Option to include 'extensions' top-level key from subgraphs

[dchukmasov]

Describe the solution you'd like

Graphql specification describes top-level 'extensions' key as a valid part of graphql server response. However, apollo router removes any extensions from the subgraph response. There is also no config to tweak this behavior. The closest include_subgraph_errors works only for 'errors' top-level key, not the 'extensions'.

Describe alternatives you've considered

Writing custom Rhai Script for the matter is an option that requires proficiency, tweaking router deployment as well as will create overhead over all responses for such a simple task as forwarding data from subgraph to the client.

[smyrick]

Thank you for raising this feature request. Let me share how to solve this today first

Returning Extensions

The Router does not return extensions because it is an object with key-value pairs rather than a list. A list, like the errors is easy to merge across subgraphs because you can just add every item to the final list even if there is duplicates. If there is a clash of keys in the extensions we would need some way to define the merging or priority logic.

To do that today you can write your own logic in customizations. Hook into the the SubgraphResponse and extract the values you want from the subgraph data and store the extensions into the context and have the logic to resolve conflicts. Then add another hook into the RouterResponse that reads the final values of the context and push those into the response setting the response.extensions field yourself.

Built-in Extensions Return

How might you imagine defining merge logic in YAML as a built in feature?

Option 1

Simple. Any conflicts get overridden, in order of the query plan

Option 2

Any conflicts get overridden, provide priority order in YAML

subgraph_extensions:
   priority_order:
      - products # Any extension key from products service will always be included
      - reviews # Any extension key from reviews service will be included above all other not named subgraphs, except products

Option 3

Do you have better ideas on how to define merge logic in YAML?