Explain how to decypher live and historic pagination tokens

Fix #12305

Author: MadLittleMods

synapse/types.py

@@ -421,22 +421,92 @@ class RoomStreamToken:
 
             s0    s1
             |     |
-        [0] V [1] V [2]
+        [0] ▼ [1] ▼ [2]
 
     Tokens can either be a point in the live event stream or a cursor going
     through historic events.
 
-    When traversing the live event stream events are ordered by when they
+    When traversing the live event stream, events are ordered by when they
     arrived at the homeserver.
 
-    When traversing historic events the events are ordered by their depth in
-    the event graph "topological_ordering" and then by when they arrived at the
-    homeserver "stream_ordering".
+    When traversing historic events, events are first ordered by their
+    "depth" ("topological_ordering" in the event graph) and tie-broken by
+    "stream_ordering" (when the event arrived at the homeserver).
+
+    ---
 
     Live tokens start with an "s" followed by the "stream_ordering" id of the
-    event it comes after. Historic tokens start with a "t" followed by the
-    "topological_ordering" id of the event it comes after, followed by "-",
-    followed by the "stream_ordering" id of the event it comes after.
+    event that comes after. The rest of the keys are joined together by
+    underscores in the following order and represent the position of various
+    data.
+
+    ex. `s2633508_17_338_6732159_1082514_541479_274711_265584_1`
+        1. `room_key`: `s2633508` -> `2633508` `stream_ordering`
+        2. `presence_key`: `17`
+        3. `typing_key`: `338`
+        4. `receipt_key`: `6732159`
+        5. `account_data_key`: `1082514`
+        6. `push_rules_key`: `541479`
+        7. `to_device_key`: `274711`
+        8. `device_list_key`: `265584`
+        9. `groups_key`: `1`
+
+    For example, you can see how many of these keys correspond to the various
+    fields in the "/sync" response:
+    ```
+    {
+        "next_batch": "s12_4_0_1_1_1_1_4_1",
+        "presence": {
+            "events": []
+        },
+        "device_lists": {
+            "changed": []
+        },
+        "rooms": {
+            "join": {
+                "!QrZlfIDQLNLdZHqTnt:hs1": {
+                    "timeline": {
+                        "events": [],
+                        "prev_batch": "s10_4_0_1_1_1_1_4_1",
+                        "limited": false
+                    },
+                    "state": {
+                        "events": []
+                    },
+                    "account_data": {
+                        "events": []
+                    },
+                    "ephemeral": {
+                        "events": []
+                    }
+                }
+            }
+        }
+    }
+    ```
+
+    ---
+
+    Historic tokens start with a "t" followed by the "depth"
+    ("topological_ordering" in the event graph) of the event that comes after,
+    followed by "-", followed by the "stream_ordering" id of the event it comes
+    after along with rest of the same keys from the live tokens.
+
+    You will see this type of token when using the "/messages" endpoint.
+
+    ex. `t426-2633508_17_338_6732159_1082514_541479_274711_265584_1`
+        1. `topological_ordering`: t426 -> `426` `depth`
+        2. `room_key`: `2633508` `stream_ordering`
+        3. `presence_key`: `17`
+        4. `typing_key`: `338`
+        5. `receipt_key`: `6732159`
+        6. `account_data_key`: `1082514`
+        7. `push_rules_key`: `541479`
+        8. `to_device_key`: `274711`
+        9. `device_list_key`: `265584`
+       10. `groups_key`: `1`
+
+    ---
 
     There is also a third mode for live tokens where the token starts with "m",
     which is sometimes used when using sharded event persisters. In this case
@@ -460,6 +530,8 @@ class RoomStreamToken:
     commonly used instance names) are at positions 58 and 59 respectively, and
     all other instances are at position 56.
 
+    ---
+
     Note: The `RoomStreamToken` cannot have both a topological part and an
     instance map.