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+This document details how various components communicate with each other. There are two kinds of components:
+ - Public-facing: exposes CS/SS API endpoints and need to be routed to via client-api-proxy or equivalent.
+ - Internal-only: exposes internal APIs and produces Kafka events.
+
+## Internal HTTP APIs
+
+Not everything can be done using Kafka logs. For example, requesting the latest events in a room is much better suited to
+a request/response model like HTTP or RPC. Therefore, components can expose "internal APIs" which sit outside of Kafka logs.
+Note in Monolith mode these are actually direct function calls and are not serialised HTTP requests.
+
+```
+   Tier 1            Sync        PublicRooms      FederationAPI      ClientAPI    MediaAPI
+Public Facing        | .-----1------`              | | |     |         | | | |
+                     2 | .-------3-----------------` | |     |         | | | |
+                     | | | .--------4----------------------------------` | | |
+                     | | | |         .---5-----------` |     |           | | |
+                     | | | |         |  .---6----------------------------` | |
+                     | | | |         |  |              |  .-----7----------` |
+                     | | | |         |  |              8  |  |               10
+                     | | | |         |  |              |  |  `---9----.      |
+                     V V V V         V  V              V  V           V      V
+   Tier 2           Roomserver     EDUServer         FedSender       AppService    KeyServer   ServerKeyAPI
+Internal only
+
+
+ Client ---> Server
+```
+- 1 (PublicRooms -> Roomserver): Calculating current auth for changing visibility
+- 2 (Sync -> Roomserver): When making backfill requests
+- 3 (FedAPI -> Roomserver): Calculating (prev/auth events) and sending new events, processing backfill/state/state_ids requests
+- 4 (ClientAPI -> Roomserver): Calculating (prev/auth events) and sending new events, processing /state requests
+- 5 (FedAPI -> EDUServer): Sending typing/send-to-device events
+- 6 (ClientAPI -> EDUServer): Sending typing/send-to-device events
+- 7 (ClientAPI -> FedSender): Handling directory lookups
+- 8 (FedAPI -> FedSender): Resetting backoffs when receiving traffic from a server. Querying joined hosts when handling alias lookup requests.
+- 9 (FedAPI -> AppService): Working out if the client is an appservice user. 
+- 10 (ClientAPI -> AppService): Working out if the client is an appservice user.
+
+## Kafka logs
+
+```
+                       .----1--------------------------------------------.
+                       V                                                 |
+   Tier 1            Sync        PublicRooms      FederationAPI      ClientAPI    MediaAPI
+Public Facing        ^   ^           ^                                 ^  
+                     |   |           |                                 |
+                     2   |           |                                 |
+                     |   `-3------------.                              |
+                     |               |  |                              |
+                     |               |  |                              |
+                     | .------4------`  |                              |
+                     | | .--------5-----|------------------------------`             
+                     | | |              |           
+   Tier 2           Roomserver     EDUServer         FedSender       AppService    KeyServer   ServerKeyAPI
+Internal only              |          |                ^ ^
+                           |          `-----6----------` |
+                           `--------------------7--------`
+
+
+Producer ----> Consumer
+```
+- 1 (ClientAPI -> Sync): For tracking account data
+- 2 (Roomserver -> Sync): For all data to send to clients
+- 3 (EDUServer -> Sync): For typing/send-to-device data to send to clients
+- 4 (Roomserver -> PublicRooms): For tracking the current room name/topic/joined count/etc.
+- 5 (Roomserver -> ClientAPI): For tracking memberships for profile updates.
+- 6 (EDUServer -> FedSender): For sending EDUs over federation
+- 7 (Roomserver -> FedSender): For sending PDUs over federation, for tracking joined hosts.