The most prevalent use case for GraphQL right now is to act as a thin API layer over a bunch of microservices (which is what Facebook originally developed it fo

The most prevalent use case for GraphQL right now is to act as a thin API layer over a bunch of microservices (which is what Facebook originally developed it for) -- but that is not the only use case. GraphQL is actually much less than a replacement for a REST API. Its really just a type system with an execution engine that can map queries against that type system to resolvers (functions). That's it. This means that you can actually use it all over the place: - Because it provides a strongly typed interface, you could use GraphQL as the basis of an RPC protocol (infact, GraphQL doesn't specify a transport mechanism, since its only job is to map queries to functions, so you could send GraphQL queries over RabbitMQ if you wanted to.. or encode them with protobuf and send them via HTTP/2). - You could use a GraphQL schema to define the internal object model of an application and use GraphQL internally as a data access layer. You don't even have to expose it outside your codebase. - You can use the tooling that comes with GraphQL (at least in the Node.js implementation) to parse incoming queries into their abstract syntax trees and to all kinds of fun stuff with them (automagically stitch multiple GraphQL schema's together into a combined API). - I'm actually working on a proxy layer that can actually sit on top of an existing GraphQL API and apply additional logic to the incoming request and outgoing responses (it would actually be a good fit as a business logic layer for a Hasura based GraphQL api) I think that in the future we'll see lots of very interesting uses of GraphQL that go beyond is current status as "REST API replacement" *To answer your question "if I sign up a user, how -- in a GraphQL world -- would I send a welcome email, typically?" -- Define a mutation (update method) in your GraphQL schema to create a new user. Inside the resolver (function that is executed) for that mutation, write code to create a user (API call? Database update?) and send an email (drop a message in a queue to your email sending serivce?). It works just like it would for any other application codebase, you just initiate process with a GraphQL query, instead of, say an HTTP Post to a REST API.