improve datasync spec

docs/spec-datasync.md

@@ -49,4 +49,39 @@ type CommentDeltaList {
 Each object of this list is a snapshot of the current state of the row/document in the database, along with the timestamps that show when it was last changed(`updatedAt`), and when it was created(`createdAt`). It also provides info on if the row was deleted(`_deleted`), in which case the `updatedAt` timestamp says when it was deleted.
 
 The objects to be fetched in the delta query can also be filtered by using the `filter` argument 
-which would work exactly like the filter in the [find](./spec-find.md) query.
+which would work exactly like the filter in the [find](./spec-find.md) query.
+
+### Server-side Conflict resolution
+
+If a client goes offline, there is a strong possiblity that the cached data may not be consistent with the data from the source. Thus it is advisable to to have some mechanism for detecting this inconsistency and some way to resolve this. For example, requiring an `updatedAt` timestamp for every mutation is a decent way to ensure this:
+
+```graphql
+input MutateCommentInput {
+  id: ID!
+  title: String
+  description: String
+  updatedAt: String!
+}
+```
+
+This could possibly detect inconsistencies when issuing mutations and possibly resolve them or inform the client about the differences, for example:
+
+```json
+{
+  "conflictInfo": {
+    "serverState": {
+      "id": "5eedae1367d72e2192561723",
+      "text": "AlreadyUpdatedTitle",
+      "_deleted": false,
+      "createdAt": "1592634899084",
+      "updatedAt": "1592634899084"
+    },
+    "clientState": {
+      "id": "5eedae1367d72e2192561723",
+      "text": "ClientSideUpdate",
+      "updatedAt": "1592634898093"
+    }
+  }
+}
+```
+