@defer: Validation rule for the unicity of path+label

[BoD]

For each @defer directive, ensure:

• if a label is present, it must be unique across the whole document
• if not, the path of where it is used must be unique

Example:

query Query1 {
    computers {
        ...ComputerFields @defer #path is computer
        ...ComputerFields2 @defer #path is computer
    }
}

2 directives have the same path computer and no label. A label must be set on at least one of them.

Less obvious example:

query Query1 {
    computers {
        ...ComputerFields @defer(label: "a")
        ...ComputerFields2 @defer(label: "b")
    }
}

fragment ComputerFields on Computer {
  screen {
    ...ScreenFields @defer # path is computer.screen
  }
}

fragment ComputerFields2 on Computer {
  screen {
    ...ScreenFields @defer # path is computer.screen
  }
}

2 directives have the same path computer.screen and no label. A label must be set on at least one of them. To detect this, the context of all the places where the fragment is used is needed...

[BoD]

On 2nd thought, not sure the 2nd example makes sense actually. Due to how each fragment has its independent Adapter this "deep" check may not be needed. To be confirmed!

[martinbonnin]

Initial thoughts are that it is needed or else each "ComputerFields" adapter cannot know whether the ScreenFields are for themselves or the other one.

[BoD]

I think you're right!

2nd example tweaked a bit:

query Query1 {
    computers {
        ...ComputerFields1 @defer(label: "a")
        ...ComputerFields2 @defer(label: "b")
    }
}

fragment ComputerFields1 on Computer {
    screen {
        ...ScreenFields1 @defer
    }
}

fragment ComputerFields2 on Computer {
    screen {
        ...ScreenFields2 @defer
    }
}

fragment ScreenFields1 on Screen {
    isColor
}

fragment ScreenFields2 on Screen {
    resolution
}

Received payloads:

// 0
{"data":{"computers":[{},{}]},"hasNext":true}
// 1
{"data":{"screen":{}},"path":["computers",0],"label":"a","hasNext":true}
// 2
{"data":{"screen":{}},"path":["computers",0],"label":"b","hasNext":true}
// 3
{"data":{"resolution":"640x480"},"path":["computers",0,"screen"],"hasNext":true}
// 4
{"data":{"screen":{}},"path":["computers",1],"label":"a","hasNext":true}
// 5
{"data":{"screen":{}},"path":["computers",1],"label":"b","hasNext":true}
// 6
{"data":{"resolution":"640x480"},"path":["computers",1,"screen"],"hasNext":true}
// 7
{"data":{"isColor":false},"path":["computers",0,"screen"],"hasNext":true}
// 8
{"data":{"isColor":false},"path":["computers",1,"screen"],"hasNext":false}

After receiving 3, inside the adapter for ComputerFields1, the test to read the fields for ScreenFields1 is "have we received the fragment for path computer.0.screen and label null?", the answer is yes, it will try to read isColor which is not there. ❌

[martinbonnin]

Life is hard 😅