How do I handle this validation scenario in a federated graph?

Hi, new to GraphQL and am currently fiddling with Apollo Federation.

Using a minimal example, I have a product service and a review service.

In my product service, I define the product type and its resolver

type Product @key(fields: "id") {
    id: ID!
    name: String!
*__resolveReference impl*

In my review service, I define the following

type Review {
  id: ID! 
  product: Product!
  comment: String!

type Product @key(fields: "id", resolvable: false) {
  id: ID!

type Mutation {
    addReview(productId: ID!): Review!

And the mutation resolver

addReview: (args) => {
    ...insert into db in the form (id, productId, comment)
    return { ... };

At this point, everything works fine since the federated graph will automatically resolve the product field of the Review by querying the product service.

My question now is what if I want to first validate that the productId provided to addReview actually exists in the product db managed the the product service?

  1. Is there a way of designing the schema to resolve this automatically?
  2. If not, it seems like I would need some form of inter-service communication, where I must call the product service from within the addReview resolver to check that the productId exists. In this case, are there any standard methods for how this is implemented?