You can pretty easily (if you have experience with the GraphQL reference implementation in JavaScript) create a GraphQL layer that sits inside of the browser, with a schema created by the UI team, that executes calls to REST APIs to resolve the data.
You could think of it as an “ORM” for the browser, which seems cool, but I wouldn’t necessarily recommend this approach (though I have done it in the past) for two reasons:
1. The “graphql” library isn’t really optimized for size so it can add a bunch of overhead to your JavaScript bundles
2. One of the benefits of GraphQL is to combine multiple requests for related data into a single query to be sent from the browser. Yes, that makes the life of the backend developer harder as they try to optimize for performance, but it makes for less data/fewer requests over the wire to the client. If you stick GraphQL in the browser, you’ve now just moved your N+1 query across the internet.
If you really want to go down that road, Apollo offers a “plugin” to their GraphQL client that allows you to call multiple REST endpoints as if they were a single GraphQL endpoint (without embedding the actual “graphql” library in the browser): https://www.apollographql.com/docs/link/links/rest/
A better approach for what you’re looking for would be to schema stitching (which allows you to combine multiple GraphQL endpoints together and treat them as one. You can even combine that with your own schema definitions to mix in whatever backend sources you want; e.g. your vendor + FedEx REST APIs): https://www.graphql-tools.com/docs/stitch-combining-schemas
Or if you don’t want to do the work yourself, check out OneGraph, which uses schema-stitching to do exactly what you describe. It’s pretty cool: https://www.onegraph.com/
I think the bigger problem there is how those multiple REST calls map to your data stores. Very rarely will there be clean separation of data between endpoints, and the stateless nature of REST makes it harder to optimize each call -- meaning, there will almost certainly be redundant queries.
That said, with GraphQL, your front end dev may not realize they are executing the equivalent of many REST calls, which is another problem :) I wouldn't say it's harder to optimize if you look at the application as a whole, though.
(in other words: caching is complicated, and may make your systems very hard to understand & debug)
I just grokked this very recently when I was reading up on Gatsby and the fact that they use GraphQL. I was confused for a bit because Gatsby is SSG, until I realized they merely use GraphQL as a generic way to query any JSON data you might have laying around, similarly to how getStaticProps() is employed in NextJS
I wish there was a client side library that implemented GraphQL as an abstraction over REST, GraphQL or other backend APIs. It would be nice to use some of the GraphQLs query/join features across several backend services without those services having to change. Imagine being able to merge a vendor API that can provide a tracking number with FedEx's API, for example.