Q:

AWS AppSync: pass arguments from parent resolver to children

In AWS AppSync, arguments send on the main query don't seem to be forwarded to all children resolvers.
type Query {
  article(id: String!, consistentRead: Boolean): Article
  book(id: String!, consistentRead: Boolean): Book
}

type Article {
  title: String!
  id: String!
}

type Book {
  articleIds: [String]!
  articles: [Article]!
  id: String!
}

when I call: 
query GetBook {
  book(id: 123, consistentRead: true) {
    articles {
      title
    }
  }
}

the first query to get the book receives the consistentRead param in $context.arguments, but the subsequent query to retrieve the article does not. ($context.arguments is empty)
I also tried articles(consistentRead: Boolean): [Article]! inside book but no luck.
Does anyone know if it's possible in AppSync to pass arguments to all queries part of the same request?

A:

It is possible to pass arguments from parent to child via the response. Let me explain ...
AppSync has several containers inside $context:

arguments
stash
source

arguments and stash are always cleared before invoking a child resolver as evident from these Cloudwatch logs:
At the very end of the parent execution  - arguments and stash data are present.
{
    "errors": [],
    "mappingTemplateType": "After Mapping",
    "path": "[getLatestDeviceState]",
    "resolverArn": "arn:aws:appsync:us-east-1:xxx:apis/yyy/types/Query/fields/getLatestDeviceState",
    "context": {
        "arguments": {
            "device": "ddddd"
        },
        "prev": {
            "result": {
                "items": [
                    {
                        "version": "849",
                        "device": "ddddd",
                        "timestamp": "2019-01-29T12:18:34.504+13:00"
                    }
                ]
            }
        },
        "stash": {"testKey": "testValue"},
        "outErrors": []
    },
    "fieldInError": false
}

and then at the very beginning of the child resolver - arguments and stash are always blank.
{
"errors": [],
"mappingTemplateType": "Before Mapping",
"path": "[getLatestDeviceState, media]",
"resolverArn": "arn:aws:appsync:us-east-1:yyy:apis/xxx/types/DeviceStatePRODConnection/fields/media",
"context": {
    "arguments": {},
    "source": {
        "items": [
            {
                "version": "849",
                "device": "ddddd",
                "timestamp": "2019-01-29T12:18:34.504+13:00"
            }
        ]
    },
    "stash": {},
    "outErrors": []
},
"fieldInError": false
}

Workaround 1 - get the argument from the previous result.
In the example above device is always present in the response of the parent resolver, so I inserted 
#set($device = $util.defaultIfNullOrBlank($ctx.args.device, $ctx.source.items[0].device))

into the request mapping template of the child resolver. It will try to get the ID it needs from the arguments and then fall back onto the previous result.
Workaround 2 - add the argument to the parent response
Modify your parent resolver response template to include the arguments:
{
    "items": $utils.toJson($context.result.items),
    "device": "${ctx.args.device}"
}

and then retrieve it in the request mapping template of the child the same way as in the first workaround.

