Stale data invalidation with delta syncing

[vladar]

GraphQL schema doesn't convey full information about data interdependencies and relations. So it is possible to have some pieces of data stale with delta sync. Quick example:

type Category {
  title: String
  totalCommentCount: Int
}

type Post {
  category: Category
  commentCount: Int
  comments: [Comment]
}

Say some comment is deleted and we are notified about it via a DELETE event. Three things are stale now:

1. Reference to this comment in Post.comments field
2. Post.commentCount field is also stale as it has to be decremented on comment delete
3. Category.totalCommentCount - same

When this happens there should be a way to update specific fields of related nodes. So in addition to UPDATE and DELETE events (targeting individual nodes), we need UPDATE_FIELDS event that can notify us about specific node fields that had changed.

This event would generate dynamic new NODE_ query that will only fetch the mentioned fields and update the node. So it will look something like this:

{
  eventName: "UPDATE_FIELDS",
  remoteTypeName: "Post",
  remoteId: { __typename: "Post", id: "1" },
  fields: ["commentCount", "comments"]
},
{
  eventName: "UPDATE_FIELDS",
  remoteTypeName: "Category",
  remoteId: { __typename: "Category", id: "1" },
  fields: ["totalCommentCount"]
},

Additionally maybe allow fields to be a fragment, i.e.:

{
  eventName: "UPDATE_FIELDS",
  remoteTypeName: "Post",
  remoteId: { __typename: "Post", id: "1" },
  fields: `fragment ChangedFields on Post { commentCount, comments { __typename } }`
},

Technically we can update related nodes fully. But that can be pretty heavy in some cases, so more targeted updates can be useful in performance-sensitive scenarios.