From 67433752ea652dce1f075b63986e6da36bdbba42 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Sun, 12 Oct 2025 23:47:30 +0800
Subject: [PATCH 01/29] Fix sync when retrieving all questions

---
 .../src/db/model/question.ts                  | 11 ++-
 .../src/leetcode/seedBatch.ts                 | 81 ++++++++-----------
 2 files changed, 37 insertions(+), 55 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/db/model/question.ts b/backend-services/leetcode-backend-service/src/db/model/question.ts
index bda27e156..b1a15bbc4 100644
--- a/backend-services/leetcode-backend-service/src/db/model/question.ts
+++ b/backend-services/leetcode-backend-service/src/db/model/question.ts
@@ -42,12 +42,11 @@ const QuestionSchema = new Schema<QuestionDoc>(
   { collection: "questions", timestamps: true },
 );
 
-QuestionSchema.index({
-  source: 1,
-  titleSlug: 1,
-  categoryTitle: 1,
-  difficulty: 1,
-});
+// Ensure uniqueness based on titleSlug
+QuestionSchema.index({ titleSlug: 1 }, { unique: true });
+
+// Compound indexes for faster queries
+QuestionSchema.index({ categoryTitle: 1, difficulty: 1 });
 
 const CursorSchema = new mongoose.Schema(
   {
diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 96bec895e..f9e1e31e3 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -38,34 +38,25 @@ export async function seedLeetCodeBatch() {
   const id = "questions";
   const cursor =
     (await SeedCursor.findById(id)) ??
-    new SeedCursor({ _id: id, nextSkip: 0, pageSize: 200, done: false });
-
-  if (cursor.done) {
-    return {
-      ok: true,
-      message: "Already completed.",
-      nextSkip: cursor.nextSkip,
-      done: true,
-    };
-  }
-
+    new SeedCursor({ _id: id, nextSkip: 0, pageSize: 200 });
   const { pageSize, nextSkip } = cursor;
 
-  const { questionList, total, initial_count } = await fetchNonPaidQuestionList(
+  // Fetch question list using the cursor's nextSkip
+  const { questionList, total } = await fetchNonPaidQuestionList(
     pageSize,
     nextSkip,
   );
 
+  // Check if there are no more questions to fetch
   if (questionList.length === 0) {
-    cursor.done = true;
     cursor.lastRunAt = new Date();
     cursor.total = total ?? cursor.total;
+    cursor.nextSkip = total + 1; // Prevent future refetching of previously fetched items
     await cursor.save();
     return {
       ok: true,
-      message: "No more questions. Marked done.",
-      nextSkip,
-      done: true,
+      message: "No more questions.",
+      nextSkip: cursor.nextSkip,
     };
   }
 
@@ -75,47 +66,40 @@ export async function seedLeetCodeBatch() {
   );
 
   const ops = questionInfos.map((q) => ({
-    updateOne: {
-      filter: { titleSlug: q.titleSlug },
-
-      update: {
-        $set: {
-          globalSlug: `leetcode:${q.titleSlug}`, // unique identifier
-          source: "leetcode",
-          titleSlug: q.titleSlug,
-          title: q.title,
-
-          // metadata
-          difficulty: q.difficulty,
-          categoryTitle: q.categoryTitle ?? null,
-          timeLimit: DIFFICULTY_TIME_LIMITS[q.difficulty] ?? 60,
-
-          // content & extras
-          content: q.content ?? null,
-          codeSnippets: q.codeSnippets ?? [],
-          hints: q.hints ?? [],
-          exampleTestcases: q.exampleTestcases ?? null,
-          updatedAt: new Date(),
-        },
-
-        $setOnInsert: {
-          createdAt: new Date(),
-        },
+    insertOne: {
+      document: {
+        globalSlug: `leetcode:${q.titleSlug}`, // unique identifier
+        source: "leetcode",
+        titleSlug: q.titleSlug,
+        title: q.title,
+
+        // metadata
+        difficulty: q.difficulty,
+        categoryTitle: q.categoryTitle ?? null,
+        timeLimit: DIFFICULTY_TIME_LIMITS[q.difficulty] ?? 60,
+
+        // content & extras
+        content: q.content ?? null,
+        codeSnippets: q.codeSnippets ?? [],
+        hints: q.hints ?? [],
+        exampleTestcases: q.exampleTestcases ?? null,
+        createdAt: new Date(),
+        updatedAt: new Date(),
       },
-
-      upsert: true,
     },
   }));
 
   const result = await Question.bulkWrite(ops, { ordered: false });
 
   // Advance cursor
-  cursor.nextSkip = nextSkip + pageSize;
+  if (nextSkip + pageSize > total) {
+    // Prevent future refetching of previously fetched items
+    cursor.nextSkip = total + 1;
+  } else {
+    cursor.nextSkip = nextSkip + pageSize;
+  }
   cursor.lastRunAt = new Date();
   cursor.total = total;
-  if (initial_count < pageSize) {
-    cursor.done = true;
-  }
   await cursor.save();
 
   return {
@@ -127,7 +111,6 @@ export async function seedLeetCodeBatch() {
     pageSize,
     nextSkip: cursor.nextSkip,
     total: cursor.total,
-    done: cursor.done,
   };
 }
 

From 51b8417ba5514382e073a3c31fc5db5c6a00d38b Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Mon, 13 Oct 2025 00:11:25 +0800
Subject: [PATCH 02/29] Replace insertOne with updateOne but with $setOnInsert
 only

---
 .../src/leetcode/seedBatch.ts                 | 42 ++++++++++---------
 1 file changed, 23 insertions(+), 19 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index f9e1e31e3..11fdaadc7 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -66,26 +66,30 @@ export async function seedLeetCodeBatch() {
   );
 
   const ops = questionInfos.map((q) => ({
-    insertOne: {
-      document: {
-        globalSlug: `leetcode:${q.titleSlug}`, // unique identifier
-        source: "leetcode",
-        titleSlug: q.titleSlug,
-        title: q.title,
-
-        // metadata
-        difficulty: q.difficulty,
-        categoryTitle: q.categoryTitle ?? null,
-        timeLimit: DIFFICULTY_TIME_LIMITS[q.difficulty] ?? 60,
-
-        // content & extras
-        content: q.content ?? null,
-        codeSnippets: q.codeSnippets ?? [],
-        hints: q.hints ?? [],
-        exampleTestcases: q.exampleTestcases ?? null,
-        createdAt: new Date(),
-        updatedAt: new Date(),
+    updateOne: {
+      filter: { titleSlug: q.titleSlug },
+      update: {
+        // setOnInsert to avoid overwriting existing entries
+        $setOnInsert: {
+          globalSlug: `leetcode:${q.titleSlug}`,
+          source: "leetcode",
+          titleSlug: q.titleSlug,
+          title: q.title,
+
+          // metadata
+          difficulty: q.difficulty,
+          categoryTitle: q.categoryTitle ?? null,
+          timeLimit: DIFFICULTY_TIME_LIMITS[q.difficulty] ?? 60,
+
+          // content & extras
+          content: q.content ?? null,
+          codeSnippets: q.codeSnippets ?? [],
+          hints: q.hints ?? [],
+          exampleTestcases: q.exampleTestcases ?? null,
+          createdAt: new Date(),
+        },
       },
+      upsert: true,
     },
   }));
 

From e2dd31b1af7bc7e53d942aab893864cffb9a9c95 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Mon, 13 Oct 2025 00:35:32 +0800
Subject: [PATCH 03/29] Remove unnecessary unique constrain enforcement

---
 .../leetcode-backend-service/src/db/model/question.ts  | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/db/model/question.ts b/backend-services/leetcode-backend-service/src/db/model/question.ts
index b1a15bbc4..46030becf 100644
--- a/backend-services/leetcode-backend-service/src/db/model/question.ts
+++ b/backend-services/leetcode-backend-service/src/db/model/question.ts
@@ -42,11 +42,13 @@ const QuestionSchema = new Schema<QuestionDoc>(
   { collection: "questions", timestamps: true },
 );
 
-// Ensure uniqueness based on titleSlug
-QuestionSchema.index({ titleSlug: 1 }, { unique: true });
-
 // Compound indexes for faster queries
-QuestionSchema.index({ categoryTitle: 1, difficulty: 1 });
+QuestionSchema.index({
+  source: 1,
+  titleSlug: 1,
+  categoryTitle: 1,
+  difficulty: 1,
+});
 
 const CursorSchema = new mongoose.Schema(
   {

From fd3cd12dd7087fd638c8d849f7f93c9e344f3af2 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Mon, 13 Oct 2025 00:36:33 +0800
Subject: [PATCH 04/29] Remove initial_count from return

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts          | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 11fdaadc7..7f9e453bd 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -191,7 +191,7 @@ export async function fetchNonPaidQuestionList(
   const { total, questions } = res.problemsetQuestionList;
   const initial_count = questions.length;
   const questionList = questions.filter((q) => !q.isPaidOnly);
-  return { questionList, total, initial_count };
+  return { questionList, total };
 }
 
 export async function getQuestionDetail(slug: string) {

From 36982edd65e84dce31223536f1aac128538bb1a6 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Mon, 13 Oct 2025 00:39:50 +0800
Subject: [PATCH 05/29] Remove fully initial_count from
 fetchNonPaidQuestionList

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts         | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 7f9e453bd..142ad0979 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -176,7 +176,6 @@ export async function fetchNonPaidQuestionList(
 ): Promise<{
   questionList: BasicInformation[];
   total: number;
-  initial_count: number;
 }> {
   const res = await gql<
     QuestionList,
@@ -189,7 +188,7 @@ export async function fetchNonPaidQuestionList(
   >(QUERY_LIST, { categorySlug: "", limit: limit, skip: skip, filters: {} });
 
   const { total, questions } = res.problemsetQuestionList;
-  const initial_count = questions.length;
+  // const initial_count = questions.length;
   const questionList = questions.filter((q) => !q.isPaidOnly);
   return { questionList, total };
 }

From 8e9081e0f8fa3f795fbdc4b95b3d15b8ce59d6b9 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Mon, 13 Oct 2025 00:47:05 +0800
Subject: [PATCH 06/29] Remove done attribute for cursor across files

---
 .../leetcode-backend-service/src/db/model/question.ts            | 1 -
 .../leetcode-backend-service/src/db/types/seedBatchResponse.ts   | 1 -
 2 files changed, 2 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/db/model/question.ts b/backend-services/leetcode-backend-service/src/db/model/question.ts
index 46030becf..a36f0750a 100644
--- a/backend-services/leetcode-backend-service/src/db/model/question.ts
+++ b/backend-services/leetcode-backend-service/src/db/model/question.ts
@@ -55,7 +55,6 @@ const CursorSchema = new mongoose.Schema(
     _id: { type: String, required: true },
     nextSkip: { type: Number, default: 0, index: true },
     pageSize: { type: Number, default: 200 },
-    done: { type: Boolean, default: false },
     lastRunAt: { type: Date },
     total: { type: Number, default: 0 },
   },
diff --git a/backend-services/leetcode-backend-service/src/db/types/seedBatchResponse.ts b/backend-services/leetcode-backend-service/src/db/types/seedBatchResponse.ts
index 47264b17a..1c82a9e07 100644
--- a/backend-services/leetcode-backend-service/src/db/types/seedBatchResponse.ts
+++ b/backend-services/leetcode-backend-service/src/db/types/seedBatchResponse.ts
@@ -2,7 +2,6 @@ export interface SeedBatchResponse {
   ok: boolean;
   message: string;
   nextSkip: number;
-  done: boolean;
   // keep room for extra fields
   [k: string]: unknown;
 }

From d2a2a45c9bb42973396f377a3cd8e6696c14cf6b Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Mon, 13 Oct 2025 00:50:26 +0800
Subject: [PATCH 07/29] Remove entirely initial_count

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts           | 1 -
 1 file changed, 1 deletion(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 142ad0979..6951d23d1 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -188,7 +188,6 @@ export async function fetchNonPaidQuestionList(
   >(QUERY_LIST, { categorySlug: "", limit: limit, skip: skip, filters: {} });
 
   const { total, questions } = res.problemsetQuestionList;
-  // const initial_count = questions.length;
   const questionList = questions.filter((q) => !q.isPaidOnly);
   return { questionList, total };
 }

From 425e37a9adc5efab718e7a67f622c76c8304e954 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Mon, 13 Oct 2025 00:52:57 +0800
Subject: [PATCH 08/29] Replace magic number total + 1 with total instead

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts      | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 6951d23d1..3b45e8c58 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -48,10 +48,10 @@ export async function seedLeetCodeBatch() {
   );
 
   // Check if there are no more questions to fetch
-  if (questionList.length === 0) {
+  if (questionList.length === 0 || nextSkip >= total) {
     cursor.lastRunAt = new Date();
     cursor.total = total ?? cursor.total;
-    cursor.nextSkip = total + 1; // Prevent future refetching of previously fetched items
+    cursor.nextSkip = total; // Prevent future refetching of previously fetched items
     await cursor.save();
     return {
       ok: true,
@@ -98,7 +98,7 @@ export async function seedLeetCodeBatch() {
   // Advance cursor
   if (nextSkip + pageSize > total) {
     // Prevent future refetching of previously fetched items
-    cursor.nextSkip = total + 1;
+    cursor.nextSkip = total;
   } else {
     cursor.nextSkip = nextSkip + pageSize;
   }

From 58bbe42f987b7c112c47198e4fd881d2f5061b7f Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Mon, 13 Oct 2025 01:06:06 +0800
Subject: [PATCH 09/29] Fix minor naming issues for comments

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts          | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 3b45e8c58..4eb778dfc 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -69,7 +69,7 @@ export async function seedLeetCodeBatch() {
     updateOne: {
       filter: { titleSlug: q.titleSlug },
       update: {
-        // setOnInsert to avoid overwriting existing entries
+        // $setOnInsert to avoid overwriting existing entries
         $setOnInsert: {
           globalSlug: `leetcode:${q.titleSlug}`,
           source: "leetcode",

From 68280a86a73f94ca9a3582eddb06461597b3c519 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Tue, 14 Oct 2025 23:17:06 +0800
Subject: [PATCH 10/29] Add question-service health check before question
 retrieval at seed-batch

---
 .../leetcode-backend-service/.env.example     |  2 +-
 .../leetcode-backend-service/src/health.ts    | 57 +++++++++++++++++++
 .../src/leetcode/seedBatch.ts                 | 13 +++++
 3 files changed, 71 insertions(+), 1 deletion(-)
 create mode 100644 backend-services/leetcode-backend-service/src/health.ts

diff --git a/backend-services/leetcode-backend-service/.env.example b/backend-services/leetcode-backend-service/.env.example
index e8f6e28ca..891b669a0 100644
--- a/backend-services/leetcode-backend-service/.env.example
+++ b/backend-services/leetcode-backend-service/.env.example
@@ -1,6 +1,6 @@
 MONGODB_URI=mongodb+srv://admin:<db-password>@cluster3219.ll0nzkk.mongodb.net/?retryWrites=true&w=majority&appName=Cluster3219
 ADMIN_TOKEN=<db-password>
-QUESTION_API_URL=http://question-backend:5275/api/v1/questions
+QUESTION_API_URL=http://localhost:5275/api/v1/questions
 # Maximum number of concurrent LeetCode detail fetches.
 # Optional: defaults to 6 if not set.
 LEETCODE_DETAIL_CONCURRENCY=6
\ No newline at end of file
diff --git a/backend-services/leetcode-backend-service/src/health.ts b/backend-services/leetcode-backend-service/src/health.ts
new file mode 100644
index 000000000..bccc1fddf
--- /dev/null
+++ b/backend-services/leetcode-backend-service/src/health.ts
@@ -0,0 +1,57 @@
+import { setTimeout as delay } from "node:timers/promises";
+
+type HealthOpts = {
+  url?: string; // health check URL
+  timeoutMs?: number; // per-attempt timeout
+  retries?: number; // number of retries after the first attempt
+};
+
+interface HealthResponse {
+  ok?: boolean;
+}
+
+export async function checkQuestionServiceHealth({
+  url = `${process.env.QUESTION_API_URL}/health`,
+  timeoutMs = 1500,
+  retries = 2,
+}: HealthOpts = {}) {
+  if (!process.env.QUESTION_API_URL)
+    throw new Error("QUESTION_API_URL is not set");
+
+  let lastErr: unknown;
+
+  for (let attempt = 0; attempt <= retries; attempt++) {
+    const controller = new AbortController();
+    const t = setTimeout(() => controller.abort(), timeoutMs);
+
+    try {
+      const res = await fetch(url, {
+        method: "GET",
+        headers: { accept: "application/json" },
+        signal: controller.signal,
+      });
+      clearTimeout(t);
+
+      if (!res.ok) {
+        lastErr = new Error(`Health endpoint returned ${res.status}`);
+      } else {
+        const body: HealthResponse = (await res.json()) as HealthResponse;
+        if (body?.ok !== true) {
+          lastErr = new Error("Health endpoint did not return ok=true");
+        } else {
+          return true;
+        }
+      }
+    } catch (err) {
+      lastErr = err;
+    }
+
+    if (attempt < retries) {
+      await delay(250 * 2 ** attempt);
+    }
+  }
+
+  throw new Error(
+    `Question service health check failed: ${(lastErr as Error)?.message ?? lastErr}`,
+  );
+}
diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 4eb778dfc..71690a7d7 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -11,6 +11,7 @@ import { QUERY_LIST, QUERY_DETAIL } from "./queries.js";
 import type { BasicInformation, QuestionList, Details } from "./types.js";
 import pLimit from "p-limit";
 import { logger } from "../logger.js";
+import { checkQuestionServiceHealth } from "../health.js";
 
 /**
  * Maximum number of concurrent requests for fetching question details.
@@ -41,6 +42,18 @@ export async function seedLeetCodeBatch() {
     new SeedCursor({ _id: id, nextSkip: 0, pageSize: 200 });
   const { pageSize, nextSkip } = cursor;
 
+  try {
+    await checkQuestionServiceHealth();
+  } catch (err) {
+    cursor.lastRunAt = new Date();
+    await cursor.save();
+    return {
+      ok: false as const,
+      message: `Aborted: question service not healthy — ${(err as Error).message}`,
+      nextSkip: cursor.nextSkip,
+    };
+  }
+
   // Fetch question list using the cursor's nextSkip
   const { questionList, total } = await fetchNonPaidQuestionList(
     pageSize,

From f18914ad19e0377e7e8c9262d2cc61af7f923049 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Thu, 16 Oct 2025 20:03:11 +0800
Subject: [PATCH 11/29] Modify API in .env.example

---
 backend-services/leetcode-backend-service/.env.example | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/backend-services/leetcode-backend-service/.env.example b/backend-services/leetcode-backend-service/.env.example
index 891b669a0..e67e9ca37 100644
--- a/backend-services/leetcode-backend-service/.env.example
+++ b/backend-services/leetcode-backend-service/.env.example
@@ -1,6 +1,6 @@
 MONGODB_URI=mongodb+srv://admin:<db-password>@cluster3219.ll0nzkk.mongodb.net/?retryWrites=true&w=majority&appName=Cluster3219
 ADMIN_TOKEN=<db-password>
-QUESTION_API_URL=http://localhost:5275/api/v1/questions
+QUESTION_API_URL=http://localhost:5275/api/v1/question-service
 # Maximum number of concurrent LeetCode detail fetches.
 # Optional: defaults to 6 if not set.
 LEETCODE_DETAIL_CONCURRENCY=6
\ No newline at end of file

From 98c127a0a82fee0ce27b5730c6059687c6db7028 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Thu, 16 Oct 2025 20:12:57 +0800
Subject: [PATCH 12/29] Reformat mongo db connection for easier logging and
 debugging

---
 .../src/db/connection.ts                      | 21 +++++++++-------
 .../src/db/connection.ts                      | 24 +++++++++++--------
 2 files changed, 27 insertions(+), 18 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/db/connection.ts b/backend-services/leetcode-backend-service/src/db/connection.ts
index 87839e84b..7d4188b6a 100644
--- a/backend-services/leetcode-backend-service/src/db/connection.ts
+++ b/backend-services/leetcode-backend-service/src/db/connection.ts
@@ -16,16 +16,21 @@ export default fp(async (app: FastifyInstance) => {
   const uri = process.env.MONGODB_URI;
   if (!uri) throw new Error("MONGODB_URI is missing");
 
-  if (
-    mongoose.connection.readyState === mongoose.ConnectionStates.disconnected
-  ) {
-    await mongoose.connect(uri, {
-      dbName: "leetcode-service",
-      serverSelectionTimeoutMS: 10000,
-    });
+  if (mongoose.connection.readyState !== mongoose.ConnectionStates.connected) {
+    try {
+      await mongoose.connect(uri, {
+        dbName: "leetcode-service",
+        serverSelectionTimeoutMS: 10000,
+      });
+      logger.info("[Mongo] Connected");
+    } catch (err) {
+      logger.error("[Mongo] Connection failed: ", err);
+      throw err;
+    }
+  } else {
+    logger.info("[Mongo] Already connected");
   }
 
-  logger.info("Mongo connected");
   app.decorate("mongo", mongoose);
 
   app.addHook("onClose", async () => {
diff --git a/backend-services/question-backend-service/src/db/connection.ts b/backend-services/question-backend-service/src/db/connection.ts
index 5ba1edf4e..88bfe45e0 100644
--- a/backend-services/question-backend-service/src/db/connection.ts
+++ b/backend-services/question-backend-service/src/db/connection.ts
@@ -16,20 +16,24 @@ export default fp(async (app: FastifyInstance) => {
   const uri = process.env.MONGODB_URI;
   if (!uri) throw new Error("MONGODB_URI is missing");
 
-  if (
-    mongoose.connection.readyState === mongoose.ConnectionStates.disconnected
-  ) {
-    await mongoose.connect(uri, {
-      dbName: "question-service",
-      serverSelectionTimeoutMS: 10000,
-    });
+  if (mongoose.connection.readyState !== mongoose.ConnectionStates.connected) {
+    try {
+      await mongoose.connect(uri, {
+        dbName: "question-service",
+        serverSelectionTimeoutMS: 10000,
+      });
+      logger.info("[Mongo] Connected");
+    } catch (err) {
+      logger.error("[Mongo] Connection failed: ", err);
+      throw err;
+    }
+  } else {
+    logger.info("[Mongo] Already connected");
   }
-
-  logger.info("Mongo connected");
   app.decorate("mongo", mongoose);
 
   app.addHook("onClose", async () => {
     await mongoose.connection.close();
-    logger.info("Mongo disconnected");
+    logger.info("[Mongo] Disconnected");
   });
 });

From afcdd048def572aa7950d99400154c0f191f2b1f Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Thu, 16 Oct 2025 20:14:18 +0800
Subject: [PATCH 13/29] Fix docker run command in question-backend-service
 README

---
 backend-services/question-backend-service/README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/backend-services/question-backend-service/README.md b/backend-services/question-backend-service/README.md
index 48b13dbd7..becd3e3d0 100644
--- a/backend-services/question-backend-service/README.md
+++ b/backend-services/question-backend-service/README.md
@@ -47,7 +47,7 @@ OR
 ```bash
 docker network create peerprep_net # If not created yet
 docker build --tag question-service .
-docker run --rm --publish 5275:5275 --env-file .env -name question-backend --network peerprep_net question-service
+docker run --rm --publish 5275:5275 --env-file .env --name question-backend --network peerprep_net question-service
 ```
 
 You should see logs like:

From a213802cfa2daa08147773f48edb8305f03ded4c Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Thu, 16 Oct 2025 23:49:59 +0800
Subject: [PATCH 14/29] Add infrastructure for failedQuestion storing

---
 .../src/db/changeStream.ts                        |  9 +++------
 .../src/db/model/failedQuestion.ts                | 15 +++++++++++++++
 .../src/db/types/failedQuestion.ts                |  8 ++++++++
 .../src/leetcode/seedBatch.ts                     | 11 +++++++++++
 4 files changed, 37 insertions(+), 6 deletions(-)
 create mode 100644 backend-services/leetcode-backend-service/src/db/model/failedQuestion.ts
 create mode 100644 backend-services/leetcode-backend-service/src/db/types/failedQuestion.ts

diff --git a/backend-services/leetcode-backend-service/src/db/changeStream.ts b/backend-services/leetcode-backend-service/src/db/changeStream.ts
index 176958935..1742a714c 100644
--- a/backend-services/leetcode-backend-service/src/db/changeStream.ts
+++ b/backend-services/leetcode-backend-service/src/db/changeStream.ts
@@ -91,18 +91,15 @@ export default fp((app: FastifyInstance) => {
           const doc = change.fullDocument as QuestionDoc;
           if (!doc) continue;
           await postDoc(doc);
-          app.log.info({ doc }, "Got changed document:");
+          logger.info("Got changed document:", { doc });
         } catch (err) {
-          app.log.error(
-            { err },
-            "[ChangeStream] Error processing change event",
-          );
+          logger.error("[ChangeStream] Error processing change event", { err });
         }
       }
       isProcessing = false;
     }
     changeStream.on("change", (change: ChangeStreamDocument) => {
-      app.log.info("[ChangeStream] Event");
+      logger.info("[ChangeStream] Event");
       changeQueue.push(change);
       void processQueue();
     });
diff --git a/backend-services/leetcode-backend-service/src/db/model/failedQuestion.ts b/backend-services/leetcode-backend-service/src/db/model/failedQuestion.ts
new file mode 100644
index 000000000..a1a6395b2
--- /dev/null
+++ b/backend-services/leetcode-backend-service/src/db/model/failedQuestion.ts
@@ -0,0 +1,15 @@
+import { Schema, model } from "mongoose";
+
+const failedQuestionSchema = new Schema(
+  {
+    provider: { type: String, required: true, default: "leetcode" },
+    leetcodeIndex: { type: Number, required: true },
+    attempts: { type: Number, default: 0 },
+    lastTriedAt: { type: Date },
+  },
+  { timestamps: true },
+);
+
+failedQuestionSchema.index({ leetcodeIndex: 1 }, { unique: true });
+
+export const FailedQuestion = model("FailedQuestion", failedQuestionSchema);
diff --git a/backend-services/leetcode-backend-service/src/db/types/failedQuestion.ts b/backend-services/leetcode-backend-service/src/db/types/failedQuestion.ts
new file mode 100644
index 000000000..4569c216f
--- /dev/null
+++ b/backend-services/leetcode-backend-service/src/db/types/failedQuestion.ts
@@ -0,0 +1,8 @@
+export interface FailedQuestion {
+  provider: string; // e.g., "leetcode"
+  leetcodeIndex: number;
+  attempts: number; // number of attempts made to fetch the question
+  lastTriedAt?: Date; // timestamp of the last attempt
+  createdAt?: Date;
+  updatedAt?: Date;
+}
diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 71690a7d7..7ef9d5055 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -60,6 +60,17 @@ export async function seedLeetCodeBatch() {
     nextSkip,
   );
 
+  // Check if total is valid, could be undefined if fail to connect to LeetCode
+  if (total === undefined || total === null) {
+    cursor.lastRunAt = new Date();
+    await cursor.save();
+    return {
+      ok: false as const,
+      message: "Failed to fetch total number of questions from leetcode.",
+      nextSkip: cursor.nextSkip,
+    };
+  }
+
   // Check if there are no more questions to fetch
   if (questionList.length === 0 || nextSkip >= total) {
     cursor.lastRunAt = new Date();

From 169d6cd279fcce9730045d466ee733c617a0897e Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Thu, 16 Oct 2025 23:58:00 +0800
Subject: [PATCH 15/29] Refactor fetchNonPaidQuestionInfo to reduce repetition

---
 .../src/leetcode/seedBatch.ts                 | 39 +++++++------------
 1 file changed, 13 insertions(+), 26 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 7ef9d5055..6dd359680 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -87,6 +87,7 @@ export async function seedLeetCodeBatch() {
   const questionInfos: QuestionDetail[] = await fetchNonPaidQuestionInfo(
     pageSize,
     nextSkip,
+    questionList,
   );
 
   const ops = questionInfos.map((q) => ({
@@ -155,35 +156,21 @@ type QuestionDetail = NonNullable<Details["question"]>;
 export async function fetchNonPaidQuestionInfo(
   limit: number,
   skip: number,
+  questionList: BasicInformation[],
 ): Promise<QuestionDetail[]> {
-  const res = await gql<
-    QuestionList,
-    {
-      categorySlug: string;
-      limit: number;
-      skip: number;
-      filters: Record<string, unknown>;
-    }
-  >(QUERY_LIST, { categorySlug: "", limit: limit, skip: skip, filters: {} });
-
-  const questionList = res.problemsetQuestionList;
-  const questions: BasicInformation[] = questionList.questions;
-
   const limitConcurrency = pLimit(DETAIL_CONCURRENCY);
 
-  const tasks = questions
-    .filter((q) => !q.isPaidOnly)
-    .map((q) =>
-      limitConcurrency(async () => {
-        try {
-          const detail = await getQuestionDetail(q.titleSlug);
-          return detail ?? null;
-        } catch {
-          logger.error(`Failed to fetch details for ${q.titleSlug}`);
-          return null;
-        }
-      }),
-    );
+  const tasks = questionList.map((q) =>
+    limitConcurrency(async () => {
+      try {
+        const detail = await getQuestionDetail(q.titleSlug);
+        return detail ?? null;
+      } catch {
+        logger.error(`Failed to fetch details for ${q.titleSlug}`);
+        return null;
+      }
+    }),
+  );
 
   const results = await Promise.all(tasks);
   return results.filter((d): d is QuestionDetail => d !== null);

From c6d5e9597f7e379d1d0a54a514c7a66cf9b3c4ec Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Thu, 16 Oct 2025 23:58:26 +0800
Subject: [PATCH 16/29] Fix comments

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts           | 1 +
 1 file changed, 1 insertion(+)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 6dd359680..e5bb79353 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -151,6 +151,7 @@ type QuestionDetail = NonNullable<Details["question"]>;
  * content of paid questions will not be accessible without a premium account.
  * @param limit - The maximum number of questions to fetch.
  * @param skip - The number of questions to skip.
+ * @param questionList - The list of basic question information to fetch details for.
  * @returns An array of non-paid question details.
  */
 export async function fetchNonPaidQuestionInfo(

From 7f2a81f1c308f8aafcb4e71f0b247f9509e102e4 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 00:09:35 +0800
Subject: [PATCH 17/29] Add debug statement

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts         | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index e5bb79353..23739a4fa 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -66,7 +66,8 @@ export async function seedLeetCodeBatch() {
     await cursor.save();
     return {
       ok: false as const,
-      message: "Failed to fetch total number of questions from leetcode.",
+      message:
+        "Failed to fetch total number of questions from leetcode. Possible failed connection to LeetCode.",
       nextSkip: cursor.nextSkip,
     };
   }

From 3d1a49205a8a08579a4c6b6ebe4bf0ea7256dd6a Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 00:32:40 +0800
Subject: [PATCH 18/29] Replace magic number with constant

---
 .../leetcode-backend-service/src/health.ts       | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/health.ts b/backend-services/leetcode-backend-service/src/health.ts
index bccc1fddf..190995676 100644
--- a/backend-services/leetcode-backend-service/src/health.ts
+++ b/backend-services/leetcode-backend-service/src/health.ts
@@ -10,10 +10,20 @@ interface HealthResponse {
   ok?: boolean;
 }
 
+const BATCH_HEALTH_TIMEOUT_MS = 1500;
+const BATCH_HEALTH_RETRIES = 2;
+const BASE_DELAY_MS = 250;
+
+/** Check the health of the question service.
+ * Throws an error if the service is unhealthy or unreachable after retries.
+ * @param opts - Options for health check.
+ * @returns A promise that resolves to true if healthy, otherwise throws an error.
+ */
+
 export async function checkQuestionServiceHealth({
   url = `${process.env.QUESTION_API_URL}/health`,
-  timeoutMs = 1500,
-  retries = 2,
+  timeoutMs = BATCH_HEALTH_TIMEOUT_MS,
+  retries = BATCH_HEALTH_RETRIES,
 }: HealthOpts = {}) {
   if (!process.env.QUESTION_API_URL)
     throw new Error("QUESTION_API_URL is not set");
@@ -47,7 +57,7 @@ export async function checkQuestionServiceHealth({
     }
 
     if (attempt < retries) {
-      await delay(250 * 2 ** attempt);
+      await delay(BASE_DELAY_MS * 2 ** attempt);
     }
   }
 

From bf5054703a6bd983363972b8554c9f1e870d94b9 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 00:33:28 +0800
Subject: [PATCH 19/29] Remove quesion and dbSchema for failedQuestion

---
 .../src/db/model/failedQuestion.ts                | 15 ---------------
 .../src/db/types/failedQuestion.ts                |  8 --------
 2 files changed, 23 deletions(-)
 delete mode 100644 backend-services/leetcode-backend-service/src/db/model/failedQuestion.ts
 delete mode 100644 backend-services/leetcode-backend-service/src/db/types/failedQuestion.ts

diff --git a/backend-services/leetcode-backend-service/src/db/model/failedQuestion.ts b/backend-services/leetcode-backend-service/src/db/model/failedQuestion.ts
deleted file mode 100644
index a1a6395b2..000000000
--- a/backend-services/leetcode-backend-service/src/db/model/failedQuestion.ts
+++ /dev/null
@@ -1,15 +0,0 @@
-import { Schema, model } from "mongoose";
-
-const failedQuestionSchema = new Schema(
-  {
-    provider: { type: String, required: true, default: "leetcode" },
-    leetcodeIndex: { type: Number, required: true },
-    attempts: { type: Number, default: 0 },
-    lastTriedAt: { type: Date },
-  },
-  { timestamps: true },
-);
-
-failedQuestionSchema.index({ leetcodeIndex: 1 }, { unique: true });
-
-export const FailedQuestion = model("FailedQuestion", failedQuestionSchema);
diff --git a/backend-services/leetcode-backend-service/src/db/types/failedQuestion.ts b/backend-services/leetcode-backend-service/src/db/types/failedQuestion.ts
deleted file mode 100644
index 4569c216f..000000000
--- a/backend-services/leetcode-backend-service/src/db/types/failedQuestion.ts
+++ /dev/null
@@ -1,8 +0,0 @@
-export interface FailedQuestion {
-  provider: string; // e.g., "leetcode"
-  leetcodeIndex: number;
-  attempts: number; // number of attempts made to fetch the question
-  lastTriedAt?: Date; // timestamp of the last attempt
-  createdAt?: Date;
-  updatedAt?: Date;
-}

From 3ed9d26c7fe5799ab00c230d329bf03d69514a52 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 00:37:21 +0800
Subject: [PATCH 20/29] Fix minor comment and error messages

---
 backend-services/leetcode-backend-service/src/health.ts       | 4 +---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts        | 2 +-
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/health.ts b/backend-services/leetcode-backend-service/src/health.ts
index 190995676..380e6896f 100644
--- a/backend-services/leetcode-backend-service/src/health.ts
+++ b/backend-services/leetcode-backend-service/src/health.ts
@@ -61,7 +61,5 @@ export async function checkQuestionServiceHealth({
     }
   }
 
-  throw new Error(
-    `Question service health check failed: ${(lastErr as Error)?.message ?? lastErr}`,
-  );
+  throw new Error(`Question service health check failed: ${String(lastErr)}`);
 }
diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 23739a4fa..a4bd70d20 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -95,7 +95,7 @@ export async function seedLeetCodeBatch() {
     updateOne: {
       filter: { titleSlug: q.titleSlug },
       update: {
-        // $setOnInsert to avoid overwriting existing entries
+        // Use $setOnInsert for all fields to ensure insert-only behavior; existing entries are never updated.
         $setOnInsert: {
           globalSlug: `leetcode:${q.titleSlug}`,
           source: "leetcode",

From 93259cd65fd59ce6e86a840ddbddb26af002ab52 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:17:41 +0800
Subject: [PATCH 21/29] Add error handling for failed leetcode connection

---
 .../src/leetcode/seedBatch.ts                 | 30 ++++++++++++-------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index a4bd70d20..7a5dd2251 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -54,25 +54,29 @@ export async function seedLeetCodeBatch() {
     };
   }
 
-  // Fetch question list using the cursor's nextSkip
-  const { questionList, total } = await fetchNonPaidQuestionList(
-    pageSize,
-    nextSkip,
-  );
+  // Fetch question list
+  let questionList: BasicInformation[] = [];
+  let total = 0;
 
-  // Check if total is valid, could be undefined if fail to connect to LeetCode
-  if (total === undefined || total === null) {
+  try {
+    const { questionList: fetchedQuestionList, total: fetchedTotal } =
+      await fetchNonPaidQuestionList(pageSize, nextSkip);
+    questionList = fetchedQuestionList;
+    total = fetchedTotal;
+  } catch (err) {
+    logger.error(
+      `Failed to fetch question list from LeetCode: ${(err as Error).message}`,
+    );
     cursor.lastRunAt = new Date();
     await cursor.save();
     return {
-      ok: false as const,
-      message:
-        "Failed to fetch total number of questions from leetcode. Possible failed connection to LeetCode.",
+      ok: false,
+      message: `Failed to fetch question list from LeetCode: ${(err as Error).message}`,
       nextSkip: cursor.nextSkip,
     };
   }
 
-  // Check if there are no more questions to fetch
+  // Check if there are more questions to process
   if (questionList.length === 0 || nextSkip >= total) {
     cursor.lastRunAt = new Date();
     cursor.total = total ?? cursor.total;
@@ -200,6 +204,10 @@ export async function fetchNonPaidQuestionList(
     }
   >(QUERY_LIST, { categorySlug: "", limit: limit, skip: skip, filters: {} });
 
+  if (!res.problemsetQuestionList) {
+    throw new Error("Failed to fetch question list from LeetCode");
+  }
+
   const { total, questions } = res.problemsetQuestionList;
   const questionList = questions.filter((q) => !q.isPaidOnly);
   return { questionList, total };

From 9929a2d547c73878d4021200a1642d0afbb74523 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:32:00 +0800
Subject: [PATCH 22/29] Fix README.md

---
 .../leetcode-backend-service/README.md        |   4 +-
 .../question-backend-service/README.md        | 107 ++++++++++--------
 2 files changed, 64 insertions(+), 47 deletions(-)

diff --git a/backend-services/leetcode-backend-service/README.md b/backend-services/leetcode-backend-service/README.md
index bd1d707bc..98d011e9f 100644
--- a/backend-services/leetcode-backend-service/README.md
+++ b/backend-services/leetcode-backend-service/README.md
@@ -49,13 +49,13 @@ Server listening on http://localhost:5285
 **Note**: Local development (e.g. `npm run dev`) is possible (though not recommended). To enable it, update the .env file by changing:
 
 ```bash
-QUESTION_API_URL=http://question-backend:5275/api/v1/questions
+QUESTION_API_URL=http://question-backend:5275/api/v1/question-service
 ```
 
 to:
 
 ```bash
-QUESTION_API_URL=http://localhost:5275/api/v1/questions
+QUESTION_API_URL=http://localhost:5275/api/v1/question-service
 ```
 
 Do change the `QUESTION_API_URL` back when using docker run.
diff --git a/backend-services/question-backend-service/README.md b/backend-services/question-backend-service/README.md
index becd3e3d0..27cf4ae3a 100644
--- a/backend-services/question-backend-service/README.md
+++ b/backend-services/question-backend-service/README.md
@@ -75,69 +75,86 @@ src/
 
 ## API
 
-Base URL: `http://localhost:5275/api/v1`
+Base URL: `http://localhost:5275/api/v1/question-service`
 
-### Questions — existence check
+### Existence Check
 
-**GET** `/question/exists`  
+**POST** `/exists-categories-difficulties`  
 Checks whether a question with the given attributes exists.
 
-Query params
+Sample reply body:
 
-- categoryTitle (string)
-- difficulty (Easy|Medium|Hard)
+```json
+{
+  "categories": {
+    "Algorithms": ["Easy", "Medium", "Hard"],
+    "CS": ["Easy"]
+  }
+}
+```
 
-Example:
+Sample response:
 
-```bash
-# For window users
-curl.exe  http://localhost:5275/api/questions/exists?categoryTitle=Algorithms&difficulty=Easy
+```json
+{
+  "Algorithms": {
+    "Easy": true,
+    "Medium": true,
+    "Hard": true
+  },
+  "CS": {
+    "Easy": false
+  }
+}
 ```
 
-### Questions — random fetch
+### Fetch random question
 
-**GET** `/question/random`  
+**POST** `/random`  
 Returns a single random question filtered by query.
 
-Query params
-
-- categoryTitle (string)
-- difficulty (Easy|Medium|Hard)
+Sample reply body:
 
-Example:
-
-```bash
-# For window users
-curl.exe http://localhost:5275/api/questions/random?categoryTitle=Algorithms&difficulty=Easy
+```json
+{
+  "categories": {
+    "Algorithms": ["Easy", "Medium", "Hard"],
+    "Database": ["Easy"]
+  }
+}
 ```
 
-### Questions — insert
-
-**POST** `/questions/post-question`
-Upserts a question document.
-
-## Data Model
+Sample response:
 
-`Question` (database: `question-service`)
-
-```ts
+```json
 {
-  titleSlug: String,
-  title: String,
-  difficulty: "Easy" | "Medium" | "Hard",
-  categoryTitle: String,
-  timeLimit: Number,
-  content: String,
-  codeSnippets: [{
-    lang: String,
-    langSlug: String,
-    code: String,
-  }],
-  hints: [String],
-  sampleTestCase: String,
-  createdAt: Date,
-  updatedAt: Date
+  "_id": "68ebac53b63b10de074be992",
+  "globalSlug": "leetcode:rearrange-products-table",
+  "__v": 0,
+  "categoryTitle": "Database",
+  "codeSnippets": [
+    {
+      "lang": "MySQL",
+      "langSlug": "mysql",
+      "code": "# Write your MySQL query statement below\n"
+    },
+  ],
+  "content": "<p>Table: <code>Products</code></p>\n....",
+  "createdAt": "2025-10-12T13:25:40.139Z",
+  "difficulty": "Easy",
+  "exampleTestcases": "{\"headers\":{\"Products\":[\"product_id\",\"store1\",\"store2\",\"store3\"]},\"rows\":{\"Products\":[[0, 95, 100, 105], [1, 70, null, 80]]}}",
+  "hints": [],
+  "source": "leetcode",
+  "timeLimit": 30,
+  "title": "Rearrange Products Table",
+  "titleSlug": "rearrange-products-table",
+  "updatedAt": "2025-10-12T13:25:40.139Z"
 }
 ```
 
+### Insert Question into Database
+
+**POST** `/question`
+Upserts a question document.
+
 ---

From 1259069560e6e12f48962e9762df3fbbb37063db Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:35:24 +0800
Subject: [PATCH 23/29] Fix naming issues

---
 .github/workflows/docker-build-push.yml | 2 +-
 .github/workflows/format-lint.yml       | 6 +++---
 .github/workflows/leetcode-update.yml   | 4 ++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/.github/workflows/docker-build-push.yml b/.github/workflows/docker-build-push.yml
index 412a70565..93b0176eb 100644
--- a/.github/workflows/docker-build-push.yml
+++ b/.github/workflows/docker-build-push.yml
@@ -21,7 +21,7 @@ jobs:
           # Add one entry per microservice
           - name: matching-ui
             context: ./ui-services/matching-ui-service
-          - name: questions-ui
+          - name: question-ui
             context: ./ui-services/question-ui-service
           - name: collab-ui
             context: ./ui-services/collab-ui-service
diff --git a/.github/workflows/format-lint.yml b/.github/workflows/format-lint.yml
index db304b101..6ef10b3af 100644
--- a/.github/workflows/format-lint.yml
+++ b/.github/workflows/format-lint.yml
@@ -67,7 +67,7 @@ jobs:
           npx eslint --format json . > eslint-report.json
           cat eslint-report.json | reviewdog -f=eslint -name="eslint" -reporter=github-pr-review -level=error
 
-  questions-ui-lint:
+  question-ui-lint:
     runs-on: ubuntu-latest
     steps:
       - name: Set REVIEWDOG_GITHUB_API_TOKEN
@@ -100,7 +100,7 @@ jobs:
           eslint_exit_code=$exit_code
 
           if [ $eslint_exit_code -ne 0 ]; then
-            echo "ESLint failed for questions-ui"
+            echo "ESLint failed for question-ui"
             exit 1
           fi
 
@@ -180,7 +180,7 @@ jobs:
           eslint_exit_code=$exit_code
 
           if [ $eslint_exit_code -ne 0 ]; then
-            echo "ESLint failed for questions-ui"
+            echo "ESLint failed for question-ui"
             exit 1
           fi
 
diff --git a/.github/workflows/leetcode-update.yml b/.github/workflows/leetcode-update.yml
index cbab04d16..fb642cb4e 100644
--- a/.github/workflows/leetcode-update.yml
+++ b/.github/workflows/leetcode-update.yml
@@ -48,7 +48,7 @@ jobs:
         run: |
           echo "Waiting for leetcode-backend to respond..."
           for i in {1..60}; do
-            code=$(curl -s -o /dev/null -w "%{http_code}" http://localhost:5285/api/v1/leetcode/health || true)
+            code=$(curl -s -o /dev/null -w "%{http_code}" http://localhost:5285/api/v1/leetcode-service/health || true)
             if [ "$code" = "200" ]; then
               echo "Leetcode backend is ready (HTTP 200)"
               exit 0
@@ -73,7 +73,7 @@ jobs:
         run: |
           echo "Waiting for question-backend to respond..."
           for i in {1..60}; do
-            code=$(curl -s -o /dev/null -w "%{http_code}" http://localhost:5275/api/v1/questions/health || true)
+            code=$(curl -s -o /dev/null -w "%{http_code}" http://localhost:5275/api/v1/question-service/health || true)
             if [ "$code" = "200" ]; then
               echo "Question backend is ready (HTTP 200)"
               exit 0

From de9469097778106edeef98ba79a7efee3b31419e Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:35:41 +0800
Subject: [PATCH 24/29] Fix README for leetcode-service

---
 backend-services/leetcode-backend-service/README.md | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/backend-services/leetcode-backend-service/README.md b/backend-services/leetcode-backend-service/README.md
index 98d011e9f..95ef16dd1 100644
--- a/backend-services/leetcode-backend-service/README.md
+++ b/backend-services/leetcode-backend-service/README.md
@@ -86,11 +86,11 @@ src/
 
 ## API
 
-Base URL: `http://localhost:5285/api/v1`
+Base URL: `http://localhost:5285/api/v1/leetcode-service`
 
 ### Seed 200 problems into Mongo
 
-**POST** `/leetcode/seed-batch`  
+**POST** `/seed-batch`  
 Fetches the next 200 problems and **upserts** to Mongo.
 
 Examples:
@@ -98,7 +98,7 @@ Examples:
 ```bash
 # Replace ADMIN_TOKEN with DB password
 # MUSt run the question-service before running the follow command
-curl.exe --request POST -H "X-Admin-Token: <>" --url "http://localhost:5285/api/v1/leetcode/seed-batch"
+curl.exe --request POST -H "X-Admin-Token: <ADMIN_TOKEN>" --url "http://localhost:5285/api/v1/leetcode-service/seed-batch"
 ```
 
 ## Data Model

From f34d83273c71777919acdd8d1cd191644a662f3c Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:38:50 +0800
Subject: [PATCH 25/29] Fix magic number issue

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts         | 3 ++-
 backend-services/question-backend-service/README.md            | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 7a5dd2251..5e32231d3 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -13,6 +13,7 @@ import pLimit from "p-limit";
 import { logger } from "../logger.js";
 import { checkQuestionServiceHealth } from "../health.js";
 
+const PAGE_SIZE = 200;
 /**
  * Maximum number of concurrent requests for fetching question details.
  * This can be configured via the LEETCODE_DETAIL_CONCURRENCY environment variable.
@@ -39,7 +40,7 @@ export async function seedLeetCodeBatch() {
   const id = "questions";
   const cursor =
     (await SeedCursor.findById(id)) ??
-    new SeedCursor({ _id: id, nextSkip: 0, pageSize: 200 });
+    new SeedCursor({ _id: id, nextSkip: 0, pageSize: PAGE_SIZE });
   const { pageSize, nextSkip } = cursor;
 
   try {
diff --git a/backend-services/question-backend-service/README.md b/backend-services/question-backend-service/README.md
index 27cf4ae3a..03b78f468 100644
--- a/backend-services/question-backend-service/README.md
+++ b/backend-services/question-backend-service/README.md
@@ -137,7 +137,7 @@ Sample response:
       "lang": "MySQL",
       "langSlug": "mysql",
       "code": "# Write your MySQL query statement below\n"
-    },
+    }
   ],
   "content": "<p>Table: <code>Products</code></p>\n....",
   "createdAt": "2025-10-12T13:25:40.139Z",

From cd2f45f5b4822ca65c03aa741b38b6b11162ce09 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:39:50 +0800
Subject: [PATCH 26/29] Remove redundant params

---
 .../leetcode-backend-service/src/leetcode/seedBatch.ts   | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 5e32231d3..4dc41f216 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -90,11 +90,8 @@ export async function seedLeetCodeBatch() {
     };
   }
 
-  const questionInfos: QuestionDetail[] = await fetchNonPaidQuestionInfo(
-    pageSize,
-    nextSkip,
-    questionList,
-  );
+  const questionInfos: QuestionDetail[] =
+    await fetchNonPaidQuestionInfo(questionList);
 
   const ops = questionInfos.map((q) => ({
     updateOne: {
@@ -161,8 +158,6 @@ type QuestionDetail = NonNullable<Details["question"]>;
  * @returns An array of non-paid question details.
  */
 export async function fetchNonPaidQuestionInfo(
-  limit: number,
-  skip: number,
   questionList: BasicInformation[],
 ): Promise<QuestionDetail[]> {
   const limitConcurrency = pLimit(DETAIL_CONCURRENCY);

From 43bffb34c47572977c5499ff2ae870a6b7aacf74 Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:51:18 +0800
Subject: [PATCH 27/29] Fix reties count and comments for setOnInsert

---
 backend-services/leetcode-backend-service/src/health.ts         | 2 +-
 .../leetcode-backend-service/src/leetcode/seedBatch.ts          | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/backend-services/leetcode-backend-service/src/health.ts b/backend-services/leetcode-backend-service/src/health.ts
index 380e6896f..402997cb0 100644
--- a/backend-services/leetcode-backend-service/src/health.ts
+++ b/backend-services/leetcode-backend-service/src/health.ts
@@ -30,7 +30,7 @@ export async function checkQuestionServiceHealth({
 
   let lastErr: unknown;
 
-  for (let attempt = 0; attempt <= retries; attempt++) {
+  for (let attempt = 0; attempt < retries; attempt++) {
     const controller = new AbortController();
     const t = setTimeout(() => controller.abort(), timeoutMs);
 
diff --git a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
index 4dc41f216..f83b63649 100644
--- a/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
+++ b/backend-services/leetcode-backend-service/src/leetcode/seedBatch.ts
@@ -97,7 +97,7 @@ export async function seedLeetCodeBatch() {
     updateOne: {
       filter: { titleSlug: q.titleSlug },
       update: {
-        // Use $setOnInsert for all fields to ensure insert-only behavior; existing entries are never updated.
+        // Use $setOnInsert for all fields to ensure insert-only behavior; existing entries' application fields are never updated (though MongoDB may update internal metadata fields).
         $setOnInsert: {
           globalSlug: `leetcode:${q.titleSlug}`,
           source: "leetcode",

From 72d9ba50973b87910c0f0b8b281365ba492c5f2e Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:55:57 +0800
Subject: [PATCH 28/29] Add new line for .env.example

---
 backend-services/leetcode-backend-service/.env.example | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/backend-services/leetcode-backend-service/.env.example b/backend-services/leetcode-backend-service/.env.example
index e67e9ca37..bdf8b680b 100644
--- a/backend-services/leetcode-backend-service/.env.example
+++ b/backend-services/leetcode-backend-service/.env.example
@@ -3,4 +3,4 @@ ADMIN_TOKEN=<db-password>
 QUESTION_API_URL=http://localhost:5275/api/v1/question-service
 # Maximum number of concurrent LeetCode detail fetches.
 # Optional: defaults to 6 if not set.
-LEETCODE_DETAIL_CONCURRENCY=6
\ No newline at end of file
+LEETCODE_DETAIL_CONCURRENCY=6

From 1f16df46c833d2f76da0f792073a248f89d4d53b Mon Sep 17 00:00:00 2001
From: "sharonsohxuanhui@hotmail.com" <sharonsohxuanhui@hotmail.com>
Date: Fri, 17 Oct 2025 23:59:28 +0800
Subject: [PATCH 29/29] Fix retries issue

---
 backend-services/leetcode-backend-service/src/health.ts | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/backend-services/leetcode-backend-service/src/health.ts b/backend-services/leetcode-backend-service/src/health.ts
index 402997cb0..ee9bac39d 100644
--- a/backend-services/leetcode-backend-service/src/health.ts
+++ b/backend-services/leetcode-backend-service/src/health.ts
@@ -30,7 +30,8 @@ export async function checkQuestionServiceHealth({
 
   let lastErr: unknown;
 
-  for (let attempt = 0; attempt < retries; attempt++) {
+  // Attempt health check with retries where retries is the number of additional attempts after the first
+  for (let attempt = 0; attempt < retries + 1; attempt++) {
     const controller = new AbortController();
     const t = setTimeout(() => controller.abort(), timeoutMs);