feat: add engine and store snapshot/restore for restart resilience

src/lib/access.js

@@ -1,8 +1,9 @@
 "use strict";
 
 class AudioAccessStore {
-  constructor() {
-    this.grants = new Map();
+  constructor(initialState) {
+    const entries = Object.entries((initialState && initialState.grants) || {});
+    this.grants = new Map(entries.map(([audioId, userIds]) => [audioId, new Set(userIds)]));
   }
 
   _key(audioId) {
@@ -71,6 +72,15 @@ class AudioAccessStore {
     this.grantAccess(audio.id, userId);
     return { unlocked: true, charged: audio.creditsCharged };
   }
+
+  exportState() {
+    const grants = {};
+    for (const [audioId, users] of this.grants.entries()) {
+      grants[audioId] = Array.from(users.values());
+    }
+
+    return { grants };
+  }
 }
 
 module.exports = {

src/lib/engine.js

@@ -6,17 +6,21 @@ const { calculateCredits } = require("./credits");
 const { extractArticleFromParent } = require("./article");
 
 class XArtAudioEngine {
-  constructor({ creditConfig }) {
+  constructor({ creditConfig, initialState }) {
     this.creditConfig = creditConfig;
-    this.wallets = new WalletStore();
-    this.access = new AudioAccessStore();
+    this.wallets = new WalletStore(initialState && initialState.wallets);
+    this.access = new AudioAccessStore(initialState && initialState.access);
 
-    this.jobs = new Map();
-    this.assets = new Map();
-    this.mentions = new Map();
+    this.jobs = new Map(Object.entries((initialState && initialState.jobs) || {}));
+    this.assets = new Map(Object.entries((initialState && initialState.assets) || {}));
+    this.mentions = new Map(Object.entries((initialState && initialState.mentions) || {}));
 
-    this.nextJobId = 1;
-    this.nextAssetId = 1;
+    this.nextJobId = Number.isInteger(initialState && initialState.nextJobId)
+      ? initialState.nextJobId
+      : 1;
+    this.nextAssetId = Number.isInteger(initialState && initialState.nextAssetId)
+      ? initialState.nextAssetId
+      : 1;
   }
 
   topUpCredits(userId, amount, idempotencyKey) {
@@ -166,6 +170,18 @@ class XArtAudioEngine {
       wallet: this.wallets,
     });
   }
+
+  exportState() {
+    return {
+      wallets: this.wallets.exportState(),
+      access: this.access.exportState(),
+      jobs: Object.fromEntries(this.jobs.entries()),
+      assets: Object.fromEntries(this.assets.entries()),
+      mentions: Object.fromEntries(this.mentions.entries()),
+      nextJobId: this.nextJobId,
+      nextAssetId: this.nextAssetId,
+    };
+  }
 }
 
 module.exports = {

src/lib/wallet.js

@@ -1,11 +1,19 @@
 "use strict";
 
 class WalletStore {
-  constructor() {
-    this.balances = new Map();
-    this.transactions = [];
-    this.byIdempotencyKey = new Map();
-    this.nextId = 1;
+  constructor(initialState) {
+    this.balances = new Map(Object.entries((initialState && initialState.balances) || {}));
+    this.transactions = Array.isArray(initialState && initialState.transactions)
+      ? [...initialState.transactions]
+      : [];
+    this.byIdempotencyKey = new Map(
+      this.transactions
+        .filter((tx) => tx.idempotencyKey)
+        .map((tx) => [tx.idempotencyKey, tx]),
+    );
+    this.nextId = Number.isInteger(initialState && initialState.nextId)
+      ? initialState.nextId
+      : (this.transactions.length + 1);
   }
 
   getBalance(userId) {
@@ -63,6 +71,14 @@ class WalletStore {
     this.balances.set(userId, newBalance);
     return tx;
   }
+
+  exportState() {
+    return {
+      balances: Object.fromEntries(this.balances.entries()),
+      transactions: [...this.transactions],
+      nextId: this.nextId,
+    };
+  }
 }
 
 module.exports = {

test/access.test.js

@@ -65,3 +65,13 @@ test("second unlock call is idempotent and does not double charge", () => {
 
   assert.equal(wallet.getBalance("u2"), 7);
 });
+
+test("restores grant state from snapshot", () => {
+  const access = new AudioAccessStore();
+  access.grantAccess("a1", "u2");
+
+  const restored = new AudioAccessStore(access.exportState());
+  const canAccess = restored.canAccess({ audio, userId: "u2" });
+  assert.equal(canAccess.allowed, true);
+  assert.equal(canAccess.reason, "grant");
+});

test/engine.test.js

@@ -154,3 +154,29 @@ test("lists jobs for user newest first and provides summary", () => {
   assert.equal(summary.totalCreditsSpent, 2);
   assert.equal(summary.balance, 8);
 });
+
+test("round-trips state snapshot across engine restart", () => {
+  const engine1 = createEngine();
+  engine1.topUpCredits("u1", 5, "topup-snapshot");
+
+  const created = engine1.processMention({
+    mentionPostId: "m-snapshot",
+    callerUserId: "u1",
+    parentPost: {
+      id: "p-snapshot",
+      authorId: "author-snapshot",
+      article: { id: "a-snapshot", title: "Snap", body: "hello world" },
+    },
+  });
+
+  const snapshot = engine1.exportState();
+  const engine2 = new XArtAudioEngine({
+    creditConfig: engine1.creditConfig,
+    initialState: snapshot,
+  });
+
+  assert.equal(engine2.getWalletBalance("u1"), 4);
+  assert.equal(engine2.getJob(created.job.id).article.title, "Snap");
+  assert.equal(engine2.getAsset(created.job.assetId).articleTitle, "Snap");
+  assert.equal(engine2.checkAudioAccess(created.job.assetId, "u1").allowed, true);
+});

test/wallet.test.js

@@ -61,3 +61,27 @@ test("is idempotent by idempotency key", () => {
   assert.equal(first.id, second.id);
   assert.equal(wallet.getBalance("u1"), 4);
 });
+
+test("can restore state from previous snapshot", () => {
+  const original = new WalletStore();
+  original.applyTransaction({
+    userId: "u1",
+    type: "credit",
+    amount: 3,
+    reason: "topup",
+    idempotencyKey: "evt-r1",
+  });
+
+  const restored = new WalletStore(original.exportState());
+  assert.equal(restored.getBalance("u1"), 3);
+
+  const duplicate = restored.applyTransaction({
+    userId: "u1",
+    type: "credit",
+    amount: 3,
+    reason: "topup",
+    idempotencyKey: "evt-r1",
+  });
+  assert.equal(duplicate.amount, 3);
+  assert.equal(restored.getBalance("u1"), 3);
+});