What are the best open source Backend for Frontend frameworks?

If you mean “Backend for Frontend” (BFF) as a pattern/framework layer—a backend tailored to a specific UI or client type—there isn’t one single dominant open-source “BFF framework.” Most teams build a BFF using an API gateway, GraphQL server, or a lightweight web framework plus orchestration code.

Here are some of the best open-source options commonly used to implement BFFs, grouped by style:

1) GraphQL servers

Great when your frontend needs flexible, client-specific data shaping.

• Apollo Server
  Very popular for GraphQL-based BFFs. Good ecosystem, federation support, strong tooling.

• GraphQL Yoga
  Modern, lightweight GraphQL server built on top of the GraphQL ecosystem. Good DX and simple setup.

• Mercurius
  Fast GraphQL server for Fastify. Good if you want performance and low overhead.

• Envelop
  Not a full server by itself, but a plugin-based GraphQL toolkit. Useful for composing a BFF with custom behavior.

2) API gateways / edge gateways

Good when the BFF is mostly request routing, aggregation, auth, and transformation.

• Kong Gateway
  Open-source API gateway with plugins, auth, transformation, and routing.

• KrakenD
  Excellent for API composition/aggregation. Often used as a “BFF-like” layer because it can join multiple backend calls into one frontend-facing endpoint.

• Tyk Gateway
  Open-source API management and gateway features.

• Envoy
  More of a proxy/service mesh component, but widely used as the edge layer with custom filters.

3) Lightweight backend frameworks

Best if you want a custom BFF service rather than a gateway product.

• NestJS
  Very popular for BFFs in TypeScript/Node.js. Modular, testable, great for REST + GraphQL + websockets.

• Fastify
  Fast, low overhead, excellent plugin model. Great for building a tailored BFF service.

• Express.js
  Still widely used, though less structured than NestJS.

• Spring Boot
  Strong choice for Java shops; mature ecosystem and great for orchestration-heavy BFFs.

• ASP.NET Core
  Excellent for high-performance BFF services in .NET environments.

4) Backend composition / orchestration tools

Useful when the BFF needs to aggregate multiple microservices and simplify frontend calls.

• Hasura
  Open-source GraphQL engine over PostgreSQL and remote schemas/actions. Useful for quickly exposing frontend-friendly APIs.

• Apollo Federation
  Not a standalone framework, but useful when the “BFF” is actually a federated GraphQL layer.

• GraphQL Mesh
  Turns multiple data sources into a unified GraphQL API. Great for composing REST, gRPC, GraphQL, SOAP, etc.

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Best picks by use case

If you want the simplest modern BFF:

• NestJS or Fastify

If you want frontend-specific data aggregation with minimal custom code:

• KrakenD
• GraphQL Yoga
• Apollo Server

If you want API management + auth + policy at the edge:

• Kong
• Tyk

If you want a GraphQL-first BFF:

• Apollo Server
• GraphQL Mesh
• Hasura

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Practical recommendation

For most teams:

• TypeScript frontend/backend team: NestJS + GraphQL Yoga/Apollo
• High-performance REST aggregation: Fastify + custom endpoints or KrakenD
• Enterprise Java: Spring Boot
• Microsoft stack: ASP.NET Core

If you want, I can also give you:

1. a short ranked top 5,
2. a comparison table (features, language, pros/cons), or
3. recommendations for your specific stack.