[Discuss] Support GraphQL DataSource

[tusharmath]

Tailcall needs to support GraphQL data source. This could be supported in various ways:

• Apollo Federation
• Open Federation
• GraphQL Fusion
• Something else?

[sujeetsr]

Putting down some thoughts around how to do Apollo Federation...

How Apollo Federation works:

• In a federated architecture, the individual GraphQL APIs are called subgraphs, and their individual schemas are composed into a supergraph.
• Each subgraph has to support the subgraph spec.
• The individual subgraph schemas are composed together into a supergraph schema.
• Clients can query this schema by querying a router, which is the public access point for the supergraph.
• The router communicates with the individual subgraphs to resolve queries, and the individual subgraphs are not queryable by clients.

Composition and Routing Options:

• Apollo provides Apollo Studio for managed federation or the Rover CLI for local composition. https://www.apollographql.com/docs/federation/quickstart/local-composition
• Apollo also provides the Apollo Router (a precompiled Rust binary) to server the composed schema.

async-graphql Federation support:

async-graphql supports the subgraph spec, so an async-graphql server can act as a subgraph in a federated architecture. (dynamic schema is supported)
https://github.com/async-graphql/examples/tree/ed1e7f28e472b27d883092f660c7632942c4f314/federation

To support Federation in Tailcall:

To enable Federation support in Tailcall, we can approach it in stages as follows:

Milestone 1: Support Tailcall server acting as a subgraph in a federated architecture.

• Do any additional steps (like entity resolver) that async-graphql does for federation
• Pass any federation related directives through to the final schema
• Compose multiple tailcall servers together with Apollo Rover and Router, and be able to serve a federated schema.

Milestone 2: Develop our own composition library (and possibly our own router).

• Basically composition will involve creating a blueprint from multiple schemas, and this blueprint can output the supergraph schema.
• We can develop our own router or continue to use Apollo router.

@tusharmath @amitksingh1490 - Let me know your thoughts.