Credit System Implementation

Overview

The credit system handles two separate credit pools:

Subscription Credits: Monthly allocation from subscription plans
Purchased Credits: One-time purchases that never expire

Key Concepts

Credit Pools

Subscription credits expire and rollover with caps
Purchased credits are permanent
Always deduct subscription credits first

Rollover Logic

Unused subscription credits roll over to next cycle
Capped based on plan (e.g., Pro caps at 200% of monthly allocation)
Purchased credits don't count toward rollover caps

Atomic Operations

All credit operations must be atomic
Use database transactions to prevent race conditions
Create audit trails for all transactions

Database Functions

Deduct Credits (Subscription First)

CREATE OR REPLACE FUNCTION deduct_credits(
  p_user_id UUID,
  p_amount INTEGER,
  p_reason TEXT DEFAULT 'image_generation'
) RETURNS TABLE(
  success BOOLEAN,
  subscription_used INTEGER,
  purchased_used INTEGER,
  new_balance_subscription INTEGER,
  new_balance_purchased INTEGER,
  transaction_id UUID
) LANGUAGE plpgsql SECURITY DEFINER AS $$
DECLARE
  v_transaction_id UUID := gen_random_uuid();
  v_subscription_balance INTEGER;
  v_purchased_balance INTEGER;
  v_subscription_to_deduct INTEGER;
  v_purchased_to_deduct INTEGER;
BEGIN
  -- Lock user credits row to prevent race conditions
  PERFORM 1 FROM user_credits WHERE user_id = p_user_id FOR UPDATE;

  -- Get current balances
  SELECT subscription_credits, purchased_credits
  INTO v_subscription_balance, v_purchased_balance
  FROM user_credits
  WHERE user_id = p_user_id;

  -- Check if enough credits
  IF v_subscription_balance + v_purchased_balance < p_amount THEN
    RETURN QUERY SELECT false, 0, 0, v_subscription_balance, v_purchased_balance, v_transaction_id;
    RETURN;
  END IF;

  -- Calculate deduction: subscription first, then purchased
  v_subscription_to_deduct := LEAST(v_subscription_balance, p_amount);
  v_purchased_to_deduct := p_amount - v_subscription_to_deduct;

  -- Update balances
  UPDATE user_credits SET
    subscription_credits = subscription_credits - v_subscription_to_deduct,
    purchased_credits = purchased_credits - v_purchased_to_deduct,
    updated_at = now()
  WHERE user_id = p_user_id;

  -- Record transaction
  INSERT INTO credit_transactions (
    id, user_id, amount, reason,
    subscription_credits_before, subscription_credits_after,
    purchased_credits_before, purchased_credits_after,
    created_at
  ) VALUES (
    v_transaction_id, p_user_id, -p_amount, p_reason,
    v_subscription_balance, v_subscription_balance - v_subscription_to_deduct,
    v_purchased_balance, v_purchased_balance - v_purchased_to_deduct,
    now()
  );

  RETURN QUERY SELECT
    true,
    v_subscription_to_deduct,
    v_purchased_to_deduct,
    v_subscription_balance - v_subscription_to_deduct,
    v_purchased_balance - v_purchased_to_deduct,
    v_transaction_id;
END;
$$;

Add Subscription Credits with Rollover

CREATE OR REPLACE FUNCTION add_subscription_credits(
  p_user_id UUID,
  p_amount INTEGER,
  p_rollover_cap INTEGER DEFAULT NULL, -- NULL = no cap
  p_reason TEXT DEFAULT 'monthly_grant'
) RETURNS TABLE(
  credits_added INTEGER,
  rollover_credits INTEGER,
  new_balance INTEGER,
  expired_credits INTEGER
) LANGUAGE plpgsql SECURITY DEFINER AS $$
DECLARE
  v_current_balance INTEGER;
  v_current_rollover INTEGER;
  v_total_before INTEGER;
  v_total_after INTEGER;
  v_expired INTEGER := 0;
  v_rollover_to_add INTEGER := 0;
  v_credits_to_add INTEGER := p_amount;
BEGIN
  -- Lock row
  PERFORM 1 FROM user_credits WHERE user_id = p_user_id FOR UPDATE;

  -- Get current state
  SELECT
    subscription_credits,
    rollover_credits,
    last_credit_reset
  INTO v_current_balance, v_current_rollover, last_credit_reset
  FROM user_credits
  WHERE user_id = p_user_id;

  -- Calculate total before
  v_total_before := v_current_balance + v_current_rollover;

  -- Handle expiration (if monthly reset)
  -- This would be called by a separate monthly reset function

  -- Add new credits
  v_current_balance := v_current_balance + p_amount;

  -- Handle rollover
  IF p_rollover_cap IS NOT NULL THEN
    -- Calculate what would be over cap
    IF v_current_balance > p_rollover_cap THEN
      v_rollover_to_add := v_current_balance - p_rollover_cap;
      v_current_balance := p_rollover_cap;
    END IF;
  END IF;

  -- Update database
  UPDATE user_credits SET
    subscription_credits = v_current_balance,
    rollover_credits = v_current_rollover + v_rollover_to_add,
    updated_at = now()
  WHERE user_id = p_user_id;

  -- Return results
  v_total_after := v_current_balance + v_current_rollover + v_rollover_to_add;

  RETURN QUERY SELECT
    v_credits_to_add,
    v_rollover_to_add,
    v_current_balance,
    v_expired;
END;
$$;

Refund Credits (Original Pool)

CREATE OR REPLACE FUNCTION refund_credits(
  p_transaction_id UUID,
  p_reason TEXT DEFAULT 'refund'
) RETURNS BOOLEAN LANGUAGE plpgsql SECURITY DEFINER AS $$
DECLARE
  v_user_id UUID;
  v_amount INTEGER;
  v_original_reason TEXT;
  v_subscription_used INTEGER;
  v_purchased_used INTEGER;
BEGIN
  -- Get original transaction details
  SELECT user_id, amount, reason, subscription_credits_used, purchased_credits_used
  INTO v_user_id, v_amount, v_original_reason, v_subscription_used, v_purchased_used
  FROM credit_transactions
  WHERE id = p_transaction_id;

  -- Lock user credits
  PERFORM 1 FROM user_credits WHERE user_id = v_user_id FOR UPDATE;

  -- Refund to original pools
  UPDATE user_credits SET
    subscription_credits = subscription_credits + v_subscription_used,
    purchased_credits = purchased_credits + v_purchased_used,
    updated_at = now()
  WHERE user_id = v_user_id;

  -- Record refund transaction
  INSERT INTO credit_transactions (
    user_id, amount, reason,
    subscription_credits_used, purchased_credits_used,
    refund_transaction_id, created_at
  ) VALUES (
    v_user_id, v_amount, p_reason,
    v_subscription_used, v_purchased_used,
    p_transaction_id, now()
  );

  RETURN true;
END;
$$;

Service Layer Patterns

Credit Transaction Service

export interface ICreditTransaction {
  id: string;
  userId: string;
  amount: number;
  reason: string;
  subscriptionCreditsUsed: number;
  purchasedCreditsUsed: number;
  createdAt: Date;
}

export interface ICreditDeductionRequest {
  userId: string;
  amount: number;
  reason?: string;
}

export interface ICreditDeductionResult {
  success: boolean;
  transactionId?: string;
  subscriptionUsed: number;
  purchasedUsed: number;
  newSubscriptionBalance: number;
  newPurchasedBalance: number;
  error?: string;
}

export class CreditTransactionService {
  /**
   * Deduct credits atomically with subscription-first logic
   */
  static async deductCredits(request: ICreditDeductionRequest): Promise<ICreditDeductionResult> {
    const { userId, amount, reason = 'usage' } = request;

    // Validate input
    if (amount <= 0) {
      return {
        success: false,
        subscriptionUsed: 0,
        purchasedUsed: 0,
        newSubscriptionBalance: 0,
        newPurchasedBalance: 0,
        error: 'Amount must be positive',
      };
    }

    try {
      // Use database transaction for atomicity
      const result = await supabase.rpc('deduct_credits', {
        p_user_id: userId,
        p_amount: amount,
        p_reason: reason,
      });

      if (result.error) {
        throw new Error(result.error.message);
      }

      const data = result.data[0];

      return {
        success: data.success,
        transactionId: data.transaction_id,
        subscriptionUsed: data.subscription_used,
        purchasedUsed: data.purchased_used,
        newSubscriptionBalance: data.new_balance_subscription,
        newPurchasedBalance: data.new_balance_purchased,
        error: data.success ? undefined : 'Insufficient credits',
      };
    } catch (error) {
      return {
        success: false,
        subscriptionUsed: 0,
        purchasedUsed: 0,
        newSubscriptionBalance: 0,
        newPurchasedBalance: 0,
        error: error.message,
      };
    }
  }

  /**
   * Refund credits to original pools
   */
  static async refundCredits(transactionId: string, reason?: string): Promise<boolean> {
    try {
      const result = await supabase.rpc('refund_credits', {
        p_transaction_id: transactionId,
        p_reason: reason || 'refund',
      });

      return !result.error && result.data;
    } catch (error) {
      console.error('Refund failed:', error);
      return false;
    }
  }

  /**
   * Get credit balance for user
   */
  static async getBalance(userId: string): Promise<{
    subscription: number;
    purchased: number;
    total: number;
  }> {
    const { data, error } = await supabase
      .from('user_credits')
      .select('subscription_credits, purchased_credits')
      .eq('user_id', userId)
      .single();

    if (error || !data) {
      return { subscription: 0, purchased: 0, total: 0 };
    }

    return {
      subscription: data.subscription_credits,
      purchased: data.purchased_credits,
      total: data.subscription_credits + data.purchased_credits,
    };
  }
}

Subscription Credit Service

export interface ISubscriptionTier {
  id: string;
  creditsPerCycle: number;
  rolloverCapPercentage: number; // e.g., 200 = 200% of monthly
}

export class SubscriptionCreditService {
  /**
   * Calculate rollover cap for tier
   */
  static calculateRolloverCap(tier: ISubscriptionTier): number {
    return Math.floor(tier.creditsPerCycle * (tier.rolloverCapPercentage / 100));
  }

  /**
   * Process monthly credit renewal
   */
  static async processMonthlyRenewal(
    userId: string,
    tier: ISubscriptionTier
  ): Promise<{
    added: number;
    rollover: number;
    expired: number;
    total: number;
  }> {
    const rolloverCap = this.calculateRolloverCap(tier);

    // Reset and add new credits with rollover logic
    const result = await supabase.rpc('add_subscription_credits', {
      p_user_id: userId,
      p_amount: tier.creditsPerCycle,
      p_rollover_cap: rolloverCap,
      p_reason: 'monthly_renewal',
    });

    if (result.error) {
      throw new Error(result.error.message);
    }

    const data = result.data[0];

    // Record audit trail
    await this.recordRenewal(userId, tier, data);

    return {
      added: data.credits_added,
      rollover: data.rollover_credits,
      expired: data.expired_credits,
      total: data.new_balance,
    };
  }

  /**
   * Handle upgrade credits (use SubscriptionCreditsService)
   */
  static async handleUpgrade(
    userId: string,
    previousTier: ISubscriptionTier,
    newTier: ISubscriptionTier,
    currentBalance: number
  ): Promise<number> {
    // Use the existing SubscriptionCreditsService for calculations
    const { SubscriptionCreditsService } = await import('./SubscriptionCredits');

    const calculation = SubscriptionCreditsService.calculateUpgradeCredits({
      currentBalance,
      previousTierCredits: previousTier.creditsPerCycle,
      newTierCredits: newTier.creditsPerCycle,
    });

    if (calculation.creditsToAdd > 0) {
      await supabase.rpc('add_subscription_credits', {
        p_user_id: userId,
        p_amount: calculation.creditsToAdd,
        p_reason: 'upgrade_bonus',
      });
    }

    return calculation.creditsToAdd;
  }

  private static async recordRenewal(
    userId: string,
    tier: ISubscriptionTier,
    data: any
  ): Promise<void> {
    // Create audit record
    await supabase.from('credit_renewals').insert({
      user_id: userId,
      tier_id: tier.id,
      credits_added: data.credits_added,
      rollover_credits: data.rollover_credits,
      expired_credits: data.expired_credits,
      new_balance: data.new_balance,
    });
  }
}

Testing Patterns

Unit Tests

describe('CreditTransactionService', () => {
  describe('deductCredits', () => {
    it('should deduct subscription credits first', async () => {
      // Setup: User has 50 subscription, 30 purchased
      // Test: Deduct 60 credits
      // Expect: 50 subscription used, 10 purchased used
    });

    it('should fail with insufficient credits', async () => {
      // Setup: User has 10 credits total
      // Test: Deduct 20 credits
      // Expect: Failure with error message
    });

    it('should handle concurrent requests safely', async () => {
      // Test: Multiple concurrent deductions
      // Expect: No race conditions, atomic operations
    });
  });

  describe('refundCredits', () => {
    it('should refund to original pools', async () => {
      // Setup: Deducted 40 credits (30 subscription, 10 purchased)
      // Test: Refund transaction
      // Expect: 30 subscription restored, 10 purchased restored
    });
  });
});

Integration Tests

describe('Credit System Integration', () => {
  it('should handle full credit lifecycle', async () => {
    // 1. Grant monthly credits
    // 2. Add purchased credits
    // 3. Deduct credits (subscription first)
    // 4. Process monthly renewal with rollover
    // 5. Verify all balances and audit trail
  });
});

Best Practices

Preventing Race Conditions

Always use SELECT ... FOR UPDATE to lock rows
Keep transactions short and focused
Use database functions for atomic operations
Implement proper retry logic

Audit Trail

Record every credit movement
Link refunds to original transactions
Store before/after balances
Include reason codes

Performance

Index user_credits.user_id
Batch credit operations when possible
Use materialized views for reporting
Cache frequently accessed balances

Error Handling

Validate all inputs
Use explicit transactions
Log all failures
Implement circuit breakers for external dependencies

Monitoring

Key Metrics

Credit deduction success rate
Average transaction time
Credit pool distribution
Rollover utilization
Refund rate by reason

Alerts

Failed credit transactions
Unusual credit balance changes
High refund rates
Database lock timeouts

credit-system

Credit System Implementation

Overview

Key Concepts

Credit Pools

Rollover Logic

Atomic Operations

Database Functions

Deduct Credits (Subscription First)

Add Subscription Credits with Rollover

Refund Credits (Original Pool)

Service Layer Patterns

Credit Transaction Service

Subscription Credit Service

Testing Patterns

Unit Tests

Integration Tests

Best Practices

Preventing Race Conditions

Audit Trail

Performance

Error Handling

Monitoring

Key Metrics

Alerts