Overlapping fields with @stream

[robrichard]

Context

It is possible to request the same field from different fragments. Normally these overlapping fields are collapsed by the graphql executor. What happens when @stream is added to normally overlapping fields?

We originally implemented execution behavior that attempted to resolve overlapping streamed fields to something that satisfied the arguments of all stream directives on that field. We received guidance from facebook recommending a validation rule to forbid this (see discussion graphql/graphql-js#2319 (comment))

Decision

We implemented the SameStreamDirective validation rule, which disallows overlapping fields with stream directives with different arguments.

Consequences

Developers who wish to create multiple streams on the same field, or to both stream and not stream, can use field aliases to ensure the incompatible fields do not overlap.

Status

• defined in spec
• implemented in graphql-js