Below is a clear, minimal-effort, high-impact Technical Roadmap to help you build your own Chargebee-like Billing & Subscription System that can work globally for SaaS products.

This roadmap focuses on MVP essentials, then Phase-2 enhancements, followed by long-term scalable architecture.
Everything is optimized for minimum development effort, maximum usability, and worldwide readiness.

✅ ⚡ MVP Roadmap

MVP = Minimum Viable Product
It means the smallest, simplest version of a product that you can launch quickly

1. Core Components (MUST-HAVE for MVP)

These are the absolute essentials your system needs to start functioning in production.

1️⃣ Customer Management

Minimum Features:

Create & update customers
Store email, name, country, and currency
Customer dashboard (basic)
Attach payment methods (via Stripe/PayPal API)

Why this matters: every SaaS needs customer identity + payment method storage.

2️⃣ Product Catalog + Plans

Minimum Features:

Create Products (e.g., CRM, API Access, etc.)
Create Plans
- Monthly / Yearly
- Price
- Currency
Support one-time charges

Optional for MVP but good:

Addons (extra users, storage, etc.)

3️⃣ Subscription Engine (Core Logic)

This is the brain of your billing system.

Minimum Functionality:

Start subscription
Cancel subscription
Pause / Resume subscription
Change plan (upgrade/downgrade)
Auto-renew billing cycle
Trial periods

Key Logic:

Billing cycle generator
Proration engine (optional for MVP)
Invoice generation engine

4️⃣ Payment Processing Integration

For fastest worldwide readiness → integrate:

Option A (single): Stripe Billing API
Option B (multi): Stripe + PayPal

Minimum Requirements:

Create checkout session
Handle webhooks
- payment_success
- payment_failed
- subscription_created
- invoice_payment_failed
Update your DB based on webhook events

5️⃣ Invoicing System (Essential but Lightweight)

Minimum Features:

Auto-generate invoices (PDF optional, HTML is enough for MVP)
Store invoice in DB
Invoice statuses:
- PAID
- UNPAID
- VOID
- UPCOMING

Optional:

Manual invoice download as PDF

6️⃣ Webhooks & Events System

Needed so that your frontend app knows:

subscription activated
payment succeeded
payment failed
subscription cancelled
invoice created

You need:

Event emitter
Webhook delivery retry system (at least 3 retries)

7️⃣ Basic Dashboard (Admin UI)

For your internal team:

Customer list
Subscription list
Plan management
Payment logs
Event logs

Make it simple, not fancy.

8️⃣ Global SaaS Readiness (Minimum)

Multi-currency pricing (USD, EUR, GBP, INR at minimum)
Tax support (basic): add custom GST/VAT %
Timezone support
Country-based pricing (optional)

⭐ MVP Architecture

Frontend (React / Next.js)
 |
API Gateway
 |
Billing Service  ---- Subscription Engine
 |                 |
 |                 ----- Invoice Generator
 |
Payment Service ---- Stripe/PayPal Integrations
 |
Event Service ---- Webhooks
 |
DB (PostgreSQL)
 |
Message Queue (Kafka/Redis Streams or SQS)

🚀 PHASE-2 FEATURES (After MVP is Live)

1. Advanced Subscription Features

Usage-based billing
Metered billing
Overages
Seat-based billing
Multi-tenancy support
Multiple payment methods

2. Revenue Recovery

Smart dunning (retry logic)
Email notifications:
- Payment failed
- Card expiring
- Invoice reminders

3. Tax & Compliance

Automated GST/VAT via TaxJar
EU VAT validation
Multi-country tax rules

4. Analytics Dashboards

MRR / ARR
Churn rate
Active subscribers
Revenue per plan

5. Integrations

Webhooks (customer apps)
Zapier
Slack
Salesforce
HubSpot

🔥 PHASE-3 (Long Term / Enterprise Level)

1. Multi-Gateway Architecture

Stripe
Razorpay
PayPal
Adyen
Apple/Google In-app billing

2. PCI-DSS Certification (For card storage)

Only required if you store card details yourself.
Recommended approach: don’t store cards — use Stripe tokens.

3. Ledger System

For finance compliance:

Immutable ledger
Double-entry accounting
Audit reports

4. Marketplace / Multiple Vendors

Like Chargebee Enterprise.

🧱 Minimum Tech Stack Recommendation

Component	Recommended
Backend	Java Spring Boot / Node.js (Fastify/NestJS)
Frontend	React / Next.js
DB	PostgreSQL (schemas for multi-tenancy)
Cache	Redis
Queue	Kafka / Redis Streams
Payments	Stripe + PayPal
Deployment	Kubernetes / ECS
Logging	ELK Stack or DataDog

🎯 MVP Feature Summary for Quick Execution

Minimum Required

Customer System
Product & Plans
Basic Subscription Engine
Stripe Payments + Webhooks
Invoice Generator
Dunning (simple retry)
Admin Dashboard
Multi-currency & basic tax
API for integration

Within 6–8 Weeks Timeline (Small Team)

Yes, this is feasible if scoped properly.

Complete Database Schema

-- Billing System Database Schema (PostgreSQL)
-- Purpose: Minimal-complete schema for a Chargebee-like SaaS billing platform.
-- Notes:
--  * Uses UUID primary keys (pgcrypto/gen_random_uuid())
--  * Uses JSONB for extensible metadata where helpful
--  * Designed for multi-currency, multi-tenant SaaS
--  * Keep proration/usage and ledger primitives for future features
-- Enable UUID function
CREATE EXTENSION IF NOT EXISTS pgcrypto;
-- Tenants (optional, for multi-tenant deployments)
CREATE TABLE tenants (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  name TEXT NOT NULL,
  slug TEXT NOT NULL UNIQUE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Customers (the buyer)
CREATE TABLE customers (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  external_id TEXT, -- optional mapping to external CRM/legacy ID
  email CITEXT NOT NULL,
  first_name TEXT,
  last_name TEXT,
  phone TEXT,
  default_currency CHAR(3) DEFAULT 'USD',
  default_payment_method_id UUID, -- FK set later to payment_methods
  billing_address JSONB,
  shipping_address JSONB,
  tax_exempt BOOLEAN DEFAULT FALSE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now(),
  UNIQUE (tenant_id, email)
);
-- Payment methods (tokenized; do NOT store raw PAN)
CREATE TABLE payment_methods (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE CASCADE,
  provider VARCHAR(50) NOT NULL, -- e.g., stripe, paypal
  provider_payment_method_id TEXT NOT NULL, -- token/id from provider
  type VARCHAR(50), -- card, ach, upi etc
  card_brand TEXT,
  card_last4 TEXT,
  card_exp_month INT,
  card_exp_year INT,
  billing_details JSONB,
  is_default BOOLEAN DEFAULT FALSE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Products (e.g., "CRM Suite")
CREATE TABLE products (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  name TEXT NOT NULL,
  description TEXT,
  metadata JSONB DEFAULT '{}'::jsonb,
  active BOOLEAN DEFAULT TRUE,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Plans (pricing tiers under products)
CREATE TABLE plans (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  product_id UUID REFERENCES products(id) ON DELETE CASCADE,
  plan_key TEXT NOT NULL, -- internal key (e.g., "starter_monthly")
  name TEXT NOT NULL,
  billing_period INTERVAL NOT NULL DEFAULT '1 month', -- 1 month, 1 year
  interval_count INT DEFAULT 1,
  trial_period_days INT DEFAULT 0,
  billing_cycle_anchor TEXT DEFAULT 'start_of_period',
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  amount_cents BIGINT NOT NULL, -- amount in smallest currency unit
  setup_fee_cents BIGINT DEFAULT 0,
  billing_behavior TEXT DEFAULT 'charge_automatically', -- other: send_invoice
  active BOOLEAN DEFAULT TRUE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now(),
  UNIQUE (tenant_id, plan_key)
);
-- Addons (optional extra charges attached to subscriptions)
CREATE TABLE addons (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  name TEXT NOT NULL,
  description TEXT,
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  billing_period INTERVAL,
  metadata JSONB DEFAULT '{}'::jsonb,
  active BOOLEAN DEFAULT TRUE,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Coupons / Discounts
CREATE TABLE coupons (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  code TEXT NOT NULL,
  description TEXT,
  discount_percent NUMERIC CHECK (discount_percent >= 0 AND discount_percent <= 100),
  discount_cents BIGINT, -- fixed amount off (in smallest currency unit)
  currency CHAR(3),
  duration TEXT DEFAULT 'once', -- once, forever, repeating
  duration_in_months INT,
  max_redemptions INT,
  expires_at TIMESTAMPTZ,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now(),
  UNIQUE (tenant_id, code)
);
CREATE TABLE coupon_redemptions (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  coupon_id UUID REFERENCES coupons(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE CASCADE,
  subscription_id UUID REFERENCES subscriptions(id) ON DELETE SET NULL,
  redeemed_at TIMESTAMPTZ DEFAULT now(),
  metadata JSONB DEFAULT '{}'::jsonb
);
-- Subscriptions
CREATE TABLE subscriptions (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE CASCADE,
  plan_id UUID REFERENCES plans(id) ON DELETE SET NULL,
  status TEXT NOT NULL DEFAULT 'active', -- active, past_due, canceled, trialing, unpaid
  quantity INT DEFAULT 1,
  current_period_start TIMESTAMPTZ,
  current_period_end TIMESTAMPTZ,
  trial_start TIMESTAMPTZ,
  trial_end TIMESTAMPTZ,
  cancel_at_period_end BOOLEAN DEFAULT FALSE,
  canceled_at TIMESTAMPTZ,
  start_date TIMESTAMPTZ DEFAULT now(),
  ended_at TIMESTAMPTZ,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Subscription items (for multiple prices or addons attached to a subscription)
CREATE TABLE subscription_items (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  subscription_id UUID REFERENCES subscriptions(id) ON DELETE CASCADE,
  plan_id UUID REFERENCES plans(id) ON DELETE SET NULL,
  addon_id UUID REFERENCES addons(id) ON DELETE SET NULL,
  quantity INT DEFAULT 1,
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  billing_period INTERVAL,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Usage records (for metered billing)
CREATE TABLE usage_records (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  subscription_item_id UUID REFERENCES subscription_items(id) ON DELETE CASCADE,
  usage_date DATE NOT NULL,
  quantity NUMERIC NOT NULL,
  description TEXT,
  processed BOOLEAN DEFAULT FALSE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now()
);
-- Invoices
CREATE TABLE invoices (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE CASCADE,
  subscription_id UUID REFERENCES subscriptions(id) ON DELETE SET NULL,
  invoice_number TEXT NOT NULL,
  status TEXT NOT NULL DEFAULT 'draft', -- draft, open, paid, void, uncollectible
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  subtotal_cents BIGINT DEFAULT 0,
  tax_cents BIGINT DEFAULT 0,
  total_cents BIGINT DEFAULT 0,
  amount_due_cents BIGINT DEFAULT 0,
  amount_paid_cents BIGINT DEFAULT 0,
  due_date TIMESTAMPTZ,
  closed_at TIMESTAMPTZ,
  pdf_url TEXT,
  lines JSONB DEFAULT '[]'::jsonb, -- optional denormalized line items
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now(),
  UNIQUE (tenant_id, invoice_number)
);
-- Invoice line items (normalized)
CREATE TABLE invoice_items (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  invoice_id UUID REFERENCES invoices(id) ON DELETE CASCADE,
  subscription_id UUID REFERENCES subscriptions(id) ON DELETE SET NULL,
  subscription_item_id UUID REFERENCES subscription_items(id) ON DELETE SET NULL,
  description TEXT,
  quantity NUMERIC DEFAULT 1,
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  period_start TIMESTAMPTZ,
  period_end TIMESTAMPTZ,
  tax_cents BIGINT DEFAULT 0,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Payments (records of payments attempted/received)
CREATE TABLE payments (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE SET NULL,
  invoice_id UUID REFERENCES invoices(id) ON DELETE SET NULL,
  payment_method_id UUID REFERENCES payment_methods(id) ON DELETE SET NULL,
  provider VARCHAR(50) NOT NULL, -- stripe, paypal
  provider_payment_id TEXT,
  status TEXT NOT NULL DEFAULT 'processing', -- processing, succeeded, failed, refunded
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  captured BOOLEAN DEFAULT TRUE,
  failure_code TEXT,
  failure_message TEXT,
  refunded_amount_cents BIGINT DEFAULT 0,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Refunds
CREATE TABLE refunds (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  payment_id UUID REFERENCES payments(id) ON DELETE SET NULL,
  provider_refund_id TEXT,
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  reason TEXT,
  status TEXT DEFAULT 'pending', -- pending, succeeded, failed
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Tax rates (basic support)
CREATE TABLE tax_rates (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  name TEXT,
  country CHAR(2),
  region TEXT,
  rate_percent NUMERIC NOT NULL,
  inclusive BOOLEAN DEFAULT FALSE,
  active BOOLEAN DEFAULT TRUE,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);
-- Dunning / Retry attempts
CREATE TABLE payment_attempts (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  invoice_id UUID REFERENCES invoices(id) ON DELETE SET NULL,
  payment_method_id UUID REFERENCES payment_methods(id) ON DELETE SET NULL,
  attempt_number INT DEFAULT 1,
  status TEXT DEFAULT 'failed', -- succeeded, failed
  failure_code TEXT,
  failure_message TEXT,
  next_retry_at TIMESTAMPTZ,
  processed_at TIMESTAMPTZ,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now()
);
-- Events / Webhooks received from payment providers
CREATE TABLE provider_events (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  provider VARCHAR(50) NOT NULL,
  provider_event_id TEXT NOT NULL,
  payload JSONB,
  processed BOOLEAN DEFAULT FALSE,
  received_at TIMESTAMPTZ DEFAULT now(),
  processed_at TIMESTAMPTZ
);
-- Outgoing webhooks to customer systems
CREATE TABLE outgoing_webhooks (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  target_url TEXT NOT NULL,
  event_type TEXT NOT NULL,
  payload JSONB NOT NULL,
  attempts INT DEFAULT 0,
  last_attempt_at TIMESTAMPTZ,
  success BOOLEAN DEFAULT FALSE,
  created_at TIMESTAMPTZ DEFAULT now()
);
-- Ledger (basic immutable ledger for accounting)
CREATE TABLE ledger_entries (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  entry_date DATE NOT NULL DEFAULT CURRENT_DATE,
  reference_type TEXT, -- invoice, payment, refund
  reference_id UUID,
  account TEXT NOT NULL, -- e.g., revenue, receivables
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  debit_or_credit TEXT NOT NULL CHECK (debit_or_credit IN ('debit','credit')),
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now()
);
-- Audit logs (who did what)
CREATE TABLE audit_logs (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  actor_id UUID, -- user/admin id
  actor_type TEXT, -- admin, system
  action TEXT NOT NULL,
  subject_type TEXT,
  subject_id UUID,
  data JSONB,
  created_at TIMESTAMPTZ DEFAULT now()
);
-- Indexes for performance (examples)
CREATE INDEX idx_customers_tenant_email ON customers(tenant_id, email);
CREATE INDEX idx_subscriptions_customer ON subscriptions(customer_id);
CREATE INDEX idx_invoices_customer_status ON invoices(customer_id, status);
CREATE INDEX idx_payments_invoice_status ON payments(invoice_id, status);
CREATE INDEX idx_usage_records_processed ON usage_records(processed);
-- Triggers to keep updated_at current (simple examples)
CREATE OR REPLACE FUNCTION touch_updated_at()
RETURNS TRIGGER AS $$
BEGIN
  NEW.updated_at = now();
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;
-- Add triggers to a few tables
CREATE TRIGGER touch_customers_updated
  BEFORE UPDATE ON customers
  FOR EACH ROW EXECUTE PROCEDURE touch_updated_at();
CREATE TRIGGER touch_subscriptions_updated
  BEFORE UPDATE ON subscriptions
  FOR EACH ROW EXECUTE PROCEDURE touch_updated_at();
CREATE TRIGGER touch_invoices_updated
  BEFORE UPDATE ON invoices
  FOR EACH ROW EXECUTE PROCEDURE touch_updated_at();
-- Sample sequences / helpers
-- A simple helper to generate readable invoice numbers (tenant-scoped)
CREATE TABLE invoice_counters (
  tenant_id UUID PRIMARY KEY REFERENCES tenants(id) ON DELETE CASCADE,
  last_serial BIGINT DEFAULT 0
);
CREATE OR REPLACE FUNCTION next_invoice_number(p_tenant_id UUID)
RETURNS TEXT LANGUAGE plpgsql AS $$
DECLARE
  s BIGINT;
BEGIN
  LOOP
    UPDATE invoice_counters SET last_serial = last_serial + 1 WHERE tenant_id = p_tenant_id RETURNING last_serial INTO s;
    IF FOUND THEN
      RETURN to_char(s, 'FM000000');
    END IF;
    BEGIN
      INSERT INTO invoice_counters (tenant_id, last_serial) VALUES (p_tenant_id, 0);
    EXCEPTION WHEN unique_violation THEN
      -- concurrent insert, loop and try again
    END;
  END LOOP;
END;
$$;
-- Example: create a function to reconcile invoice totals (simplified)
CREATE OR REPLACE FUNCTION recompute_invoice_totals(p_invoice_id UUID)
RETURNS VOID LANGUAGE plpgsql AS $$
DECLARE
  s_subtotal BIGINT := 0;
  s_tax BIGINT := 0;
  s_total BIGINT := 0;
  s_paid BIGINT := 0;
BEGIN
  SELECT COALESCE(SUM(amount_cents),0) INTO s_subtotal FROM invoice_items WHERE invoice_id = p_invoice_id;
  SELECT COALESCE(SUM(tax_cents),0) INTO s_tax FROM invoice_items WHERE invoice_id = p_invoice_id;
  s_total := s_subtotal + s_tax;
  SELECT COALESCE(SUM(amount_cents),0) INTO s_paid FROM payments WHERE invoice_id = p_invoice_id AND status = 'succeeded';
  UPDATE invoices SET subtotal_cents = s_subtotal, tax_cents = s_tax, total_cents = s_total, amount_paid_cents = s_paid, amount_due_cents = GREATEST(s_total - s_paid, 0), updated_at = now() WHERE id = p_invoice_id;
END;
$$;
-- End of schema
-- You can extend this schema with: tenant-specific settings, feature flags, advanced tax tables, currency conversion rates, localization tables, and compliance fields (GDPR, consent timestamps).
\

-- Billing System Database Schema (PostgreSQL)
-- Purpose: Minimal-complete schema for a Chargebee-like SaaS billing platform.
-- Notes:
--  * Uses UUID primary keys (pgcrypto/gen_random_uuid())
--  * Uses JSONB for extensible metadata where helpful
--  * Designed for multi-currency, multi-tenant SaaS
--  * Keep proration/usage and ledger primitives for future features

-- Enable UUID function
CREATE EXTENSION IF NOT EXISTS pgcrypto;

-- Tenants (optional, for multi-tenant deployments)
CREATE TABLE tenants (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  name TEXT NOT NULL,
  slug TEXT NOT NULL UNIQUE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Customers (the buyer)
CREATE TABLE customers (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  external_id TEXT, -- optional mapping to external CRM/legacy ID
  email CITEXT NOT NULL,
  first_name TEXT,
  last_name TEXT,
  phone TEXT,
  default_currency CHAR(3) DEFAULT 'USD',
  default_payment_method_id UUID, -- FK set later to payment_methods
  billing_address JSONB,
  shipping_address JSONB,
  tax_exempt BOOLEAN DEFAULT FALSE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now(),
  UNIQUE (tenant_id, email)
);

-- Payment methods (tokenized; do NOT store raw PAN)
CREATE TABLE payment_methods (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE CASCADE,
  provider VARCHAR(50) NOT NULL, -- e.g., stripe, paypal
  provider_payment_method_id TEXT NOT NULL, -- token/id from provider
  type VARCHAR(50), -- card, ach, upi etc
  card_brand TEXT,
  card_last4 TEXT,
  card_exp_month INT,
  card_exp_year INT,
  billing_details JSONB,
  is_default BOOLEAN DEFAULT FALSE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Products (e.g., "CRM Suite")
CREATE TABLE products (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  name TEXT NOT NULL,
  description TEXT,
  metadata JSONB DEFAULT '{}'::jsonb,
  active BOOLEAN DEFAULT TRUE,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Plans (pricing tiers under products)
CREATE TABLE plans (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  product_id UUID REFERENCES products(id) ON DELETE CASCADE,
  plan_key TEXT NOT NULL, -- internal key (e.g., "starter_monthly")
  name TEXT NOT NULL,
  billing_period INTERVAL NOT NULL DEFAULT '1 month', -- 1 month, 1 year
  interval_count INT DEFAULT 1,
  trial_period_days INT DEFAULT 0,
  billing_cycle_anchor TEXT DEFAULT 'start_of_period',
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  amount_cents BIGINT NOT NULL, -- amount in smallest currency unit
  setup_fee_cents BIGINT DEFAULT 0,
  billing_behavior TEXT DEFAULT 'charge_automatically', -- other: send_invoice
  active BOOLEAN DEFAULT TRUE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now(),
  UNIQUE (tenant_id, plan_key)
);

-- Addons (optional extra charges attached to subscriptions)
CREATE TABLE addons (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  name TEXT NOT NULL,
  description TEXT,
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  billing_period INTERVAL,
  metadata JSONB DEFAULT '{}'::jsonb,
  active BOOLEAN DEFAULT TRUE,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Coupons / Discounts
CREATE TABLE coupons (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  code TEXT NOT NULL,
  description TEXT,
  discount_percent NUMERIC CHECK (discount_percent >= 0 AND discount_percent <= 100),
  discount_cents BIGINT, -- fixed amount off (in smallest currency unit)
  currency CHAR(3),
  duration TEXT DEFAULT 'once', -- once, forever, repeating
  duration_in_months INT,
  max_redemptions INT,
  expires_at TIMESTAMPTZ,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now(),
  UNIQUE (tenant_id, code)
);

CREATE TABLE coupon_redemptions (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  coupon_id UUID REFERENCES coupons(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE CASCADE,
  subscription_id UUID REFERENCES subscriptions(id) ON DELETE SET NULL,
  redeemed_at TIMESTAMPTZ DEFAULT now(),
  metadata JSONB DEFAULT '{}'::jsonb
);

-- Subscriptions
CREATE TABLE subscriptions (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE CASCADE,
  plan_id UUID REFERENCES plans(id) ON DELETE SET NULL,
  status TEXT NOT NULL DEFAULT 'active', -- active, past_due, canceled, trialing, unpaid
  quantity INT DEFAULT 1,
  current_period_start TIMESTAMPTZ,
  current_period_end TIMESTAMPTZ,
  trial_start TIMESTAMPTZ,
  trial_end TIMESTAMPTZ,
  cancel_at_period_end BOOLEAN DEFAULT FALSE,
  canceled_at TIMESTAMPTZ,
  start_date TIMESTAMPTZ DEFAULT now(),
  ended_at TIMESTAMPTZ,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Subscription items (for multiple prices or addons attached to a subscription)
CREATE TABLE subscription_items (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  subscription_id UUID REFERENCES subscriptions(id) ON DELETE CASCADE,
  plan_id UUID REFERENCES plans(id) ON DELETE SET NULL,
  addon_id UUID REFERENCES addons(id) ON DELETE SET NULL,
  quantity INT DEFAULT 1,
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  billing_period INTERVAL,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Usage records (for metered billing)
CREATE TABLE usage_records (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  subscription_item_id UUID REFERENCES subscription_items(id) ON DELETE CASCADE,
  usage_date DATE NOT NULL,
  quantity NUMERIC NOT NULL,
  description TEXT,
  processed BOOLEAN DEFAULT FALSE,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now()
);

-- Invoices
CREATE TABLE invoices (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE CASCADE,
  subscription_id UUID REFERENCES subscriptions(id) ON DELETE SET NULL,
  invoice_number TEXT NOT NULL,
  status TEXT NOT NULL DEFAULT 'draft', -- draft, open, paid, void, uncollectible
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  subtotal_cents BIGINT DEFAULT 0,
  tax_cents BIGINT DEFAULT 0,
  total_cents BIGINT DEFAULT 0,
  amount_due_cents BIGINT DEFAULT 0,
  amount_paid_cents BIGINT DEFAULT 0,
  due_date TIMESTAMPTZ,
  closed_at TIMESTAMPTZ,
  pdf_url TEXT,
  lines JSONB DEFAULT '[]'::jsonb, -- optional denormalized line items
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now(),
  UNIQUE (tenant_id, invoice_number)
);

-- Invoice line items (normalized)
CREATE TABLE invoice_items (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  invoice_id UUID REFERENCES invoices(id) ON DELETE CASCADE,
  subscription_id UUID REFERENCES subscriptions(id) ON DELETE SET NULL,
  subscription_item_id UUID REFERENCES subscription_items(id) ON DELETE SET NULL,
  description TEXT,
  quantity NUMERIC DEFAULT 1,
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  period_start TIMESTAMPTZ,
  period_end TIMESTAMPTZ,
  tax_cents BIGINT DEFAULT 0,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Payments (records of payments attempted/received)
CREATE TABLE payments (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  customer_id UUID REFERENCES customers(id) ON DELETE SET NULL,
  invoice_id UUID REFERENCES invoices(id) ON DELETE SET NULL,
  payment_method_id UUID REFERENCES payment_methods(id) ON DELETE SET NULL,
  provider VARCHAR(50) NOT NULL, -- stripe, paypal
  provider_payment_id TEXT,
  status TEXT NOT NULL DEFAULT 'processing', -- processing, succeeded, failed, refunded
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  captured BOOLEAN DEFAULT TRUE,
  failure_code TEXT,
  failure_message TEXT,
  refunded_amount_cents BIGINT DEFAULT 0,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Refunds
CREATE TABLE refunds (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  payment_id UUID REFERENCES payments(id) ON DELETE SET NULL,
  provider_refund_id TEXT,
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  reason TEXT,
  status TEXT DEFAULT 'pending', -- pending, succeeded, failed
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Tax rates (basic support)
CREATE TABLE tax_rates (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  name TEXT,
  country CHAR(2),
  region TEXT,
  rate_percent NUMERIC NOT NULL,
  inclusive BOOLEAN DEFAULT FALSE,
  active BOOLEAN DEFAULT TRUE,
  created_at TIMESTAMPTZ DEFAULT now(),
  updated_at TIMESTAMPTZ DEFAULT now()
);

-- Dunning / Retry attempts
CREATE TABLE payment_attempts (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  invoice_id UUID REFERENCES invoices(id) ON DELETE SET NULL,
  payment_method_id UUID REFERENCES payment_methods(id) ON DELETE SET NULL,
  attempt_number INT DEFAULT 1,
  status TEXT DEFAULT 'failed', -- succeeded, failed
  failure_code TEXT,
  failure_message TEXT,
  next_retry_at TIMESTAMPTZ,
  processed_at TIMESTAMPTZ,
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now()
);

-- Events / Webhooks received from payment providers
CREATE TABLE provider_events (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  provider VARCHAR(50) NOT NULL,
  provider_event_id TEXT NOT NULL,
  payload JSONB,
  processed BOOLEAN DEFAULT FALSE,
  received_at TIMESTAMPTZ DEFAULT now(),
  processed_at TIMESTAMPTZ
);

-- Outgoing webhooks to customer systems
CREATE TABLE outgoing_webhooks (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  target_url TEXT NOT NULL,
  event_type TEXT NOT NULL,
  payload JSONB NOT NULL,
  attempts INT DEFAULT 0,
  last_attempt_at TIMESTAMPTZ,
  success BOOLEAN DEFAULT FALSE,
  created_at TIMESTAMPTZ DEFAULT now()
);

-- Ledger (basic immutable ledger for accounting)
CREATE TABLE ledger_entries (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  entry_date DATE NOT NULL DEFAULT CURRENT_DATE,
  reference_type TEXT, -- invoice, payment, refund
  reference_id UUID,
  account TEXT NOT NULL, -- e.g., revenue, receivables
  amount_cents BIGINT NOT NULL,
  currency CHAR(3) NOT NULL DEFAULT 'USD',
  debit_or_credit TEXT NOT NULL CHECK (debit_or_credit IN ('debit','credit')),
  metadata JSONB DEFAULT '{}'::jsonb,
  created_at TIMESTAMPTZ DEFAULT now()
);

-- Audit logs (who did what)
CREATE TABLE audit_logs (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  tenant_id UUID REFERENCES tenants(id) ON DELETE CASCADE,
  actor_id UUID, -- user/admin id
  actor_type TEXT, -- admin, system
  action TEXT NOT NULL,
  subject_type TEXT,
  subject_id UUID,
  data JSONB,
  created_at TIMESTAMPTZ DEFAULT now()
);

-- Indexes for performance (examples)
CREATE INDEX idx_customers_tenant_email ON customers(tenant_id, email);
CREATE INDEX idx_subscriptions_customer ON subscriptions(customer_id);
CREATE INDEX idx_invoices_customer_status ON invoices(customer_id, status);
CREATE INDEX idx_payments_invoice_status ON payments(invoice_id, status);
CREATE INDEX idx_usage_records_processed ON usage_records(processed);

-- Triggers to keep updated_at current (simple examples)
CREATE OR REPLACE FUNCTION touch_updated_at()
RETURNS TRIGGER AS $$
BEGIN
  NEW.updated_at = now();
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

-- Add triggers to a few tables
CREATE TRIGGER touch_customers_updated
  BEFORE UPDATE ON customers
  FOR EACH ROW EXECUTE PROCEDURE touch_updated_at();

CREATE TRIGGER touch_subscriptions_updated
  BEFORE UPDATE ON subscriptions
  FOR EACH ROW EXECUTE PROCEDURE touch_updated_at();

CREATE TRIGGER touch_invoices_updated
  BEFORE UPDATE ON invoices
  FOR EACH ROW EXECUTE PROCEDURE touch_updated_at();

-- Sample sequences / helpers
-- A simple helper to generate readable invoice numbers (tenant-scoped)
CREATE TABLE invoice_counters (
  tenant_id UUID PRIMARY KEY REFERENCES tenants(id) ON DELETE CASCADE,
  last_serial BIGINT DEFAULT 0
);

CREATE OR REPLACE FUNCTION next_invoice_number(p_tenant_id UUID)
RETURNS TEXT LANGUAGE plpgsql AS $$
DECLARE
  s BIGINT;
BEGIN
  LOOP
    UPDATE invoice_counters SET last_serial = last_serial + 1 WHERE tenant_id = p_tenant_id RETURNING last_serial INTO s;
    IF FOUND THEN
      RETURN to_char(s, 'FM000000');
    END IF;
    BEGIN
      INSERT INTO invoice_counters (tenant_id, last_serial) VALUES (p_tenant_id, 0);
    EXCEPTION WHEN unique_violation THEN
      -- concurrent insert, loop and try again
    END;
  END LOOP;
END;
$$;

-- Example: create a function to reconcile invoice totals (simplified)
CREATE OR REPLACE FUNCTION recompute_invoice_totals(p_invoice_id UUID)
RETURNS VOID LANGUAGE plpgsql AS $$
DECLARE
  s_subtotal BIGINT := 0;
  s_tax BIGINT := 0;
  s_total BIGINT := 0;
  s_paid BIGINT := 0;
BEGIN
  SELECT COALESCE(SUM(amount_cents),0) INTO s_subtotal FROM invoice_items WHERE invoice_id = p_invoice_id;
  SELECT COALESCE(SUM(tax_cents),0) INTO s_tax FROM invoice_items WHERE invoice_id = p_invoice_id;
  s_total := s_subtotal + s_tax;
  SELECT COALESCE(SUM(amount_cents),0) INTO s_paid FROM payments WHERE invoice_id = p_invoice_id AND status = 'succeeded';
  UPDATE invoices SET subtotal_cents = s_subtotal, tax_cents = s_tax, total_cents = s_total, amount_paid_cents = s_paid, amount_due_cents = GREATEST(s_total - s_paid, 0), updated_at = now() WHERE id = p_invoice_id;
END;
$$;

-- End of schema
-- You can extend this schema with: tenant-specific settings, feature flags, advanced tax tables, currency conversion rates, localization tables, and compliance fields (GDPR, consent timestamps).
\

Complete System Design Diagram

Security & Compliance

PCI: do not store card PAN/CVV. Use Stripe Elements or tokenization. If you must store cards (not recommended) — follow PCI-DSS and apply for SAQ/DSS.
TLS everywhere (mTLS for internal services optional).
Role-based access control (RBAC) for admin console.
Secrets: use HashiCorp Vault / cloud KMS.
Data encryption at rest for DB and object storage.
Audit logs for all financial operations (immutable where possible).
GDPR: customer deletion flows — anonymize data with retention windows, keep ledger entries as required by law (mark PII redacted).

Observability & SLOs

Instrumentation:

Distributed tracing: OpenTelemetry -> Jaeger/Tempo.
Metrics: Prometheus, Grafana dashboards (MRR, failed payments rate, invoice generation time, queue lag).
Logs: structured logs to ELK / Datadog / Splunk.
Alerting: high failure-rate for payments, pod crash loops, DB replication lag, queue consumer lag.

SLO examples:

Payment success flow end-to-end: 99.9% within 10s (excluding external provider delays).
Webhook processing: 99.5% within 30s.
Invoice generation latency: 99% under 2 minutes.

CI/CD & Release strategy

Git per service (mono-repo optional with clear build pipelines).
Build → unit tests → integration tests (with test containers for DB) → deploy to staging.
Canary/Grey deployments with feature flags (LaunchDarkly).
Blue/Green or Rolling for critical services (Payment Service).
Migration strategy: use versioned DB migrations (Flyway / Liquibase).

Multi-region & Global considerations

Localize currency and date/time formatting at UI and invoices.
For GDPR/EU and local tax: host tenant data in region if requested (data residency) — design by making tenant->region mapping.
Consider read-replicas in target regions; write region ideally single region per tenant.
Use CDN for static assets and invoice PDFs.

Developer UX — SDKs & API design

Provide REST + OpenAPI spec and client SDKs (Node, Java, Python) to increase adoption.
Design APIs to be intuitive:
- POST /v1/subscriptions with idempotency-key
- GET /v1/customers/{id}
- POST /v1/invoices/{id}/pay
Provide webhooks docs and test sandbox.

Operational Runbook (short)

Payment webhook fails: look up provider_event, reconcile payment state, replay events from provider UI.
Message backlog grows: check consumer lag, scale consumers, inspect slow consumer logs.
Invoice numbers gap: investigate invoice_counters; use atomic serial function (already in schema).
Refunds stuck: check provider refund status, reconcile with ledger entries.

Suggested Tech Stack (practical)

Backend: Java (Spring Boot) or Node.js (NestJS) or Go — choose based on team skill.
API Gateway: Kong / Nginx / AWS ALB + OIDC provider.
Messaging: Kafka (high throughput) or Redis Streams (simpler).
DB: PostgreSQL (per service), Timescale for metrics if needed.
Cache: Redis.
Storage: Amazon S3 / MinIO.
Infra: Kubernetes (EKS/GKE/AKS) or ECS/Fargate.
Observability: Prometheus + Grafana + Jaeger + ELK.
CI/CD: GitHub Actions / GitLab CI / Jenkins.
Payment Providers: Stripe (primary), PayPal / Razorpay / Adyen (regional).

Phased implementation plan (suggested)

Phase 4: Enterprise features (multi-tenant isolation, marketplace).

Phase 0: Infra, CI/CD, API Gateway, Auth Service.

Phase 1 (MVP): Catalog, Customer, Subscription, Payment connector (Stripe), Invoice, minimal Admin UI, Event Bus, Outbox.

Phase 2: Dunning, Usage Metering, Webhook Dispatcher, Ledger, Refunds.

Phase 3: Multi-gateway, tax automation, analytics, multi-region, PCI roadmap if needed.

Below are the major real-world challenges you will face while building and operating a Billing & Subscription Management System (Chargebee-like).
This list includes engineering, business, finance, and compliance challenges — based on how leading billing platforms operate.

🚨 Top Challenges in Building a Billing System

1. Complexity of Subscription Lifecycle

Subscriptions are not just “create and renew.” You must handle:

Plan changes (upgrade, downgrade)
Proration (mid-cycle billing)
Trials, extensions, free credits
Scheduled changes (future plan switches)
Pause / resume
Auto-cancel rules
Multi-quantity subscriptions
Metered/usage-based billing

These flows multiply quickly and become challenging to keep consistent and bug-free.

2. Multi-Currency & Global Billing Rules

Billing systems must support worldwide SaaS:

Different currencies
Localized invoice formats
Currency conversion at correct FX rate
Rounding rules (varies by country)
Region-specific tax laws (GST, VAT, EU VAT, GST-AU)

Each of these requires domain expertise and continuous updates.

3. Tax Compliance (VAT, GST, Sales Tax)

Tax rules change frequently. Challenges:

Different tax rates by country/state
Reverse charge rules
EU OSS rules
B2B vs B2C tax differences
Digital service tax in 40+ countries
Exemptions and VAT/GST ID validation
Accurate invoice representation

This is extremely hard to maintain without integrating providers like TaxJar / Avalara.

4. Payment Gateway Variations & Failures

Every payment gateway behaves differently:

Different APIs (Stripe, Razorpay, PayPal, Adyen)
Different webhook formats
Different payment states
Random downtime
Card declines for many reasons
Currency support differences

Ensuring idempotent, fault-tolerant payment processing is a major challenge.

5. Dunning & Payment Recovery Logic

Recovering failed payments is critical for SaaS revenue. Challenges:

Smart retry strategy
Handling soft/hard declines
Timezone-aware retry windows
Notifications and escalations
Grace periods
Suspensions vs cancellations
Plan downgrade after failure

Bad dunning directly = revenue loss.

6. Handling Usage & Metered Billing at Scale

Usage billing seems simple, but challenges include:

Billions of usage records
Duplicate submissions
Idempotency
Real-time aggregation
Backfill & correction
Delayed reporting
Large tenants with high throughput

Netflix, AWS, Twilio use extremely complex metering pipelines.

7. Invoice Generation & Numbering

Invoices must be:

Legally compliant
Sequentially numbered
Immutable after finalization
Localized (PDF, language, currency)
Accurate tax amounts
Correct line-item breakdown

A single mistake breaks compliance in many countries.

8. Ledger & Financial Accuracy

A billing system must maintain audit-grade accounting integrity:

Rigid double-entry ledger
No negative invoices
Refund adjustments
Invoice-credit-note accounting
FX gain/loss accounting
Immutable audit logs

This is the hardest part of enterprise-grade billing systems.

9. Multi-Tenancy Challenges

If your system supports multiple SaaS companies:

Tenant isolation in DB
Rate limits per tenant
Custom features per tenant
Custom taxes/currencies
Security boundaries
Global outages affecting all tenants

Most failures come from poor multi-tenancy design.

10. Event Consistency & State Conflicts

Your billing platform depends on async events:

subscription.created
invoice.generated
payment.succeeded
payment.failed

Challenges:

Duplicate events
Out-of-order events
Lost webhooks
Race conditions
Event replay issues

Requires patterns like transactional outbox, idempotency, message versioning.

11. Compliance & Security Requirements

Billing touches financial data; must meet standards:

PCI-DSS (if storing cards)
SOC2
GDPR (data privacy & deletion)
Data residency (EU, AU, US)
Secure token storage
Audit trails
PII encryption

You must prove that your system is compliant to enterprise buyers.

12. Pricing Experiments & Flexibility

SaaS companies frequently change pricing:

Challenges include:

Plan versioning
Grandfathering old customers
Custom contracts for enterprise clients
Usage-based pricing variations
Introductory discounts
Coupons, promo codes
Volume-based discounts

Your system must allow these without breaking old subscriptions.

13. Recovering from Webhook Failures

Payment gateways send critical webhooks:

Payment success
Payment failure
Refund processed
Chargeback initiated

Challenges:

Retries
Idempotency
Out-of-order processing
Webhook signature verification
Webhook replay logic

A single missed webhook → incorrect invoice → angry customer.

14. Scaling Events & Batch Jobs

Billing cycles (monthly/weekly/daily) create load spikes:

Millions of invoices generated at midnight UTC
Batch payment runs
Dunning queues
Usage aggregation windows

Requires distributed schedulers and horizontal scaling.

15. Customer Expectations

Today’s customers demand:

Instant provisioning
Real-time dashboards
Reliable invoices
Smooth failed payment recovery
Clear transactional emails
Integrations with Zapier, Slack, Quickbooks, CRM systems

Meeting all expectations is hard.

16. Global Availability & Latency

If your SaaS is worldwide:

Users in US, EU, India, Australia need low-latency API access
Payment providers region-specific
Data residency rules
Multi-region failovers

Setting up multi-region architecture is expensive & complex.

17. Maintaining Backward Compatibility

Billing events and APIs cannot be changed easily — customers rely on them.

Challenges:

Versioning REST APIs
Versioning events
Backward compatibility for integrations
Deprecation timelines

Billing is a core part of revenue — break nothing.

18. Integrations

Billing system must integrate with many external systems:

CRM (HubSpot, Salesforce)
Accounting (Quickbooks, Xero)
Payment gateways
Tax systems
Analytics platforms
Custom internal ERPs

Every integration brings complexity, rate limits, and event-mapping issues.

19. Real-Time Reporting

Customers want dashboards showing:

MRR / ARR
Churn
Expansion revenue
Cohorts
Usage consumption

Challenge:

Large computations
Real-time updates
Multi-tenant isolation
Aggregation from multiple microservices

20. Cost Control & Performance

Billing systems process heavy workloads.
Challenges include:

Kafka cluster cost
High DB IOPS
PDF generation cost
Caching infrastructure
Multi-region traffic costs
Large invoice history storage

Optimize without compromising reliability.

🔥 Final Summary — The Hardest Parts

The top 3 hardest challenges in building a billing system are:

1️⃣ Maintaining accurate financial & tax compliance
2️⃣ Ensuring payment reliability & dunning
3️⃣ Handling complex subscription lifecycle + proration + usage

If you solve these properly, your billing system becomes world-class.

Table of Contents

✅ ⚡ MVP Roadmap

1. Core Components (MUST-HAVE for MVP)

1️⃣ Customer Management

2️⃣ Product Catalog + Plans

3️⃣ Subscription Engine (Core Logic)

4️⃣ Payment Processing Integration

5️⃣ Invoicing System (Essential but Lightweight)

6️⃣ Webhooks & Events System

7️⃣ Basic Dashboard (Admin UI)

8️⃣ Global SaaS Readiness (Minimum)

⭐ MVP Architecture

🚀 PHASE-2 FEATURES (After MVP is Live)

1. Advanced Subscription Features

2. Revenue Recovery

3. Tax & Compliance

4. Analytics Dashboards

5. Integrations

🔥 PHASE-3 (Long Term / Enterprise Level)

1. Multi-Gateway Architecture

2. PCI-DSS Certification (For card storage)

3. Ledger System

4. Marketplace / Multiple Vendors

🧱 Minimum Tech Stack Recommendation

🎯 MVP Feature Summary for Quick Execution

Minimum Required

Within 6–8 Weeks Timeline (Small Team)

Complete Database Schema

Complete System Design Diagram

Security & Compliance

Observability & SLOs

CI/CD & Release strategy

Multi-region & Global considerations

Developer UX — SDKs & API design

Operational Runbook (short)

Suggested Tech Stack (practical)

Phased implementation plan (suggested)

🚨 Top Challenges in Building a Billing System

1. Complexity of Subscription Lifecycle

2. Multi-Currency & Global Billing Rules

3. Tax Compliance (VAT, GST, Sales Tax)

4. Payment Gateway Variations & Failures

5. Dunning & Payment Recovery Logic

6. Handling Usage & Metered Billing at Scale

7. Invoice Generation & Numbering

8. Ledger & Financial Accuracy

9. Multi-Tenancy Challenges

10. Event Consistency & State Conflicts

11. Compliance & Security Requirements

12. Pricing Experiments & Flexibility

13. Recovering from Webhook Failures

14. Scaling Events & Batch Jobs

15. Customer Expectations

16. Global Availability & Latency

17. Maintaining Backward Compatibility

18. Integrations

19. Real-Time Reporting

20. Cost Control & Performance

🔥 Final Summary — The Hardest Parts