package main

import (
    "context"
    "time"
    "go.mongodb.org/mongo-driver/mongo"
    "go.mongodb.org/mongo-driver/mongo/options"
    "go.mongodb.org/mongo-driver/mongo/readpref"
)

func connectReplicaSet() *mongo.Client {
    // Driver tự discover toàn bộ topology từ bất kỳ seed nào
    uri := "mongodb://mongo1:27017,mongo2:27017,mongo3:27017/?replicaSet=rs0"

    opts := options.Client().ApplyURI(uri).
        SetMaxPoolSize(100).       // Max connections per node
        SetMinPoolSize(5).        // Warmup connections
        SetMaxConnIdleTime(10 * time.Minute).
        SetServerSelectionTimeout(5 * time.Second).
        SetHeartbeatInterval(10 * time.Second).  // Client-side monitoring
        SetReadPreference(readpref.SecondaryPreferred()).  // Read from secondary nếu có
        SetWriteConcern(/* w:majority, j:true */)

    client, _ := mongo.Connect(context.Background(), opts)

    // Driver ngay lập tức bắt đầu Server Monitoring (SDAM):
    // 1. Connect đến seed nodes
    // 2. Chạy "isMaster" command → nhận topology info
    // 3. Discover tất cả replica set members
    // 4. Monitor liên tục (heartbeat interval)

    return client
}

// ── OP_MSG Frame Structure ─────────────────────────────────
// MsgHeader (16 bytes)
messageLength:  4 bytes  // Total message size
requestID:     4 bytes  // Client-assigned, dùng để match response
responseTo:    4 bytes  // 0 cho requests, requestID của request cho responses
opCode:        4 bytes  // 2013 (0x07DD) = OP_MSG

// OP_MSG body
flagBits:      4 bytes  // checksumPresent(bit0), moreToCome(bit1)

// Section type 0 — Command document (BSON)
// Ví dụ InsertOne command:
{
  "insert":    "users",          // collection name
  "documents": [{ "name":"Alice", "age":30 }],
  "ordered":   true,
  "writeConcern": { "w": "majority", "j": true, "wtimeout": 5000 },
  "lsid":      { "id": /* UUID */ },    // Logical session ID
  "$db":       "mydb",
  "$readPreference": { "mode": "primary" }
}

// Server response (OP_MSG):
// { "n": 1, "ok": 1.0 }  — thành công
// { "ok": 0, "code": 11000, "errmsg": "duplicate key..." }  — lỗi

storage:
  engine: wiredTiger
  wiredTiger:
    engineConfig:
      # Cache size — critical tuning parameter
      # Rule: (RAM - 1GB) * 0.5, nhưng để lại cho OS file cache
      cacheSizeGB: 4          # Với 16GB RAM → 4-6GB WT cache

      journalCompressor: snappy  # snappy | zlib | none

    collectionConfig:
      blockCompressor: snappy    # Compression cho collection data
      # snappy: fast + decent compression (default)
      # zlib: slower + better compression (archive data)
      # zstd: best ratio (MongoDB 4.2+)

    indexConfig:
      prefixCompression: true    # Compress index keys với common prefix

# Kiểm tra cache usage:
# db.serverStatus().wiredTiger.cache
# "bytes currently in the cache" / "maximum bytes configured" = eviction pressure
# Nếu > 80%: tăng cacheSizeGB hoặc scale RAM

// Xem replication status
rs.printReplicationInfo()
// Output:
// configured oplog size:   10240 MB
// log length start to end: 86400 secs (24.0 hrs)
// oplog first event time:  2024-01-15 00:00:00 GMT
// oplog last event time:   2024-01-16 00:00:00 GMT
// now:                     2024-01-16 00:00:00 GMT

rs.printSecondaryReplicationInfo()
// source: mongo2:27017
//         syncedTo: Mon Jan 16 2024 09:59:58 GMT
//         0 secs (0 hrs) behind the primary

// Chi tiết lag per member:
rs.status().members.forEach(m => {
  if (m.state === 2) {  // 2 = SECONDARY
    lag = m.optimeDate - rs.status().members.find(x => x.state===1).optimeDate;
    print(m.name + " lag: " + lag/1000 + "s");
  }
});

// Alert nếu lag > threshold (ví dụ từ monitoring script)
// Lag cao → secondaries không kịp apply → đọc stale data khi dùng readPreference:secondary

// ── Read Preference Options ────────────────────────────────

// 1. primary (default) — luôn đọc từ Primary
//    Đảm bảo đọc data mới nhất, không lag
opts := options.Find().SetReadPreference(readpref.Primary())

// 2. secondary — chỉ đọc từ Secondary
//    Có thể lag → stale data. Tăng read throughput.
opts := options.Find().SetReadPreference(readpref.Secondary())

// 3. secondaryPreferred — Secondary nếu có, fallback Primary
//    Best for read-heavy workloads với acceptable staleness
opts := options.Find().SetReadPreference(readpref.SecondaryPreferred())

// 4. nearest — node có RTT thấp nhất (Primary hoặc Secondary)
//    Dùng cho geo-distributed clusters
opts := options.Find().SetReadPreference(readpref.Nearest())

// 5. primaryPreferred — Primary nếu available, fallback Secondary
//    Graceful degradation khi Primary overloaded
opts := options.Find().SetReadPreference(readpref.PrimaryPreferred())

// ── Với MaxStaleness (giới hạn stale data) ────────────────
rp, _ := readpref.New(
    readpref.SecondaryMode,
    readpref.WithMaxStaleness(90*time.Second),  // Tối đa lag 90s
)
// Driver sẽ không route đến Secondary có lag > 90s