call it directly; server.js wires it to node:http.
-let nextId = 1;
-const notes = new Map();
+const { createNotesRepository } = require('./notes-repository');
+
+const repo = createNotesRepository(process.env.NOTES_STORE || 'memory');
 
 function handleRequest(method, path, body) {
   if (method === 'GET' && path === '/notes') {
-    return { status: 200, body: [...notes.values()] };
+    return { status: 200, body: repo.findAll() };
   }
   if (method === 'POST' && path === '/notes') {
     if (!body || typeof body.text !== 'string' || !body.text.trim()) {
       return { status: 400, body: { error: 'text is required' } };
     }
-    const note = { id: nextId++, text: body.text.trim() };
-    notes.set(note.id, note);
-    return { status: 201, body: note };
+    return { status: 201, body: repo.create({ text: body.text.trim() }) };
+  }
+  const match = /^\/notes\/(\d+)$/.exec(path);
+  if (method === 'DELETE' && match) {
+    return repo.deleteById(Number(match[1]))
+      ? { status: 204, body: undefined }
+      : { status: 404, body: { error: 'not found' } };
   }
   return { status: 404, body: { error: 'not found' } };
 }
 
 function resetForTests() {
-  nextId = 1;
-  notes.clear();
+  repo.clear();
 }
 
 module.exports = { handleRequest, resetForTests };
diff --git a/notes-repository.js b/notes-repository.js
new file mode 100644
index 0000000..ce12933
--- /dev/null
+++ b/notes-repository.js
@@ -0,0 +1,55 @@
+// Storage abstraction so the API can swap backends (memory, sqlite, redis)
+// without touching request handling.
+class NotesRepository {
+  findAll() { throw new Error('not implemented'); }
+  findById(_id) { throw new Error('not implemented'); }
+  create(_fields) { throw new Error('not implemented'); }
+  update(_id, _fields) { throw new Error('not implemented'); }
+  deleteById(_id) { throw new Error('not implemented'); }
+  clear() { throw new Error('not implemented'); }
+}
+
+class InMemoryNotesRepository extends NotesRepository {
+  constructor() {
+    super();
+    this.nextId = 1;
+    this.notes = new Map();
+  }
+
+  findAll() { return [...this.notes.values()]; }
+
+  findById(id) { return this.notes.get(id) ?? null; }
+
+  create(fields) {
+    const note = { id: this.nextId++, ...fields };
+    this.notes.set(note.id, note);
+    return note;
+  }
+
+  update(id, fields) {
+    const existing = this.findById(id);
+    if (!existing) return null;
+    const updated = { ...existing, ...fields, id };
+    this.notes.set(id, updated);
+    return updated;
+  }
+
+  deleteById(id) { return this.notes.delete(id); }
+
+  clear() {
+    this.nextId = 1;
+    this.notes.clear();
+  }
+}
+
+const REPOSITORIES = {
+  memory: () => new InMemoryNotesRepository(),
+};
+
+function createNotesRepository(kind = 'memory') {
+  const factory = REPOSITORIES[kind];
+  if (!factory) throw new Error(`unknown notes store: ${kind}`);
+  return factory();
+}
+
+module.exports = { NotesRepository, InMemoryNotesRepository, createNotesRepository };
diff --git a/run-tests.js b/run-tests.js
index 3278093..d1d19c1 100644
--- a/run-tests.js
+++ b/run-tests.js
@@ -14,4 +14,8 @@ assert.strictEqual(listed.body.length, 1);
 assert.strictEqual(handleRequest('POST', '/notes', {}).status, 400);
 assert.strictEqual(handleRequest('GET', '/nope', null).status, 404);
 
+assert.strictEqual(handleRequest('DELETE', `/notes/${created.body.id}`, null).status, 204);
+assert.strictEqual(handleRequest('GET', '/notes', null).body.length, 0);
+assert.strictEqual(handleRequest('DELETE', '/notes/42', null).status, 404);
+
 console.log('all notes tests passed');