// system_design.arch System Design Architecture Mastery 🏗️

# Nginx Load Balancer — Round Robin አሰራር
upstream backend {
    server app1.internal:3000;
    server app2.internal:3000;
    server app3.internal:3000;
    # ሁሉም ጥያቄዎች (Requests) በየተራ 1→2→3→1→2→3 እያለ ይከፋፈላል!
}

server {
    listen 80;
    location / {
        proxy_pass http://backend;
        proxy_set_header X-Real-IP $remote_addr;

        # Health check — የሞተ (የተበላሸ) ሰርቨርን ከስራ ውጭ ያደርጋል
        proxy_connect_timeout 5s;
        proxy_read_timeout 30s;
    }
}

# Sticky Sessions — አንድ ተጠቃሚ ሁልጊዜ ተመሳሳይ ሰርቨር ላይ እንዲቆይ:
upstream backend_sticky {
    ip_hash; ← የተጠቃሚውን IP መነሻ ያደርጋል
    server app1.internal:3000;
    server app2.internal:3000;
}

// ── 1. Cache-Aside (Lazy Loading) ── ← በጣም የተለመደው
async function getUser(id) {
  const cached = await redis.get(`user:${id}`);
  if (cached) return JSON.parse(cached); // Cache Hit! (ካገኘው ይመልሳል)

  const user = await db.findUser(id);     // Cache Miss (ካላገኘው ወደ DB ይሄዳል)
  await redis.setEx(`user:${id}`, 3600, JSON.stringify(user));
  return user;
}

// ── 2. Write-Through ── ወደ DB እና Cache በተመሳሳይ ጊዜ መፃፍ
async function updateUser(id, data) {
  await db.updateUser(id, data);
  await redis.setEx(`user:${id}`, 3600, JSON.stringify(data));
  // DB እና Cache ሁልጊዜ ተመሳሳይ ዳታ ይኖራቸዋል!
}

// ── 3. Cache Invalidation — ዳታ ሲቀየር የድሮውን Cache ማጥፋት
async function deleteUser(id) {
  await db.deleteUser(id);
  await redis.del(`user:${id}`);     // Invalidate (አጥፋው)!
  await redis.del(`users:list`);    // ተያያዥነት ያላቸውንም Cache ያጠፋል!
}

// ── 4. Cache Stampede Prevention — ብዙ ጥያቄዎች በአንድ ጊዜ Cache Miss ሲያደርጉ
async function getUserSafe(id) {
  const lockKey = `lock:user:${id}`;
  const lock = await redis.set(lockKey, 1, 'NX', 'EX', 10);
  if (!lock) {
    // ሌላ ጥያቄ (request) ዳታውን እያመጣ ነው — ስለዚህ ትንሽ ጠብቆ እንደገና ይሞክራል
    await sleep(100);
    return getUser(id);
  }
  // ማንም አልያዘውም — ስለዚህ ከ DB አምጥቶ Cache ያደርገዋል
}

-- ❌ Slow: Full table scan — 1 ሚሊዮንም ሮው (rows) ካሉ ሁሉንም ያነባል
SELECT * FROM posts WHERE user_id = 42;
-- Execution: 850ms (ያለ index)

-- ✅ መጀመሪያ Index ፍጠሩ!
CREATE INDEX idx_posts_user_id ON posts(user_id);
-- አሁን ያው ጥያቄ: 2ms ብቻ! 425 እጥፍ ፈጣን ነው!

-- Compound Index — ለብዙ columns በአንድ ጊዜ
CREATE INDEX idx_posts_user_created
  ON posts(user_id, created_at DESC);
-- "የአቤልን posts — አዲሶቹን መጀመሪያ" — በቅፅበት ይመልሳል!

-- ❌ N+1 Query Problem — በቃለመጠይቅ ጊዜ ሁሌ የሚነሳ!
const users = await prisma.user.findMany();
for (const user of users) {
  const posts = await prisma.post.findMany({where:{authorId:user.id}});
  // 100 users ቢኖሩ = 101 queries ወደ DB ይላካሉ! (N+1 ችግር)
}

-- ✅ Fix: include መጠቀም (በ 1 query ብቻ ያልቃል!)
const users = await prisma.user.findMany({
  include: { posts: true } // JOIN በማድረግ በ 1 query ይመልሰዋል!
});

-- Read Replica — አዲስ መረጃ ለመፃፍ ወደ Primary፣ ለማንበብ ወደ Replica
const writePrisma = new PrismaClient({ datasourceUrl: PRIMARY_URL });
const readPrisma  = new PrismaClient({ datasourceUrl: REPLICA_URL });
-- ለማንበብ → replica (የማንበብ አቅምን 10x ያሳድጋል!)
-- ለመፃፍ → primary (Consistencyን ለመጠበቅ!)