Update Delta Feature

update-delta.md

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+# Update Delta
+
+## 1. Abstract
+When an RDF resource is updated on a storage server, a notification is sent to a subscribing client, but this notification does not tell the client how the resource was updated. As a result, the client must make another request to the storage server to retieve the document and reconcile its updates. Update Delta is a notifications feature that tells the client change, or delta between the document pre-update and post-update in the notification activity.
+
+## 2. Status of the Document
+TODO
+
+## 3. Notification Feature
+Update Delta is a notification feature. As such, any compliant storage server MUST list it among other features during the notification discover phase as the URL `http://www.w3.org/ns/solid/notifications#updateDelta`.
+
+For example, a compliant storage server could have a document like the following:
+
+```turtle
+@prefix notify: <http://www.w3.org/ns/solid/notifications#> .
+<>
+  notify:notificationChannel <#websocketNotification> .
+<#websocketNotification>
+  a notify:WebSocketSubscription2021 ;
+  notify:subscription <https://websocket.example/subscription> ;
+  notify:features notify:state, notify:rate, notify:expiration, notify:updateDelta .
+```
+
+## 4. Update Activity
+
+Any Update activity (including updates to containers) MUST include the [`result`](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-result) field.
+
+> Is the `result` field the most appropriate one for this? I didn't see an ActivityStreams property that was really appropriate. Should we make a new property?
+
+The `result` field MUST be mapped to [`Add` type](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-add) and/or [`Remove` type](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-remove) activities.
+
+The `object` field of the `Add` and `Remove` activities MUST map to a graph distinct from the default graph.
+
+Each graph referenced by the `Add` and `Remove` activities must contain the respective triples that were added and removed.
+
+
+In this example, the triple `<http://example.com/prince> <http://xmlns.com/foaf/0.1/name> "Prince Rogers Nelson"^^xsd:string` was removed, and the triple `<http://example.com/prince> <http://xmlns.com/foaf/0.1/name> "Ƭ̵̬̊"^^xsd:string` was added.
+
+```json
+{
+  "@context": [
+    "https://www.w3.org/ns/activitystreams",
+    "https://www.w3.org/ns/solid/notification/v1"
+  ],
+  "id":"urn:uuid:<uuid>",
+  "type": [
+    "Update"
+  ],
+  "actor": [
+    "<WebID>"
+  ],
+  "object": {
+    "id":"https://example.org/<user>/some-container/resource1.ttl",
+     "type": [
+      "http://www.w3.org/ns/ldp#Resource"
+    ]
+  },
+  "published":"2021-08-05T01:01:49.550044Z",
+  "result": [
+    {
+      "type": "Remove",
+      "object": {
+        "@id": "<Graph1Name>",
+        "@graph": [
+          {
+            "@id": "http://example.com/prince",
+            "http://xmlns.com/foaf/0.1/name": "Prince Rogers Nelson"
+          }
+        ]
+      }
+    },
+    {
+      "type": "Add",
+      "object": {
+        "@id": "<Graph2Name>",
+        "@graph": [
+          {
+            "@id": "http://example.com/prince",
+            "http://xmlns.com/foaf/0.1/name": "Ƭ̵̬̊"
+          }
+        ]
+      }
+    }
+  ]
+}
+```
+
+> Alternatively, we could create some terms outside of Activity Streams. The example would look like this:
+
+```json
+{
+  "@context": [
+    "https://www.w3.org/ns/activitystreams",
+    "https://www.w3.org/ns/solid/notification/v1"
+  ],
+  "id":"urn:uuid:<uuid>",
+  "type": [
+    "Update"
+  ],
+  "actor": [
+    "<WebID>"
+  ],
+  "object": {
+    "id":"https://example.org/<user>/some-container/resource1.ttl",
+     "type": [
+      "http://www.w3.org/ns/ldp#Resource"
+    ]
+  },
+  "published":"2021-08-05T01:01:49.550044Z",
+  "added": {
+    "@id": "<Graph1Name>",
+    "@graph": [
+      {
+        "@id": "http://example.com/prince",
+        "http://xmlns.com/foaf/0.1/name": "Prince Rogers Nelson"
+      }
+    ]
+  },
+  "removed": {
+    "@id": "<Graph2Name>",
+    "@graph": [
+      {
+        "@id": "http://example.com/prince",
+        "http://xmlns.com/foaf/0.1/name": "Ƭ̵̬̊"
+      }
+    ]
+  }
+}
+```