Kill Bill

Kill Bill is a modular, Java‑based subscription billing and payments platform that empowers developers to build highly customized billing pipelines without vendor lock‑in. At its core, it exposes a RESTful API and an event‑driven plugin architecture that let you plug in payment gateways, tax engines, analytics tools, or any custom logic. The system is designed to be self‑hosted, yet it ships with a one‑click deployer for AWS and Docker images that can be orchestrated by Kubernetes or any container runtime.

Technical Stack

• Language & Runtime: Java 17+ (Spring Boot‑style services) with a lightweight OSGi module system for runtime extensibility.
• Data Layer: PostgreSQL (primary) and optional Cassandra/Redis for caching; the persistence layer is built on JPA/Hibernate, exposing a well‑defined set of database tables for accounts, subscriptions, invoices, and payments.
• Messaging: Kafka (or RabbitMQ) for event streams; events are serialized as JSON and can be consumed by external services or internal plugins.
• API Layer: A versioned REST API (/api/v1/...) plus a GraphQL fallback for complex queries. Authentication is JWT‑based, with OAuth2 support for third‑party integrations.
• UI: The Kaui web UI is a single‑page React application that talks to the REST API; it can be deployed independently or bundled with Kill Bill.

Core Capabilities

• Subscription Management: Create plans, tiers, usage‑based billing, proration, trials, and promotional discounts.
• Payment Processing: Abstracted payment gateway interface; supports Adyen, Stripe, Braintree, PayPal, and any custom gateway via the plugin API.
• Invoicing & Reporting: Real‑time invoice generation, PDF export, and built‑in analytics dashboards.
• Event Hooks & Webhooks: Expose event streams (invoice created, payment succeeded, subscription cancelled) to external systems; also allow inbound webhooks for gateway callbacks.
• Extensibility: Plugins are OSGi bundles that can add new payment methods, tax calculations, or custom workflows. The plugin system is hot‑reloadable and can be managed through the admin UI.

Deployment & Infrastructure

Kill Bill ships as a Docker image (killbill/killbill) and is fully Kubernetes‑ready. A Helm chart is available for automated deployment, including optional sidecar containers for Kafka and a PostgreSQL operator. The platform scales horizontally by adding more API nodes; the database is the primary scaling bottleneck, but sharding or read replicas can be employed for high throughput. For developers who prefer a single‑VM setup, the one‑click AWS deployer provisions an EC2 instance with all dependencies pre‑installed.

Integration & Extensibility

• Plugin SDK: Java APIs (org.killbill.billing.plugin) expose lifecycle hooks for subscription events, payment processing, and invoicing.
• REST/Webhooks: Third‑party services can consume or push events via standard HTTP.
• Custom UI Components: The Kaui app can be extended with custom React components that consume the same API, allowing bespoke dashboards.
• Data Export: CSV/JSON dumps of accounts and invoices are available; developers can also tap into the Kafka stream for real‑time analytics.

Developer Experience

• Documentation: Comprehensive docs on docs.killbill.io, including API reference, plugin tutorials, and deployment guides.
• Community: Active GitHub org with over 400 contributors; Slack channel and regular virtual meetups.
• Configuration: YAML/Properties files for runtime settings; environment variables can override defaults, making CI/CD pipelines straightforward.
• Testing: Unit tests are written in JUnit 5; integration tests run against an embedded H2 database and can be executed with Maven or Gradle.

Use Cases

1. SaaS Subscription Platforms – Build a custom billing engine that supports multiple pricing models and integrates with internal accounting systems.
2. Marketplace Gateways – Use Kill Bill as a central billing hub while delegating payment collection to different gateway plugins per vendor.
3. Enterprise SaaS – Deploy a self‑hosted solution to meet data sovereignty requirements while still leveraging modern payment providers.
4. Hybrid Billing – Start with Kill Bill for core subscriptions, then gradually replace or augment specific modules (e.g., tax calculation) with proprietary logic.

Advantages

• Vendor‑agnostic: No lock‑in to a single payment processor; swap plugins as business needs evolve.
• Open Source & Extensible: Apache 2.0 license allows free modification; the OSGi plugin system gives fine‑grained control over features.
• Performance & Scalability: Java + Kafka provides low‑latency event handling; horizontal scaling is straightforward.
• Rich Feature Set: Out‑of‑the‑box support for usage‑based billing, proration, and complex discounting outperforms many SaaS alternatives.
• Community & Support: A mature ecosystem with active contributors, making it easier to find help and reusable plugins.

Kill Bill is thus an ideal choice for developers who need a battle‑tested, highly customizable billing platform that can grow from a startup prototype to an enterprise‑grade system without relinquishing control over data or functionality.