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# Dendrite
Dendrite will be a matrix homeserver written in go.
# Design
*
## Log Based Architecture
### Decomposition and Decoupling
A matrix homeserver can be built around append-only event logs built from the
messages, receipts, presence, typing notifications, device messages and other
events sent by users on the homeservers or by other homeservers.
The server would then decompose into two categories: writers that add new
entries to the logs and readers that read those entries.
The event logs then serve to decouple the two components, the writers and
readers need only agree on the format of the entries in the event log.
This format could be largely dervived from the wire format of the events used
in the client and federation protocols:
C-S API +---------+ Event Log +---------+ C-S API
---------> | |+ (e.g. kafka) | |+ --------->
| Writers || =============> | Readers ||
---------> | || | || --------->
S-S API +---------+| +---------+| S-S API
+---------+ +---------+
However the way matrix handles state events in a room creates a few
complications for this model.
1) Writers require the room state at an event to check if it is allowed.
2) Readers require the room state at an event to determine the users and
servers that are allowed to see the event.
3) A client can query the current state of the room from a reader.
The writers and readers cannot extract the necessary information directly from
the event logs because it would take to long to extract the information as the
state is built up by collecting individual state events from the event history.
The writers and readers therefore need access to something that stores copies
of the event state in a form that can be efficiently queried. One possibility
would be for the readers and writers to maintain copies of the current state
in local databases. A second possibility would be to add a dedicated component
that maintained the state of the room and exposed an API that the readers and
writers could query to get the state. The second has the advantage that the
state is calculated and stored in a single location.
C-S API +---------+ Log +--------+ Log +---------+ C-S API
---------> | |+ ======> | | ======> | |+ --------->
| Writers || | Room | | Readers ||
---------> | || <------ | Server | ------> | || --------->
S-S API +---------+| Query | | Query +---------+| S-S API
+---------+ +--------+ +---------+
The room server can annotate the events it logs to the readers with room state
so that the readers can avoid querying the room server unnecessarily.
[This architecture can be extended to cover most of the APIs.](WIRING.md)
# TODO
- [ ] gomatrixlib
- [x] Canonical JSON.
- [x] Signed JSON.
- [ ] Event hashing.
- [ ] Event signing.
- [x] Federation server discovery.
- [x] Federation key lookup.
- [ ] Federation request signing.
- [x] Event authentication.
- [ ] Event visibility.
- [ ] State resolution.
- [ ] Third party invites authentication.
- [ ] Room Server
- [ ] Inputing new events from logs.
- [ ] Inputing backfilled events from logs.
- [ ] Outputing events and current state to logs.
- [ ] Querying state at an event.
- [ ] Querying current forward extremities and state.
- [ ] Querying message history.
- [ ] Exporting/importing messages from other servers.
- [ ] Other Room Server stuff.
- [ ] Client Room Send
- [ ] Handling /client/r0/room/... HTTP PUTs
- [ ] Talk to remote servers for joins to remote servers.
- [ ] Outputting new events to logs.
- [ ] Updating the last active time in presence.
- [ ] Other Client Room Send stuff.
- [ ] Client Sync
- [ ] Inputing new room events and state from the logs.
- [ ] Handling /client/r0/sync HTTP GETs
- [ ] Outputing whether the client is syncing to the logs.
- [ ] Other Client Sync Stuff.
- [ ] Other Components.

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# Wiring
The diagram is incomplete. The following things aren't shown on the diagram:
- [ ] Device Messages
- [ ] User Profiles
- [ ] Notification Counts
- [ ] Sending federation.
- [ ] Querying federation.
- [ ] Other things that aren't shown on the diagram.
Diagram:
W -> Writer
S -> Server/Store/Service/Something/Stuff
R -> Reader
+---+ +---+ +---+
+----------| W | +----------| S | +--------| R |
| +---+ | Receipts +---+ | Client +---+
| Federation |>=========================================>| Server |>=====================>| Sync |
| Send | | | | |
| | +---+ | | | |
| | +--------| W | | | | |
| | | Client +---+ | | | |
| | | Receipt |>=====>| | | |
| | | Updater | | | | |
| | +----------+ | | | |
| | | | | |
| | +---+ +---+ | | +---+ | |
| | +------------| W | +------| S | | | +--------| R | | |
| | | Federation +---+ | Room +---+ | | | Client +---+ | |
| | | Backfill |>=====>| Server |>=====>| |>=====>| Push | | |
| | +--------------+ | | +------------+ | | | |
| | | | | | | |
| | | |>==========================>| | | |
| | | | +----------+ | |
| | | | | |
| | | | +---+ | |
| | | | +--------| R | | |
| | | | | Client +---+ | |
| |>========================>| |>==========================>| Search | | |
| | | | | | | |
| | | | +----------+ | |
| | | | | |
| | | |>==========================================>| |
| | | | | |
| | +---+ | | +---+ | |
| | +--------| W | | | +----------| S | | |
| | | Client +---+ | | | Presence +---+ | |
| | | Room |>=====>| |>=====>| Server |>=====================>| |
| | | Send | +--------+ | | | |
| | | | | | | |
| | | |>======================>| |<=====================<| |
| | +----------+ | | | |
| | | | | |
| | +---+ | | | |
| | +--------| W | | | | |
| | | Client +---+ | | | |
| | | Presence |>=====>| | | |
| | | Setter | | | | |
| | +----------+ | | | |
| | | | | |
| | | | | |
| |>=========================================>| | | |
| | +------------+ | |
| | | |
| | +---+ | |
| | +----------| S | | |
| | | Typing +---+ | |
| |>=========================================>| Server |>=====================>| |
+------------+ | | +----------+
+---+ | |
+--------| W | | |
| Client +---+ | |
| Typing |>=====>| |
| Setter | | |
+----------+ +------------+
# Component Descriptions
Many of the components are logical rather than physical. For example it is
possible that all of the client API writers will end up being glued together
and always deployed as a single unit.
Outbound federation requests will probably need to be funnelled through a
choke-point to implement ratelimiting and backoff correctly.
## Federation Send
* Handles `/federation/v1/send/` requests.
* Fetches missing ``prev_events`` from the remote server if needed.
* Fetches missing room state from the remote server if needed.
* Checks signatures on remote events, downloading keys if needed.
* Queries information needed to process events from the Room Server.
* Writes room events to logs.
* Writes presence updates to logs.
* Writes receipt updates to logs.
* Writes typing updates to logs.
* Writes other updates to logs.
## Client Room Send
* Handles puts to `/client/v1/rooms/` that create room events.
* Queries information needed to process events from the Room Server.
* Talks to remote servers if needed for joins and invites.
* Writes room event pdus.
* Writes presence updates to logs.
## Client Presence Setter
* Handles puts to whatever the client API path for presence is?
* Writes presence updates to logs.
## Client Typing Setter
* Handles puts to whatever the client API path for typing is?
* Writes typing updates to logs.
## Client Receipt Updater
* Handles puts to whatever the client API path for receipts is?
* Writes typing updates to logs.
## Federation Backfill
* Backfills events from other servers
* Writes the resulting room events to logs.
* Is a different component from the room server itself cause it'll
be easier if the room server component isn't making outbound HTTP requests
to remote servers
## Room Server
* Reads new and backfilled room events from the logs written by FS, FB and CRS.
* Tracks the current state of the room and the state at each event.
* Probably does auth checks on the incoming events.
* Handles state resolution as part of working out the current state and the
* state at each event.
* Writes updates to the current state and new events to logs.
* Shards by room ID.
## Receipt Server
* Reads new updates to receipts from the logs written by the FS and CRU.
* Somehow learns enough information from the room server to workout how the
current receipt markers move with each update.
* Writes the new marker positions to logs
* Shards by room ID?
* It may be impossible to implement without folding it into the Room Server
forever coupling the components together.
## Typing Server
* Reads new updates to typing from the logs written by the FS and CTS.
* Updates the current list of people typing in a room.
* Writes the current list of people typing in a room to the logs.
* Shards by room ID?
## Presence Server
* Reads the current state of the rooms from the logs to track the intersection
of room membership between users.
* Reads updates to presence from the logs writen by the FS and the CPS.
* Reads when clients sync from the logs from the Client Sync.
* Tracks any timers for users.
* Writes the changes to presence state to the logs.
* Shards by user ID somehow?
## Client Sync
* Handle /client/v2/sync requests.
* Reads new events and the current state of the rooms from logs written by the Room Server.
* Reads new receipts positions from the logs written by the Receipts Server.
* Reads changes to presence from the logs written by the Presence Server.
* Reads changes to typing from the logs written by the Typing Server.
* Writes when a client starts and stops syncing to the logs.
## Client Search
* Handle whatever the client API path for event search is?
* Reads new events and the current state of the rooms from logs writeen by the Room Server.
* Maintains a full text search index of somekind.
## Client Push
* Pushes unread messages to remote push servers.
* Reads new events and the current state of the rooms from logs writeen by the Room Server.
* Reads the position of the read marker from the Receipts Server.
* Makes outbound HTTP hits to the push server for the client device.