[docs-only] add client protocol overview

docs/architecture/protocol-changes.md

@@ -0,0 +1,40 @@
+---
+title: "Protocol changes"
+date: 2022-05-17T08:46:00+01:00
+weight: 30
+geekdocRepo: https://github.com/owncloud/ocis
+geekdocEditPath: edit/master/docs/architecture
+geekdocFilePath: protocol-changes.md
+---
+
+The spaces concept allows clients to look up the space endpoints a user has access to and then do individual sync discoveries. Technically, we introduce an indirection that allows clients to rely on server provided URLs instead of hardcoded `/webdav` or `/dav/files/{username}` paths, that may change over time.
+
+## Space discovery
+
+{{<mermaid class="text-center">}}
+%%{init: {"sequence": { "showSequenceNumbers":true, "messageFontFamily":"courier", "messageFontWeight":"normal", "messageFontSize":"11"}}}%%
+%% font weight is a css bug: https://github.com/mermaid-js/mermaid/issues/1976
+%% edit this diagram by pasting it into eg. https://mermaid.live
+sequenceDiagram
+    participant Client
+    participant Graph
+    participant SpaceA
+    participant SpaceB
+    links Client: {"web": "https://owncloud.dev/clients/web/", "RClone": "https://owncloud.dev/clients/rclone/"}
+    link Graph: Documentation @ https://owncloud.dev/extensions/graph/
+
+    Note left of Client:  First, a clients looks<br/>up the spaces a user has access to
+    opt space lookup
+        Client->>+Graph: GET /me/drives
+        Graph-->>-Client: 200 OK JSON list of spaces, say A, B and C,<br/> each with a dedicated webDavURL, etag and quota
+    end
+
+    Note left of Client: Then it can do a parallel<br/>sync discovery on spaces<br/>whose etag changed
+    par Client to Space A
+        Client->>+SpaceA: PROPFIND {webDavURL for Space A}
+        SpaceA-->>-Client: 207 Multistatus PROPFIND response
+    and Client to Space B
+        Client->>+SpaceB: PROPFIND {webDavURL for space B}
+        SpaceB-->>-Client: 207 Multistatus PROPFIND response
+    end  
+{{</mermaid>}}