Coupon and Discount Service Development

Coupon and Discount Service Development

Coupons and discounts aren't just "enter promo code" on checkout. This is rules system managing pricing: who, when, what, how much. Poorly designed system becomes loopholes and analytics chaos. Well designed — precision marketing tool.

Discount Types

Before coding, fix model:

Coupons (Promo Codes) — user enters code or link applies auto. Code unique or reusable, bound to discount rule.

Automatic Discounts — apply without code meeting conditions: "category X 15% off Friday", "500 rubles off 3000+ order".

Loyalty Programs — discount depends on purchase history (cashback, points, tiers).

Group Discounts — B2B bulk rates, employee discounts, partner pricing.

Database Model

discount_rules (
  id, name, type,              -- coupon | automatic | loyalty
  discount_type,               -- percentage | fixed_amount | free_shipping | bxgy
  discount_value NUMERIC,
  min_order_amount NUMERIC,
  min_qty INT,
  max_uses INT,                -- NULL = unlimited
  max_uses_per_user INT,
  starts_at TIMESTAMPTZ,
  ends_at TIMESTAMPTZ,
  is_active BOOLEAN,
  stackable BOOLEAN            -- combine with other discounts
)

discount_conditions (
  id, rule_id,
  condition_type,              -- product | category | tag | user_group | first_order
  condition_operator,          -- in | not_in | gte | lte
  condition_value JSONB
)

coupons (
  id, rule_id, code VARCHAR(32),
  usage_count INT DEFAULT 0,
  is_single_use BOOLEAN
)

coupon_uses (
  id, coupon_id, order_id, user_id, used_at,
  discount_amount NUMERIC      -- recorded at use time
)

Splitting discount_rules and coupons allows one rule multiple codes (bulk email) or one code different limits.

Coupon Generation Batches

For email campaigns need unique codes — one per recipient:

function generateCouponBatch(int $ruleId, int $count): array {
    $codes = [];
    while (count($codes) < $count) {
        $code = strtoupper(Str::random(8)); // A-Z0-9, 8 chars
        if (!Coupon::where('code', $code)->exists()) {
            $codes[] = ['rule_id' => $ruleId, 'code' => $code, 'is_single_use' => true];
        }
    }
    Coupon::insert($codes);
    return array_column($codes, 'code');
}

For large batches (100k+ codes) generate upfront with index uniqueness check, not SELECT EXISTS in loop.

Coupon Validation and Application

On checkout code entry check:

1. Code exists and active
2. Start/end date
3. Usage limit not exceeded (max_uses)
4. User didn't exhaust limit (max_uses_per_user)
5. Cart sum >= min_order_amount
6. Cart items match conditions (discount_conditions)

Check atomically on apply — race condition possible, both requests apply last coupon simultaneously. Solution:

UPDATE coupons
SET usage_count = usage_count + 1
WHERE code = :code
  AND usage_count < max_uses  -- for single-use: usage_count < 1
RETURNING id;
-- if 0 rows — coupon exhausted

UPDATE ... RETURNING inside transaction eliminates race.

Cart Discount Calculation

Server calculates, never trust client. Algorithm:

1. Get applied rules (automatic + coupon)
2. For each rule determine eligible items (apply conditions)
3. Apply in priority order
4. If stackable=false — apply largest discount only
5. Return breakdown: which discount applied to each item

Breakdown important for display ("–500 rubles SAVE500 coupon") and analytics.

BxGy (Buy X Get Y) — "buy 3 get 4th free". Separate rule type: qty >= X add item Y zero price or reduce N-th unit price.

Automatic Discounts and Priorities

Multiple automatic rules can trigger. Need policy:

• First matched — apply highest priority rule
• Best discount — apply rule giving most benefit
• All compatible — apply all with stackable=true

Policy set per-store, may vary by rule type.

Analytics and Reporting

Without analytics marketing flies blind. Basic metrics:

For marketers — dashboard with period, discount type, channel (utm source) filters.

Abuse Prevention

• One coupon per order — standard, but if stacking allowed, explicit config
• Email verification before "new customer" discount — else create 100 accounts
• Rate limiting on coupon apply endpoint — protect from brute force
• Alerts on sharp usage spike — possible leak

Marketer Cabinet

Interface for discount management must allow:

• Create discount rules with visual condition builder
• Generate and export CSV coupon batches
• Real-time stats per campaign
• Deactivate campaign one-click (important for errors)

Timeline

• Basic coupon system (promo code, percent/amount, end date): 1–2 weeks
• Full system (category/product conditions, automatic, BxGy, analytics, marketer cabinet): 3–5 weeks
• Loyalty program with points and tiers adds 3–4 weeks