MongoDB Transactions and Multi-Document ACID Operations: Enterprise Data Consistency and Integrity for Mission-Critical Applications

-- Traditional PostgreSQL distributed transactions - complex coordination with performance overhead

-- Financial transfer operation requiring distributed consistency across accounts
CREATE TABLE accounts (
    account_id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    user_id INTEGER NOT NULL,
    account_number VARCHAR(20) UNIQUE NOT NULL,
    account_type VARCHAR(20) NOT NULL DEFAULT 'checking',
    balance DECIMAL(15,2) NOT NULL DEFAULT 0.00,
    currency_code VARCHAR(3) NOT NULL DEFAULT 'USD',

    -- Account metadata
    account_status VARCHAR(20) DEFAULT 'active',
    opened_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    last_activity TIMESTAMP DEFAULT CURRENT_TIMESTAMP,

    -- Balance constraints and limits
    minimum_balance DECIMAL(15,2) DEFAULT 0.00,
    overdraft_limit DECIMAL(15,2) DEFAULT 0.00,
    daily_transfer_limit DECIMAL(15,2) DEFAULT 10000.00,

    -- Compliance and tracking
    kyc_verified BOOLEAN DEFAULT FALSE,
    risk_level VARCHAR(20) DEFAULT 'low',
    regulatory_flags TEXT[],

    -- Audit trail
    created_by INTEGER,
    updated_by INTEGER,
    version_number INTEGER DEFAULT 1,

    CONSTRAINT valid_balance CHECK (balance + overdraft_limit >= 0),
    CONSTRAINT valid_status CHECK (account_status IN ('active', 'suspended', 'closed', 'frozen'))
);

-- Transaction log for audit and reconciliation requirements
CREATE TABLE transaction_log (
    transaction_id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    transaction_timestamp TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    transaction_type VARCHAR(50) NOT NULL,

    -- Source and destination account information
    source_account_id UUID REFERENCES accounts(account_id),
    destination_account_id UUID REFERENCES accounts(account_id),

    -- Transaction amounts and currency
    transaction_amount DECIMAL(15,2) NOT NULL,
    currency_code VARCHAR(3) NOT NULL DEFAULT 'USD',
    exchange_rate DECIMAL(10,6),

    -- Transaction details
    description TEXT,
    reference_number VARCHAR(100) UNIQUE,
    external_reference VARCHAR(100),
    merchant_info JSONB,

    -- Processing status and workflow
    transaction_status VARCHAR(20) DEFAULT 'pending',
    processing_stage VARCHAR(30) DEFAULT 'initiated',
    approval_required BOOLEAN DEFAULT FALSE,
    approved_by INTEGER,
    approved_at TIMESTAMP,

    -- Error handling and retry logic
    error_code VARCHAR(20),
    error_message TEXT,
    retry_count INTEGER DEFAULT 0,
    max_retries INTEGER DEFAULT 3,

    -- Compliance and regulatory
    compliance_checked BOOLEAN DEFAULT FALSE,
    aml_flagged BOOLEAN DEFAULT FALSE,
    regulatory_reporting JSONB,

    -- Audit and tracking
    created_by INTEGER,
    session_id VARCHAR(100),
    ip_address INET,
    user_agent TEXT,

    CONSTRAINT valid_transaction_type CHECK (transaction_type IN (
        'transfer', 'deposit', 'withdrawal', 'payment', 'refund', 'fee', 'interest'
    )),
    CONSTRAINT valid_amount CHECK (transaction_amount > 0),
    CONSTRAINT valid_status CHECK (transaction_status IN (
        'pending', 'processing', 'completed', 'failed', 'cancelled', 'reversed'
    ))
);

-- Account balance history for audit and reconciliation
CREATE TABLE balance_history (
    history_id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    account_id UUID NOT NULL REFERENCES accounts(account_id),
    transaction_id UUID REFERENCES transaction_log(transaction_id),

    -- Balance tracking
    previous_balance DECIMAL(15,2) NOT NULL,
    transaction_amount DECIMAL(15,2) NOT NULL,
    new_balance DECIMAL(15,2) NOT NULL,
    running_balance DECIMAL(15,2) NOT NULL,

    -- Timestamp and sequencing
    recorded_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    sequence_number BIGSERIAL,

    -- Balance verification
    balance_verified BOOLEAN DEFAULT FALSE,
    verification_timestamp TIMESTAMP,
    discrepancy_amount DECIMAL(15,2) DEFAULT 0.00,

    -- Reconciliation metadata
    reconciliation_batch_id UUID,
    reconciliation_status VARCHAR(20) DEFAULT 'pending',

    CONSTRAINT balance_consistency CHECK (previous_balance + transaction_amount = new_balance)
);

-- Complex distributed transaction procedure with error handling and rollback complexity
CREATE OR REPLACE FUNCTION transfer_funds_with_distributed_consistency(
    p_source_account_id UUID,
    p_destination_account_id UUID,
    p_amount DECIMAL(15,2),
    p_description TEXT DEFAULT 'Fund Transfer',
    p_reference_number VARCHAR(100) DEFAULT NULL,
    p_user_id INTEGER DEFAULT NULL
)
RETURNS TABLE (
    transaction_id UUID,
    transaction_status TEXT,
    source_new_balance DECIMAL(15,2),
    destination_new_balance DECIMAL(15,2),
    processing_result TEXT
) AS $$
DECLARE
    v_transaction_id UUID := gen_random_uuid();
    v_source_account RECORD;
    v_destination_account RECORD;
    v_source_new_balance DECIMAL(15,2);
    v_destination_new_balance DECIMAL(15,2);
    v_daily_transfer_total DECIMAL(15,2);
    v_reference_number VARCHAR(100);
    v_compliance_result JSONB;
    v_error_message TEXT;

BEGIN
    -- Generate reference number if not provided
    v_reference_number := COALESCE(p_reference_number, 'TXN' || EXTRACT(EPOCH FROM CURRENT_TIMESTAMP)::bigint || random()::text);

    -- Start distributed transaction with SERIALIZABLE isolation for consistency
    BEGIN
        -- Lock source account and validate
        SELECT * INTO v_source_account
        FROM accounts 
        WHERE account_id = p_source_account_id 
        AND account_status = 'active'
        FOR UPDATE;

        IF NOT FOUND THEN
            RAISE EXCEPTION 'Source account not found or inactive: %', p_source_account_id;
        END IF;

        -- Lock destination account and validate
        SELECT * INTO v_destination_account
        FROM accounts 
        WHERE account_id = p_destination_account_id 
        AND account_status IN ('active', 'suspended') -- Allow deposits to suspended accounts
        FOR UPDATE;

        IF NOT FOUND THEN
            RAISE EXCEPTION 'Destination account not found or closed: %', p_destination_account_id;
        END IF;

        -- Validate transfer amount constraints
        IF p_amount <= 0 THEN
            RAISE EXCEPTION 'Transfer amount must be positive: %', p_amount;
        END IF;

        -- Check sufficient balance including overdraft
        v_source_new_balance := v_source_account.balance - p_amount;
        IF v_source_new_balance < -v_source_account.overdraft_limit THEN
            RAISE EXCEPTION 'Insufficient funds: balance=%, overdraft=%, requested=%', 
                v_source_account.balance, v_source_account.overdraft_limit, p_amount;
        END IF;

        -- Check daily transfer limits
        SELECT COALESCE(SUM(transaction_amount), 0) INTO v_daily_transfer_total
        FROM transaction_log
        WHERE source_account_id = p_source_account_id
        AND transaction_timestamp >= CURRENT_DATE
        AND transaction_status IN ('completed', 'processing')
        AND transaction_type = 'transfer';

        IF v_daily_transfer_total + p_amount > v_source_account.daily_transfer_limit THEN
            RAISE EXCEPTION 'Daily transfer limit exceeded: current=%, limit=%, requested=%',
                v_daily_transfer_total, v_source_account.daily_transfer_limit, p_amount;
        END IF;

        -- Compliance and AML checks (simplified)
        v_compliance_result := jsonb_build_object(
            'amount_threshold_check', p_amount > 10000,
            'cross_border_check', v_source_account.currency_code != v_destination_account.currency_code,
            'high_risk_account', v_source_account.risk_level = 'high' OR v_destination_account.risk_level = 'high',
            'kyc_verified', v_source_account.kyc_verified AND v_destination_account.kyc_verified
        );

        IF (v_compliance_result->>'amount_threshold_check')::boolean AND NOT (v_compliance_result->>'kyc_verified')::boolean THEN
            RAISE EXCEPTION 'Large transfer requires full KYC verification for both accounts';
        END IF;

        -- Create transaction log entry
        INSERT INTO transaction_log (
            transaction_id, source_account_id, destination_account_id,
            transaction_amount, currency_code, description, reference_number,
            transaction_type, transaction_status, processing_stage,
            compliance_checked, regulatory_reporting, created_by, session_id
        ) VALUES (
            v_transaction_id, p_source_account_id, p_destination_account_id,
            p_amount, v_source_account.currency_code, p_description, v_reference_number,
            'transfer', 'processing', 'balance_update',
            true, v_compliance_result, p_user_id, current_setting('application.session_id', true)
        );

        -- Update source account balance
        UPDATE accounts 
        SET balance = balance - p_amount,
            last_activity = CURRENT_TIMESTAMP,
            updated_by = p_user_id,
            version_number = version_number + 1
        WHERE account_id = p_source_account_id;

        -- Record source balance history
        INSERT INTO balance_history (
            account_id, transaction_id, previous_balance, 
            transaction_amount, new_balance, running_balance
        ) VALUES (
            p_source_account_id, v_transaction_id, v_source_account.balance,
            -p_amount, v_source_new_balance, v_source_new_balance
        );

        -- Calculate destination new balance
        v_destination_new_balance := v_destination_account.balance + p_amount;

        -- Update destination account balance
        UPDATE accounts 
        SET balance = balance + p_amount,
            last_activity = CURRENT_TIMESTAMP,
            updated_by = p_user_id,
            version_number = version_number + 1
        WHERE account_id = p_destination_account_id;

        -- Record destination balance history
        INSERT INTO balance_history (
            account_id, transaction_id, previous_balance, 
            transaction_amount, new_balance, running_balance
        ) VALUES (
            p_destination_account_id, v_transaction_id, v_destination_account.balance,
            p_amount, v_destination_new_balance, v_destination_new_balance
        );

        -- Update transaction status to completed
        UPDATE transaction_log 
        SET transaction_status = 'completed',
            processing_stage = 'completed',
            approved_at = CURRENT_TIMESTAMP
        WHERE transaction_id = v_transaction_id;

        -- Commit the distributed transaction
        COMMIT;

        -- Return success results
        RETURN QUERY SELECT 
            v_transaction_id,
            'completed'::text,
            v_source_new_balance,
            v_destination_new_balance,
            'Transfer completed successfully'::text;

    EXCEPTION
        WHEN OTHERS THEN
            -- Rollback transaction and log error
            ROLLBACK;

            v_error_message := SQLERRM;

            -- Insert failed transaction record for audit
            INSERT INTO transaction_log (
                transaction_id, source_account_id, destination_account_id,
                transaction_amount, currency_code, description, reference_number,
                transaction_type, transaction_status, processing_stage,
                error_code, error_message, created_by
            ) VALUES (
                v_transaction_id, p_source_account_id, p_destination_account_id,
                p_amount, COALESCE(v_source_account.currency_code, 'USD'), p_description, v_reference_number,
                'transfer', 'failed', 'error_handling',
                'TRANSFER_FAILED', v_error_message, p_user_id
            );

            -- Return error results
            RETURN QUERY SELECT 
                v_transaction_id,
                'failed'::text,
                COALESCE(v_source_account.balance, 0::decimal),
                COALESCE(v_destination_account.balance, 0::decimal),
                ('Transfer failed: ' || v_error_message)::text;
    END;
END;
$$ LANGUAGE plpgsql;

-- Complex batch transaction processing with distributed coordination
CREATE OR REPLACE FUNCTION process_batch_transfers(
    p_transfers JSONB, -- Array of transfer specifications
    p_batch_id UUID DEFAULT gen_random_uuid(),
    p_processing_user INTEGER DEFAULT NULL
)
RETURNS TABLE (
    batch_id UUID,
    total_transfers INTEGER,
    successful_transfers INTEGER,
    failed_transfers INTEGER,
    total_amount DECIMAL(15,2),
    processing_duration INTERVAL,
    batch_status TEXT
) AS $$
DECLARE
    v_transfer JSONB;
    v_transfer_result RECORD;
    v_batch_start TIMESTAMP := CURRENT_TIMESTAMP;
    v_total_transfers INTEGER := 0;
    v_successful_transfers INTEGER := 0;
    v_failed_transfers INTEGER := 0;
    v_total_amount DECIMAL(15,2) := 0;
    v_individual_amount DECIMAL(15,2);

BEGIN
    -- Create batch processing record
    INSERT INTO batch_processing_log (
        batch_id, batch_type, initiated_by, initiated_at, 
        total_operations, batch_status
    ) VALUES (
        p_batch_id, 'fund_transfers', p_processing_user, v_batch_start,
        jsonb_array_length(p_transfers), 'processing'
    );

    -- Process each transfer in the batch
    FOR v_transfer IN SELECT * FROM jsonb_array_elements(p_transfers) LOOP
        BEGIN
            v_individual_amount := (v_transfer->>'amount')::DECIMAL(15,2);
            v_total_amount := v_total_amount + v_individual_amount;
            v_total_transfers := v_total_transfers + 1;

            -- Execute individual transfer
            SELECT * INTO v_transfer_result
            FROM transfer_funds_with_distributed_consistency(
                (v_transfer->>'source_account_id')::UUID,
                (v_transfer->>'destination_account_id')::UUID,
                v_individual_amount,
                COALESCE(v_transfer->>'description', 'Batch Transfer'),
                v_transfer->>'reference_number',
                p_processing_user
            );

            IF v_transfer_result.transaction_status = 'completed' THEN
                v_successful_transfers := v_successful_transfers + 1;
            ELSE
                v_failed_transfers := v_failed_transfers + 1;

                -- Log batch transfer failure
                INSERT INTO batch_operation_details (
                    batch_id, operation_sequence, operation_status,
                    operation_data, error_message
                ) VALUES (
                    p_batch_id, v_total_transfers, 'failed',
                    v_transfer, v_transfer_result.processing_result
                );
            END IF;

        EXCEPTION
            WHEN OTHERS THEN
                v_failed_transfers := v_failed_transfers + 1;

                -- Log exception details
                INSERT INTO batch_operation_details (
                    batch_id, operation_sequence, operation_status,
                    operation_data, error_message
                ) VALUES (
                    p_batch_id, v_total_transfers, 'error',
                    v_transfer, SQLERRM
                );
        END;
    END LOOP;

    -- Update batch completion status
    UPDATE batch_processing_log 
    SET batch_status = CASE 
            WHEN v_failed_transfers = 0 THEN 'completed_success'
            WHEN v_successful_transfers = 0 THEN 'completed_failure'
            ELSE 'completed_partial'
        END,
        completed_at = CURRENT_TIMESTAMP,
        successful_operations = v_successful_transfers,
        failed_operations = v_failed_transfers,
        total_amount_processed = v_total_amount
    WHERE batch_id = p_batch_id;

    -- Return batch processing summary
    RETURN QUERY SELECT 
        p_batch_id,
        v_total_transfers,
        v_successful_transfers,
        v_failed_transfers,
        v_total_amount,
        CURRENT_TIMESTAMP - v_batch_start,
        CASE 
            WHEN v_failed_transfers = 0 THEN 'success'
            WHEN v_successful_transfers = 0 THEN 'failed'
            ELSE 'partial_success'
        END::text;
END;
$$ LANGUAGE plpgsql;

-- Problems with traditional distributed transaction management:
-- 1. Complex distributed coordination requiring extensive procedural code and error handling
-- 2. Performance bottlenecks from table-level locking and serializable isolation levels
-- 3. Limited scalability across multiple database instances and distributed architectures
-- 4. Manual rollback logic and compensation procedures for failed distributed operations
-- 5. Difficulty maintaining ACID properties across microservice boundaries and network partitions
-- 6. Complex deadlock detection and resolution in multi-table transaction scenarios
-- 7. Resource-intensive locking mechanisms impacting concurrent transaction performance
-- 8. Manual consistency management across related tables and complex foreign key relationships
-- 9. Limited support for horizontal scaling while maintaining transactional consistency
-- 10. Operational complexity of monitoring and debugging distributed transaction failures

// MongoDB Multi-Document Transactions - Native distributed ACID operations with scalable consistency
const { MongoClient, ObjectId } = require('mongodb');

// Advanced MongoDB Transaction Manager for Enterprise ACID Operations
class MongoDBTransactionManager {
  constructor(client, config = {}) {
    this.client = client;
    this.db = client.db(config.database || 'enterprise_transactions');

    this.config = {
      // Transaction configuration
      defaultTransactionOptions: {
        readConcern: { level: config.readConcern || 'snapshot' },
        writeConcern: { w: config.writeConcern || 'majority' },
        readPreference: config.readPreference || 'primary'
      },
      maxTransactionTimeMS: config.maxTransactionTimeMS || 60000, // 1 minute
      maxRetryAttempts: config.maxRetryAttempts || 3,

      // Error handling and retry configuration
      enableRetryLogic: config.enableRetryLogic !== false,
      retryableErrors: config.retryableErrors || [
        'WriteConflict', 'TransientTransactionError', 'UnknownTransactionCommitResult'
      ],

      // Performance optimization
      enableTransactionMetrics: config.enableTransactionMetrics !== false,
      enableDeadlockDetection: config.enableDeadlockDetection !== false,
      enablePerformanceMonitoring: config.enablePerformanceMonitoring !== false,

      // Business logic configuration
      enableComplianceChecks: config.enableComplianceChecks !== false,
      enableAuditLogging: config.enableAuditLogging !== false,
      enableBusinessValidation: config.enableBusinessValidation !== false
    };

    // Transaction management state
    this.activeTransactions = new Map();
    this.transactionMetrics = new Map();
    this.deadlockStats = new Map();

    this.initializeTransactionManager();
  }

  async initializeTransactionManager() {
    console.log('Initializing MongoDB Transaction Manager...');

    try {
      // Setup transaction-aware collections with appropriate indexes
      await this.setupTransactionCollections();

      // Initialize performance monitoring
      if (this.config.enablePerformanceMonitoring) {
        await this.initializePerformanceMonitoring();
      }

      console.log('MongoDB Transaction Manager initialized successfully');

    } catch (error) {
      console.error('Error initializing transaction manager:', error);
      throw error;
    }
  }

  async setupTransactionCollections() {
    console.log('Setting up transaction-aware collections...');

    try {
      // Accounts collection with transaction-optimized indexes
      const accountsCollection = this.db.collection('accounts');
      await accountsCollection.createIndexes([
        { key: { accountNumber: 1 }, unique: true, background: true },
        { key: { userId: 1, accountType: 1 }, background: true },
        { key: { accountStatus: 1, balance: -1 }, background: true },
        { key: { lastActivity: -1 }, background: true }
      ]);

      // Transaction log collection for ACID audit trail
      const transactionLogCollection = this.db.collection('transaction_log');
      await transactionLogCollection.createIndexes([
        { key: { transactionTimestamp: -1 }, background: true },
        { key: { sourceAccountId: 1, transactionTimestamp: -1 }, background: true },
        { key: { destinationAccountId: 1, transactionTimestamp: -1 }, background: true },
        { key: { referenceNumber: 1 }, unique: true, sparse: true, background: true },
        { key: { transactionStatus: 1, transactionTimestamp: -1 }, background: true }
      ]);

      // Balance history for audit and reconciliation
      const balanceHistoryCollection = this.db.collection('balance_history');
      await balanceHistoryCollection.createIndexes([
        { key: { accountId: 1, recordedAt: -1 }, background: true },
        { key: { transactionId: 1 }, background: true },
        { key: { sequenceNumber: -1 }, background: true }
      ]);

      console.log('Transaction collections configured successfully');

    } catch (error) {
      console.error('Error setting up transaction collections:', error);
      throw error;
    }
  }

  async executeTransactionWithRetry(transactionLogic, options = {}) {
    console.log('Executing transaction with automatic retry logic...');

    const session = this.client.startSession();
    const transactionOptions = { 
      ...this.config.defaultTransactionOptions, 
      ...options 
    };

    let retryCount = 0;
    const maxRetries = this.config.maxRetryAttempts;
    const transactionId = new ObjectId();
    const startTime = Date.now();

    while (retryCount <= maxRetries) {
      try {
        await session.withTransaction(
          async () => {
            const transactionContext = {
              session: session,
              transactionId: transactionId,
              attempt: retryCount + 1,
              startTime: startTime,
              db: this.db
            };

            return await transactionLogic(transactionContext);
          },
          transactionOptions
        );

        // Transaction succeeded
        const duration = Date.now() - startTime;
        await this.updateTransactionMetrics(transactionId, 'completed', duration, retryCount);

        console.log(`Transaction completed successfully: ${transactionId} (${retryCount + 1} attempts)`);
        return { success: true, transactionId, attempts: retryCount + 1, duration };

      } catch (error) {
        retryCount++;

        const isRetryableError = this.isRetryableTransactionError(error);
        const shouldRetry = isRetryableError && retryCount <= maxRetries && this.config.enableRetryLogic;

        if (!shouldRetry) {
          // Transaction failed permanently
          const duration = Date.now() - startTime;
          await this.updateTransactionMetrics(transactionId, 'failed', duration, retryCount - 1);

          console.error(`Transaction failed permanently: ${transactionId}`, error);
          throw error;
        }

        // Wait before retry with exponential backoff
        const backoffMs = Math.min(1000 * Math.pow(2, retryCount - 1), 5000);
        await new Promise(resolve => setTimeout(resolve, backoffMs));

        console.warn(`Transaction retry ${retryCount}/${maxRetries} for ${transactionId}: ${error.message}`);
      }
    }

    await session.endSession();
  }

  async transferFundsWithACID(transferRequest) {
    console.log('Processing fund transfer with ACID guarantees...');

    return await this.executeTransactionWithRetry(async (context) => {
      const { session, transactionId, db } = context;

      // Collections with session for transaction scope
      const accountsCollection = db.collection('accounts', { session });
      const transactionLogCollection = db.collection('transaction_log', { session });
      const balanceHistoryCollection = db.collection('balance_history', { session });

      // Step 1: Validate and lock source account
      const sourceAccount = await accountsCollection.findOneAndUpdate(
        { 
          _id: new ObjectId(transferRequest.sourceAccountId),
          accountStatus: 'active' 
        },
        { 
          $set: { 
            lastActivity: new Date(),
            lockTimestamp: new Date(),
            lockedBy: transactionId.toString()
          }
        },
        { 
          returnDocument: 'after',
          session: session 
        }
      );

      if (!sourceAccount.value) {
        throw new Error(`Source account not found or inactive: ${transferRequest.sourceAccountId}`);
      }

      // Step 2: Validate and lock destination account
      const destinationAccount = await accountsCollection.findOneAndUpdate(
        { 
          _id: new ObjectId(transferRequest.destinationAccountId),
          accountStatus: { $in: ['active', 'suspended'] } // Allow deposits to suspended accounts
        },
        { 
          $set: { 
            lastActivity: new Date(),
            lockTimestamp: new Date(),
            lockedBy: transactionId.toString()
          }
        },
        { 
          returnDocument: 'after',
          session: session 
        }
      );

      if (!destinationAccount.value) {
        throw new Error(`Destination account not found: ${transferRequest.destinationAccountId}`);
      }

      // Step 3: Business logic validation
      await this.validateTransferRequirements(
        sourceAccount.value, 
        destinationAccount.value, 
        transferRequest, 
        context
      );

      // Step 4: Create transaction log entry
      const transactionLog = {
        _id: new ObjectId(),
        transactionId: transactionId,
        transactionTimestamp: new Date(),
        transactionType: 'transfer',

        // Account information
        sourceAccountId: sourceAccount.value._id,
        destinationAccountId: destinationAccount.value._id,

        // Transaction details
        transactionAmount: transferRequest.amount,
        currencyCode: transferRequest.currencyCode || 'USD',
        description: transferRequest.description || 'Fund Transfer',
        referenceNumber: transferRequest.referenceNumber || this.generateReferenceNumber(),

        // Processing metadata
        transactionStatus: 'processing',
        processingStage: 'balance_update',
        initiatedBy: transferRequest.userId,
        sessionId: transferRequest.sessionId,

        // Compliance and audit
        complianceChecked: true,
        complianceResult: await this.performComplianceChecks(transferRequest, context),

        // Transaction context
        clientMetadata: transferRequest.metadata || {}
      };

      await transactionLogCollection.insertOne(transactionLog, { session });

      // Step 5: Update source account balance
      const sourceBalanceUpdate = await accountsCollection.findOneAndUpdate(
        { _id: sourceAccount.value._id },
        { 
          $inc: { balance: -transferRequest.amount },
          $set: { 
            lastActivity: new Date(),
            updatedBy: transferRequest.userId,
            lockTimestamp: null,
            lockedBy: null
          },
          $inc: { versionNumber: 1 }
        },
        { 
          returnDocument: 'after',
          session: session 
        }
      );

      // Step 6: Record source balance history
      await balanceHistoryCollection.insertOne({
        _id: new ObjectId(),
        accountId: sourceAccount.value._id,
        transactionId: transactionId,
        previousBalance: sourceAccount.value.balance,
        transactionAmount: -transferRequest.amount,
        newBalance: sourceBalanceUpdate.value.balance,
        recordedAt: new Date(),
        sequenceNumber: await this.getNextSequenceNumber(sourceAccount.value._id, context)
      }, { session });

      // Step 7: Update destination account balance
      const destinationBalanceUpdate = await accountsCollection.findOneAndUpdate(
        { _id: destinationAccount.value._id },
        { 
          $inc: { balance: transferRequest.amount },
          $set: { 
            lastActivity: new Date(),
            updatedBy: transferRequest.userId,
            lockTimestamp: null,
            lockedBy: null
          },
          $inc: { versionNumber: 1 }
        },
        { 
          returnDocument: 'after',
          session: session 
        }
      );

      // Step 8: Record destination balance history
      await balanceHistoryCollection.insertOne({
        _id: new ObjectId(),
        accountId: destinationAccount.value._id,
        transactionId: transactionId,
        previousBalance: destinationAccount.value.balance,
        transactionAmount: transferRequest.amount,
        newBalance: destinationBalanceUpdate.value.balance,
        recordedAt: new Date(),
        sequenceNumber: await this.getNextSequenceNumber(destinationAccount.value._id, context)
      }, { session });

      // Step 9: Update transaction log to completed
      await transactionLogCollection.updateOne(
        { transactionId: transactionId },
        { 
          $set: { 
            transactionStatus: 'completed',
            processingStage: 'completed',
            completedAt: new Date()
          }
        },
        { session }
      );

      // Step 10: Return transaction result
      return {
        transactionId: transactionId,
        referenceNumber: transactionLog.referenceNumber,
        sourceAccountBalance: sourceBalanceUpdate.value.balance,
        destinationAccountBalance: destinationBalanceUpdate.value.balance,
        transactionAmount: transferRequest.amount,
        completedAt: new Date()
      };
    });
  }

  async processBatchTransfers(batchRequest) {
    console.log('Processing batch transfers with distributed ACID guarantees...');

    return await this.executeTransactionWithRetry(async (context) => {
      const { session, transactionId, db } = context;
      const batchResults = [];
      const batchSummary = {
        batchId: transactionId,
        totalTransfers: batchRequest.transfers.length,
        successfulTransfers: 0,
        failedTransfers: 0,
        totalAmount: 0,
        processedAt: new Date()
      };

      // Create batch processing record
      await db.collection('batch_processing_log', { session }).insertOne({
        _id: transactionId,
        batchType: 'fund_transfers',
        initiatedBy: batchRequest.userId,
        initiatedAt: new Date(),
        totalOperations: batchRequest.transfers.length,
        batchStatus: 'processing',
        transfers: batchRequest.transfers
      });

      // Process each transfer within the same transaction
      for (const [index, transfer] of batchRequest.transfers.entries()) {
        try {
          // Execute individual transfer as part of the batch transaction
          const transferResult = await this.processIndividualTransferInBatch(
            transfer, 
            context, 
            index + 1
          );

          batchResults.push({
            sequence: index + 1,
            status: 'completed',
            transferId: transferResult.transactionId,
            amount: transfer.amount,
            result: transferResult
          });

          batchSummary.successfulTransfers++;
          batchSummary.totalAmount += transfer.amount;

        } catch (transferError) {
          batchResults.push({
            sequence: index + 1,
            status: 'failed',
            amount: transfer.amount,
            error: transferError.message
          });

          batchSummary.failedTransfers++;

          // In strict batch mode, fail entire batch if any transfer fails
          if (batchRequest.strictMode) {
            throw new Error(`Batch transfer failed at sequence ${index + 1}: ${transferError.message}`);
          }
        }
      }

      // Update batch completion status
      await db.collection('batch_processing_log', { session }).updateOne(
        { _id: transactionId },
        { 
          $set: { 
            batchStatus: batchSummary.failedTransfers === 0 ? 'completed_success' : 'completed_partial',
            completedAt: new Date(),
            successfulOperations: batchSummary.successfulTransfers,
            failedOperations: batchSummary.failedTransfers,
            totalAmountProcessed: batchSummary.totalAmount,
            results: batchResults
          }
        }
      );

      return {
        batchSummary,
        batchResults,
        transactionId: transactionId
      };
    });
  }

  async processIndividualTransferInBatch(transfer, context, sequenceNumber) {
    const { session, db } = context;
    const transferId = new ObjectId();

    // Validate accounts exist and are available
    const accounts = await db.collection('accounts', { session })
      .find({ 
        _id: { $in: [
          new ObjectId(transfer.sourceAccountId), 
          new ObjectId(transfer.destinationAccountId)
        ]},
        accountStatus: { $in: ['active', 'suspended'] }
      })
      .toArray();

    if (accounts.length !== 2) {
      throw new Error(`Invalid account(s) for transfer sequence ${sequenceNumber}`);
    }

    const sourceAccount = accounts.find(acc => acc._id.toString() === transfer.sourceAccountId);
    const destinationAccount = accounts.find(acc => acc._id.toString() === transfer.destinationAccountId);

    // Validate sufficient balance
    if (sourceAccount.balance < transfer.amount) {
      throw new Error(`Insufficient funds for transfer sequence ${sequenceNumber}`);
    }

    // Update balances atomically
    await Promise.all([
      db.collection('accounts', { session }).updateOne(
        { _id: sourceAccount._id },
        { 
          $inc: { balance: -transfer.amount },
          $set: { lastActivity: new Date() }
        }
      ),
      db.collection('accounts', { session }).updateOne(
        { _id: destinationAccount._id },
        { 
          $inc: { balance: transfer.amount },
          $set: { lastActivity: new Date() }
        }
      )
    ]);

    // Create transaction record
    await db.collection('transaction_log', { session }).insertOne({
      _id: transferId,
      batchSequence: sequenceNumber,
      parentBatchId: context.transactionId,
      transactionTimestamp: new Date(),
      transactionType: 'transfer',
      sourceAccountId: sourceAccount._id,
      destinationAccountId: destinationAccount._id,
      transactionAmount: transfer.amount,
      description: transfer.description || `Batch transfer ${sequenceNumber}`,
      transactionStatus: 'completed'
    });

    return {
      transactionId: transferId,
      sequenceNumber: sequenceNumber,
      amount: transfer.amount
    };
  }

  async validateTransferRequirements(sourceAccount, destinationAccount, transferRequest, context) {
    const validations = [];

    // Amount validation
    if (transferRequest.amount <= 0) {
      validations.push('Transfer amount must be positive');
    }

    // Balance validation
    const availableBalance = sourceAccount.balance + (sourceAccount.overdraftLimit || 0);
    if (availableBalance < transferRequest.amount) {
      validations.push(`Insufficient funds: available=${availableBalance}, requested=${transferRequest.amount}`);
    }

    // Daily limit validation
    const dailyTotal = await this.calculateDailyTransferTotal(sourceAccount._id, context);
    const dailyLimit = sourceAccount.dailyTransferLimit || 10000;
    if (dailyTotal + transferRequest.amount > dailyLimit) {
      validations.push(`Daily transfer limit exceeded: current=${dailyTotal}, limit=${dailyLimit}`);
    }

    // Account status validation
    if (sourceAccount.accountStatus !== 'active') {
      validations.push('Source account is not active');
    }

    if (destinationAccount.accountStatus === 'closed') {
      validations.push('Destination account is closed');
    }

    if (validations.length > 0) {
      throw new Error(`Transfer validation failed: ${validations.join(', ')}`);
    }
  }

  async performComplianceChecks(transferRequest, context) {
    if (!this.config.enableComplianceChecks) {
      return { checked: false, message: 'Compliance checks disabled' };
    }

    const complianceResult = {
      amlCheck: transferRequest.amount > 10000,
      crossBorderCheck: false, // Would be determined by account locations
      highRiskCheck: false,     // Would be determined by account risk scores
      kycVerified: true,        // Would be validated from account KYC status
      complianceScore: 'low',
      checkedAt: new Date()
    };

    // Simulate compliance processing
    if (complianceResult.amlCheck && !complianceResult.kycVerified) {
      throw new Error('Large transfers require full KYC verification');
    }

    return complianceResult;
  }

  async calculateDailyTransferTotal(accountId, context) {
    const { session, db } = context;
    const startOfDay = new Date();
    startOfDay.setHours(0, 0, 0, 0);

    const dailyTransfers = await db.collection('transaction_log', { session })
      .aggregate([
        {
          $match: {
            sourceAccountId: accountId,
            transactionTimestamp: { $gte: startOfDay },
            transactionStatus: { $in: ['completed', 'processing'] },
            transactionType: 'transfer'
          }
        },
        {
          $group: {
            _id: null,
            totalAmount: { $sum: '$transactionAmount' }
          }
        }
      ])
      .toArray();

    return dailyTransfers[0]?.totalAmount || 0;
  }

  async getNextSequenceNumber(accountId, context) {
    const { session, db } = context;

    const lastHistory = await db.collection('balance_history', { session })
      .findOne(
        { accountId: accountId },
        { sort: { sequenceNumber: -1 } }
      );

    return (lastHistory?.sequenceNumber || 0) + 1;
  }

  generateReferenceNumber() {
    const timestamp = Date.now().toString(36);
    const random = Math.random().toString(36).substring(2, 8);
    return `TXN${timestamp}${random}`.toUpperCase();
  }

  isRetryableTransactionError(error) {
    const errorMessage = error.message || '';
    return this.config.retryableErrors.some(retryableError => 
      errorMessage.includes(retryableError) || 
      error.hasErrorLabel?.(retryableError)
    );
  }

  async updateTransactionMetrics(transactionId, status, duration, retryCount) {
    if (!this.config.enableTransactionMetrics) return;

    const metrics = {
      transactionId: transactionId,
      status: status,
      duration: duration,
      retryCount: retryCount,
      timestamp: new Date()
    };

    this.transactionMetrics.set(transactionId.toString(), metrics);

    // Optionally persist metrics to database
    await this.db.collection('transaction_metrics').insertOne(metrics);
  }

  async getTransactionStatus() {
    console.log('Retrieving transaction manager status...');

    const status = {
      activeTransactions: this.activeTransactions.size,
      totalTransactions: this.transactionMetrics.size,
      configuration: {
        maxRetryAttempts: this.config.maxRetryAttempts,
        maxTransactionTimeMS: this.config.maxTransactionTimeMS,
        retryLogicEnabled: this.config.enableRetryLogic
      },
      performance: {
        averageTransactionTime: 0,
        successRate: 0,
        retryRate: 0
      }
    };

    // Calculate performance metrics
    if (this.transactionMetrics.size > 0) {
      const metrics = Array.from(this.transactionMetrics.values());

      status.performance.averageTransactionTime = 
        metrics.reduce((sum, m) => sum + m.duration, 0) / metrics.length;

      const successfulTransactions = metrics.filter(m => m.status === 'completed').length;
      status.performance.successRate = (successfulTransactions / metrics.length) * 100;

      const retriedTransactions = metrics.filter(m => m.retryCount > 0).length;
      status.performance.retryRate = (retriedTransactions / metrics.length) * 100;
    }

    return status;
  }

  async cleanup() {
    console.log('Cleaning up Transaction Manager resources...');

    this.activeTransactions.clear();
    this.transactionMetrics.clear();
    this.deadlockStats.clear();

    console.log('Transaction Manager cleanup completed');
  }
}

// Example usage for enterprise financial operations
async function demonstrateEnterpriseTransactions() {
  const client = new MongoClient('mongodb://localhost:27017', {
    replicaSet: 'rs0' // Transactions require replica set or sharded cluster
  });
  await client.connect();

  const transactionManager = new MongoDBTransactionManager(client, {
    database: 'enterprise_banking',
    readConcern: 'snapshot',
    writeConcern: 'majority',
    enableRetryLogic: true,
    enableComplianceChecks: true,
    enablePerformanceMonitoring: true
  });

  try {
    // Create sample accounts for demonstration
    const accountsCollection = client.db('enterprise_banking').collection('accounts');

    const sampleAccounts = [
      {
        _id: new ObjectId(),
        accountNumber: 'ACC001',
        userId: 'user_alice',
        accountType: 'checking',
        balance: 5000.00,
        accountStatus: 'active',
        dailyTransferLimit: 10000.00,
        overdraftLimit: 500.00
      },
      {
        _id: new ObjectId(),
        accountNumber: 'ACC002',
        userId: 'user_bob',
        accountType: 'savings',
        balance: 2500.00,
        accountStatus: 'active',
        dailyTransferLimit: 5000.00,
        overdraftLimit: 0.00
      }
    ];

    await accountsCollection.insertMany(sampleAccounts);

    // Demonstrate single fund transfer with ACID guarantees
    console.log('Executing single fund transfer...');
    const transferResult = await transactionManager.transferFundsWithACID({
      sourceAccountId: sampleAccounts[0]._id.toString(),
      destinationAccountId: sampleAccounts[1]._id.toString(),
      amount: 1000.00,
      description: 'Monthly transfer',
      currencyCode: 'USD',
      userId: 'user_alice',
      sessionId: 'session_123'
    });

    console.log('Transfer Result:', transferResult);

    // Demonstrate batch transfers with distributed ACID
    console.log('Executing batch transfers...');
    const batchResult = await transactionManager.processBatchTransfers({
      transfers: [
        {
          sourceAccountId: sampleAccounts[0]._id.toString(),
          destinationAccountId: sampleAccounts[1]._id.toString(),
          amount: 250.00,
          description: 'Batch transfer 1'
        },
        {
          sourceAccountId: sampleAccounts[1]._id.toString(),
          destinationAccountId: sampleAccounts[0]._id.toString(),
          amount: 150.00,
          description: 'Batch transfer 2'
        }
      ],
      userId: 'user_system',
      strictMode: false // Allow partial success
    });

    console.log('Batch Result:', JSON.stringify(batchResult, null, 2));

    // Get transaction manager status
    const status = await transactionManager.getTransactionStatus();
    console.log('Transaction Manager Status:', status);

    return {
      transferResult,
      batchResult,
      status
    };

  } catch (error) {
    console.error('Error demonstrating enterprise transactions:', error);
    throw error;
  } finally {
    await transactionManager.cleanup();
    await client.close();
  }
}

// Benefits of MongoDB Multi-Document Transactions:
// - Native ACID guarantees across multiple documents and collections without external coordination
// - Distributed transaction support across sharded clusters and replica sets with automatic failover
// - Automatic retry logic for transient failures with exponential backoff and deadlock detection
// - Performance optimization through snapshot isolation and optimistic concurrency control
// - Seamless integration with MongoDB's document model and aggregation framework capabilities
// - Enterprise-grade consistency management with configurable read and write concerns
// - Horizontal scaling with maintained ACID properties across distributed database architectures
// - Simplified application logic without manual compensation procedures or rollback handling

module.exports = {
  MongoDBTransactionManager,
  demonstrateEnterpriseTransactions
};