Updated Client Sync Server Design (markdown)

Client-Sync-Server-Design.md

@@ -38,20 +38,23 @@ Database:
 - For each room, get the latest N messages: `SELECT * FROM timeline WHERE room_id=? ORDER BY pkey_id DESC LIMIT N`
 - "Roll back" the current state by the number of timeline entries and return that along with the `pkey_id` stream token to Alice.
 
-#### Alice hits /sync with a token with no new events
+#### Alice hits /sync with a token
 - Get all rooms Alice is joined to: `SELECT room_id FROM room_state WHERE type="m.room.member" AND state_key="@alice:localhost" AND membership="join";` - Cache this information in-memory.
+- We need to know if this token is going to make us do a lot of work. Work out if we are `<M` or not:
+
+   ```
+   SELECT COUNT(*) FROM timeline WHERE room_id=ANY([?,?,?]) ORDER BY pkey_id DESC LIMIT M
+   ```
+
+If number of entries == 0 then we need to wait for a new event:
 - Wait for new event from Kafka log.
 - On new event, if `room_id` matches one that Alice is joined to, return event to Alice along with new `pkey_id`.
 
-##### Alice hits /sync with a token (incremental sync)
-We need to know if this token is going to make us do a lot of work. Work out if we are `<M` or not:
- - `SELECT * FROM timeline WHERE room_id=ANY([?,?,?]) ORDER BY pkey_id DESC LIMIT M`
-
-If number of entries < M:
+Else if number of entries < M:
  - For each room, roll back current state by the timeline entries, up to `N`.
  - Return that along with the latest `pkey_id` to Alice.
 
-If number of entries >= M:
+Else if number of entries >= M:
 - Ask the room server:
   * Which rooms Alice is in.
   * For each room, get current state and latest N events.