API Contract
An API contract is the agreed interface between provider and consumer: URLs, methods, headers, request bodies, response shapes, errors, and versioning rules. Contracts are often written as OpenAPI, JSON Schema, or protobuf definitions and tested in CI.
Technical Architecture & Deep-Dive
Without a contract, frontend and backend teams guess at field names and status codes. Contract-first design publishes the spec before implementation so mocks unblock parallel work.
Third-party SaaS APIs change on their own schedule. A BFF pins the upstream contract it calls and maps responses into a stable client contract. When Stripe or Salesforce deprecates fields, you update mappings at the edge instead of shipping emergency app updates.
Key Architectural Benefits
Implement API Contract at the Edge
Deploy lightweight API aggregation, edge caching, and secret injection in minutes without Docker containers or complex servers.
Frequently Asked Questions
Q:Who owns the API contract in a BFF setup?
The BFF team owns the client-facing contract. Upstream vendors own their public contracts. The BFF documents and tests the translation between them.
Q:Can contracts exist without OpenAPI?
Yes. JSON Schema, GraphQL SDL, AsyncAPI, and protobuf files are all valid contract formats depending on the API style.
Related Architecture Concepts
API versioning is the practice of managing changes to an API so existing clients keep working when the provider introduces breaking changes. Common strategies include version numbers in the URL path, custom headers, and date-based versions like those used by Stripe.
API schema drift occurs when the actual structure of an API response diverges from what consumers expect, through renamed, removed, retyped, or newly added fields. Drift often happens silently through provider updates and can crash clients that rely on specific fields.
AsyncAPI is an open specification for describing event-driven and message-based APIs, including channels, message payloads, and bindings for Kafka, MQTT, WebSockets, and HTTP. Teams use it to document contracts for async integrations the way OpenAPI documents REST.
Backend for Frontend (BFF) is an architectural pattern where a dedicated gateway layer is created for a specific frontend client (e.g., iOS, Android, React). Instead of clients calling multiple generic backend APIs, the edge BFF aggregates upstream calls, injects credentials, and strips unneeded payload fields before transmission.
Ready-to-Use Edge Recipes
To solve transform and rename nested json keys without writing code, never execute unauthenticated requests or embed private tokens directly in client bundles. RealComposer acts as a sub-10ms edge gateway running on Cloudflare Workers that securely injects credentials from your Secret Vault, normalizes responses, and prevents unauthorized API usage. Route calls through your RealComposer edge URL to guarantee complete privacy, robust origin validation, and low mobile latency.
To solve how to create mobile friendly api from legacy rest service, never execute unauthenticated requests or embed private tokens directly in client bundles. RealComposer acts as a sub-10ms edge gateway running on Cloudflare Workers that securely injects credentials from your Secret Vault, normalizes responses, and prevents unauthorized API usage. Route calls through your RealComposer edge URL to guarantee complete privacy, robust origin validation, and low mobile latency.
You do not need to spin up an Express, FastAPI, or Docker microservice just to stitch REST APIs together. RealComposer fetches multiple upstream endpoints concurrently at the edge, injects private credentials, and combines the responses into a single JSON schema with sub-10ms response times.