Contract as a Service (CaaS)

Contract-as-a-Service (CaaS) is a runtime engine that reads an OpenAPI contract and turns it into a fully functional CRUD API. Instead of generating code from the contract ("contract-first") or documenting code after the fact ("code-first"), the contract itself becomes the product: a single source of truth that the platform executes.

You provide a well designed openapi contract. The CaaS runtime reads that contract and provides a functional API. Your development team spends less time implementing endpoints and more time designing clear, consistent contracts and focusing on business logic.

What is Contract as a Service?

Contract-as-a-Service (CaaS) is an architecture in which a generic runtime engine interprets an API contract (typically an OpenAPI document) at runtime to expose a production-ready API, with most behavior specified declaratively in the contract.

Instead of generating code from the contract or documenting code after the fact, the contract itself becomes the executable specification that drives the runtime.

Key Principles

Contract is the primary source of truth - The OpenAPI contract defines resources, operations, schemas, validations, and runtime behavior
Runtime is contract-agnostic - A single, reusable runtime can host many APIs across different teams and domains
Business logic lives at the edges - Complex workflows are moved to event-driven services, not embedded in API controllers

Why CaaS?

Most organizations struggle with:

API sprawl – dozens or hundreds of services, each slightly different
High maintenance cost – every service re-implements the same security, logging, and database interactions
Slow time-to-market – weeks or months to stand up a "simple" CRUD API

CaaS solves these problems by moving behavior into the contract and centralizing the runtime. When all repetitive boilerplate is handled once in the platform, teams can ship new APIs dramatically faster.

Benefits

Time-to-market - Go from idea to working API in minutes instead of weeks
Lower maintenance - Evolve contracts, not scattered service code
Consistency by design - Security, logging, monitoring applied consistently to every API
Clear separation of concerns - CRUD operations in the runtime, business logic in event-driven services

Getting Started

Problem & Motivation - Understand the challenges CaaS addresses
Core Concepts - Learn the fundamental ideas behind CaaS
Overview - High-level introduction to CaaS

Design & Implementation

Contract Design - How to design contracts for CaaS
Business Logic Patterns - Where business logic fits in the CaaS model
Architecture & Runtime - How CaaS fits into your system
Adoption - How to adopt CaaS in your organization

Implementations

CaaS is a design pattern and architecture. Multiple implementations can exist:

APIstry - A Node.js/Fastify-based CaaS runtime with MongoDB integration