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140 lines
6.4 KiB
Markdown
140 lines
6.4 KiB
Markdown
# Design
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## Log Based Architecture
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### Decomposition and Decoupling
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A matrix homeserver can be built around append-only event logs built from the
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messages, receipts, presence, typing notifications, device messages and other
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events sent by users on the homeservers or by other homeservers.
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The server would then decompose into two categories: writers that add new
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entries to the logs and readers that read those entries.
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The event logs then serve to decouple the two components, the writers and
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readers need only agree on the format of the entries in the event log.
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This format could be largely derived from the wire format of the events used
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in the client and federation protocols:
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C-S API +---------+ Event Log +---------+ C-S API
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---------> | |+ (e.g. kafka) | |+ --------->
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| Writers || =============> | Readers ||
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---------> | || | || --------->
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S-S API +---------+| +---------+| S-S API
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+---------+ +---------+
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However the way matrix handles state events in a room creates a few
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complications for this model.
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1) Writers require the room state at an event to check if it is allowed.
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2) Readers require the room state at an event to determine the users and
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servers that are allowed to see the event.
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3) A client can query the current state of the room from a reader.
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The writers and readers cannot extract the necessary information directly from
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the event logs because it would take too long to extract the information as the
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state is built up by collecting individual state events from the event history.
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The writers and readers therefore need access to something that stores copies
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of the event state in a form that can be efficiently queried. One possibility
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would be for the readers and writers to maintain copies of the current state
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in local databases. A second possibility would be to add a dedicated component
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that maintained the state of the room and exposed an API that the readers and
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writers could query to get the state. The second has the advantage that the
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state is calculated and stored in a single location.
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C-S API +---------+ Log +--------+ Log +---------+ C-S API
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---------> | |+ ======> | | ======> | |+ --------->
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| Writers || | Room | | Readers ||
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---------> | || <------ | Server | ------> | || --------->
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S-S API +---------+| Query | | Query +---------+| S-S API
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+---------+ +--------+ +---------+
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The room server can annotate the events it logs to the readers with room state
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so that the readers can avoid querying the room server unnecessarily.
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[This architecture can be extended to cover most of the APIs.](WIRING.md)
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## How things are supposed to work.
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### Local client sends an event in an existing room.
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0) The client sends a PUT `/_matrix/client/r0/rooms/{roomId}/send` request
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and an HTTP loadbalancer routes the request to a ClientAPI.
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1) The ClientAPI:
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* Authenticates the local user using the `access_token` sent in the HTTP
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request.
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* Checks if it has already processed or is processing a request with the
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same `txnID`.
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* Calculates which state events are needed to auth the request.
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* Queries the necessary state events and the latest events in the room
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from the RoomServer.
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* Confirms that the room exists and checks whether the event is allowed by
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the auth checks.
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* Builds and signs the events.
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* Writes the event to a "InputRoomEvent" kafka topic.
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* Send a `200 OK` response to the client.
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2) The RoomServer reads the event from "InputRoomEvent" kafka topic:
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* Checks if it has already has a copy of the event.
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* Checks if the event is allowed by the auth checks using the auth events
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at the event.
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* Calculates the room state at the event.
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* Works out what the latest events in the room after processing this event
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are.
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* Calculate how the changes in the latest events affect the current state
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of the room.
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* TODO: Workout what events determine the visibility of this event to other
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users
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* Writes the event along with the changes in current state to an
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"OutputRoomEvent" kafka topic. It writes all the events for a room to
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the same kafka partition.
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3a) The ClientSync reads the event from the "OutputRoomEvent" kafka topic:
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* Updates its copy of the current state for the room.
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* Works out which users need to be notified about the event.
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* Wakes up any pending `/_matrix/client/r0/sync` requests for those users.
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* Adds the event to the recent timeline events for the room.
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3b) The FederationSender reads the event from the "OutputRoomEvent" kafka topic:
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* Updates its copy of the current state for the room.
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* Works out which remote servers need to be notified about the event.
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* Sends a `/_matrix/federation/v1/send` request to those servers.
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* Or if there is a request in progress then add the event to a queue to be
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sent when the previous request finishes.
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### Remote server sends an event in an existing room.
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0) The remote server sends a `PUT /_matrix/federation/v1/send` request and an
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HTTP loadbalancer routes the request to a FederationReceiver.
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1) The FederationReceiver:
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* Authenticates the remote server using the "X-Matrix" authorisation header.
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* Checks if it has already processed or is processing a request with the
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same `txnID`.
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* Checks the signatures for the events.
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Fetches the ed25519 keys for the event senders if necessary.
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* Queries the RoomServer for a copy of the state of the room at each event.
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* If the RoomServer doesn't know the state of the room at an event then
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query the state of the room at the event from the remote server using
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`GET /_matrix/federation/v1/state_ids` falling back to
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`GET /_matrix/federation/v1/state` if necessary.
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* Once the state at each event is known check whether the events are
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allowed by the auth checks against the state at each event.
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* For each event that is allowed write the event to the "InputRoomEvent"
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kafka topic.
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* Send a 200 OK response to the remote server listing which events were
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successfully processed and which events failed
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2) The RoomServer processes the event the same as it would a local event.
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3a) The ClientSync processes the event the same as it would a local event.
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