feat: add TTL to GameSessionStore, replay protection and session cleanup to evaluateAnswer

apps/api/src/gameSessionStore/GameSessionStore.ts

@@ -1,7 +1,11 @@
 export type GameSessionData = { answers: Map<string, number> };
 
 export interface GameSessionStore {
-  create(sessionId: string, data: GameSessionData): Promise<void>;
+  create(
+    sessionId: string,
+    data: GameSessionData,
+    ttlMs: number,
+  ): Promise<void>;
   get(sessionId: string): Promise<GameSessionData | null>;
   delete(sessionId: string): Promise<void>;
 }

apps/api/src/gameSessionStore/InMemoryGameSessionStore.test.ts

@@ -0,0 +1,45 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import { InMemoryGameSessionStore } from "./InMemoryGameSessionStore.js";
+
+describe("InMemoryGameSessionStore", () => {
+  let store: InMemoryGameSessionStore;
+
+  beforeEach(() => {
+    store = new InMemoryGameSessionStore();
+  });
+
+  it("returns null for a non-existent session", async () => {
+    const result = await store.get("00000000-0000-0000-0000-000000000000");
+    expect(result).toBeNull();
+  });
+
+  it("returns session data after creation", async () => {
+    const data = { answers: new Map([["q1", 2]]) };
+    await store.create("session-1", data, 60_000);
+    const result = await store.get("session-1");
+    expect(result).toEqual(data);
+  });
+
+  it("returns null after the session is deleted", async () => {
+    const data = { answers: new Map([["q1", 2]]) };
+    await store.create("session-1", data, 60_000);
+    await store.delete("session-1");
+    const result = await store.get("session-1");
+    expect(result).toBeNull();
+  });
+
+  it("returns null after TTL expires", async () => {
+    const data = { answers: new Map([["q1", 2]]) };
+    await store.create("session-1", data, 1);
+    await new Promise((resolve) => setTimeout(resolve, 10));
+    const result = await store.get("session-1");
+    expect(result).toBeNull();
+  });
+
+  it("returns session data before TTL expires", async () => {
+    const data = { answers: new Map([["q1", 2]]) };
+    await store.create("session-1", data, 60_000);
+    const result = await store.get("session-1");
+    expect(result).not.toBeNull();
+  });
+});

apps/api/src/gameSessionStore/InMemoryGameSessionStore.ts

@@ -1,15 +1,27 @@
 import type { GameSessionStore, GameSessionData } from "./GameSessionStore.js";
 
-export class InMemoryGameSessionStore implements GameSessionStore {
-  private sessions = new Map<string, GameSessionData>();
+type SessionEntry = { data: GameSessionData; expiresAt: number };
 
-  create(sessionId: string, data: GameSessionData): Promise<void> {
-    this.sessions.set(sessionId, data);
+export class InMemoryGameSessionStore implements GameSessionStore {
+  private sessions = new Map<string, SessionEntry>();
+
+  create(
+    sessionId: string,
+    data: GameSessionData,
+    ttlMs: number,
+  ): Promise<void> {
+    this.sessions.set(sessionId, { data, expiresAt: Date.now() + ttlMs });
     return Promise.resolve();
   }
 
   get(sessionId: string): Promise<GameSessionData | null> {
-    return Promise.resolve(this.sessions.get(sessionId) ?? null);
+    const entry = this.sessions.get(sessionId);
+    if (!entry) return Promise.resolve(null);
+    if (Date.now() > entry.expiresAt) {
+      this.sessions.delete(sessionId);
+      return Promise.resolve(null);
+    }
+    return Promise.resolve(entry.data);
   }
 
   delete(sessionId: string): Promise<void> {

apps/api/src/services/gameService.test.ts

@@ -219,4 +219,55 @@ describe("evaluateAnswer", () => {
       "Question not found",
     );
   });
+
+  it("throws NotFoundError when the same question is submitted twice", async () => {
+    const session = await createGameSession(validRequest, store);
+    const question = session.questions[0]!;
+
+    await evaluateAnswer(
+      {
+        sessionId: session.sessionId,
+        questionId: question.questionId,
+        selectedOptionId: 0,
+      },
+      store,
+    );
+
+    await expect(
+      evaluateAnswer(
+        {
+          sessionId: session.sessionId,
+          questionId: question.questionId,
+          selectedOptionId: 0,
+        },
+        store,
+      ),
+    ).rejects.toThrow("Question not found");
+  });
+
+  it("deletes the session after the last question is answered", async () => {
+    const session = await createGameSession(validRequest, store);
+
+    for (const question of session.questions) {
+      await evaluateAnswer(
+        {
+          sessionId: session.sessionId,
+          questionId: question.questionId,
+          selectedOptionId: 0,
+        },
+        store,
+      );
+    }
+
+    await expect(
+      evaluateAnswer(
+        {
+          sessionId: session.sessionId,
+          questionId: session.questions[0]!.questionId,
+          selectedOptionId: 0,
+        },
+        store,
+      ),
+    ).rejects.toThrow("Game session not found");
+  });
 });

apps/api/src/services/gameService.ts

@@ -59,7 +59,7 @@ export const createGameSession = async (
   );
 
   const sessionId = randomUUID();
-  await store.create(sessionId, { answers: answerKey });
+  await store.create(sessionId, { answers: answerKey }, 30 * 60 * 1000);
 
   return { sessionId, questions };
 };
@@ -80,6 +80,12 @@ export const evaluateAnswer = async (
     throw new NotFoundError(`Question not found: ${submission.questionId}`);
   }
 
+  session.answers.delete(submission.questionId);
+
+  if (session.answers.size === 0) {
+    await store.delete(submission.sessionId);
+  }
+
   return {
     questionId: submission.questionId,
     isCorrect: submission.selectedOptionId === correctOptionId,

documentation/tickets/t00005.md

@@ -0,0 +1,93 @@
+# ADR: Session lifecycle — TTL and replay protection
+
+## Status
+
+Accepted
+
+## Date
+
+2026-04-28
+
+## Context
+
+`InMemoryGameSessionStore` had no TTL and no cleanup mechanism. Every session created stayed in memory until the process restarted. Additionally, `evaluateAnswer` never removed a question from the answer key after evaluating it, meaning the same question could be submitted multiple times and receive a valid result each time — a potential exploit in multiplayer and a correctness bug in singleplayer.
+
+## Decision
+
+Add a `ttlMs` parameter to `GameSessionStore.create()` so both the in-memory and future Valkey implementations handle expiry consistently. Delete questions from the answer key after evaluation. Delete the session when the last question is answered.
+
+## Options considered
+
+### Option A — Delete on last answer only
+
+Simple. Covers replay protection and normal session completion. Abandoned sessions (player starts game, never finishes) still leak memory.
+
+### Option B — Delete on last answer + TTL on the interface ✅
+
+Delete on answer covers normal flow. TTL covers abandoned sessions. TTL on the interface means `ValKeyGameSessionStore` can use Redis-native `EXPIRE` without any interface changes during migration.
+
+Chosen because it closes the memory leak entirely and makes the Valkey migration a zero-interface-change operation.
+
+### Option C — TTL hardcoded inside InMemoryGameSessionStore only
+
+Simpler short-term. But the interface wouldn't carry the TTL parameter, so `ValKeyGameSessionStore` would need a different mechanism — inconsistency between implementations.
+
+## Consequences
+
+- Sessions expire after 30 minutes of inactivity regardless of completion state
+- Submitting the same question twice throws `NotFoundError` on the second attempt
+- Sessions are deleted automatically when the last question is answered
+- `GameSessionStore.create()` now requires a `ttlMs` argument — any future implementation must honour it
+- `ValKeyGameSessionStore` can implement TTL via Redis `EXPIRE` with no interface changes
+- `InMemoryGameSessionStore` stores `{ data, expiresAt }` entries instead of raw `GameSessionData` — expiry is checked lazily on `get()`
+
+## Affected files
+
+- `apps/api/src/gameSessionStore/GameSessionStore.ts` — `ttlMs` added to `create`
+- `apps/api/src/gameSessionStore/InMemoryGameSessionStore.ts` — TTL implementation
+- `apps/api/src/gameSessionStore/InMemoryGameSessionStore.test.ts` — new test file
+- `apps/api/src/services/gameService.ts` — passes TTL to `store.create`, deletes question after evaluation, deletes session when empty
+- `apps/api/src/services/gameService.test.ts` — replay protection and session cleanup tests added
+
+## References
+
+- [Redis EXPIRE command](https://redis.io/commands/expire/)
+
+---
+
+## Setup guide / implementation notes
+
+1. `GameSessionStore.ts` — add `ttlMs` to `create`:
+
+   ```ts
+   create(sessionId: string, data: GameSessionData, ttlMs: number): Promise<void>;
+   ```
+
+2. `InMemoryGameSessionStore.ts` — wrap stored data with expiry:
+
+   ```ts
+   type SessionEntry = { data: GameSessionData; expiresAt: number };
+   ```
+
+   Check expiry on `get()`, delete expired entries lazily.
+
+3. `gameService.ts` — pass TTL when creating session:
+
+   ```ts
+   await store.create(sessionId, { answers: answerKey }, 30 * 60 * 1000);
+   ```
+
+   After evaluating an answer:
+
+   ```ts
+   session.answers.delete(submission.questionId);
+   if (session.answers.size === 0) {
+     await store.delete(submission.sessionId);
+   }
+   ```
+
+4. When implementing `ValKeyGameSessionStore`, pass `ttlMs` to Redis `EXPIRE`:
+
+   ```ts
+   await valkey.set(sessionId, serialize(data), "EX", Math.ceil(ttlMs / 1000));
+   ```