diff options
Diffstat (limited to 'docs/development/synapse_architecture/faster_joins.md')
| -rw-r--r-- | docs/development/synapse_architecture/faster_joins.md | 375 |
1 files changed, 375 insertions, 0 deletions
diff --git a/docs/development/synapse_architecture/faster_joins.md b/docs/development/synapse_architecture/faster_joins.md new file mode 100644 index 0000000000..c32d713b8a --- /dev/null +++ b/docs/development/synapse_architecture/faster_joins.md @@ -0,0 +1,375 @@ +# How do faster joins work? + +This is a work-in-progress set of notes with two goals: +- act as a reference, explaining how Synapse implements faster joins; and +- record the rationale behind our choices. + +See also [MSC3902](https://github.com/matrix-org/matrix-spec-proposals/pull/3902). + +The key idea is described by [MSC706](https://github.com/matrix-org/matrix-spec-proposals/pull/3902). This allows servers to +request a lightweight response to the federation `/send_join` endpoint. +This is called a **faster join**, also known as a **partial join**. In these +notes we'll usually use the word "partial" as it matches the database schema. + +## Overview: processing events in a partially-joined room + +The response to a partial join consists of +- the requested join event `J`, +- a list of the servers in the room (according to the state before `J`), +- a subset of the state of the room before `J`, +- the full auth chain of that state subset. + +Synapse marks the room as partially joined by adding a row to the database table +`partial_state_rooms`. It also marks the join event `J` as "partially stated", +meaning that we have neither received nor computed the full state before/after +`J`. This is done by adding a row to `partial_state_events`. + +<details><summary>DB schema</summary> + +``` +matrix=> \d partial_state_events +Table "matrix.partial_state_events" + Column │ Type │ Collation │ Nullable │ Default +══════════╪══════╪═══════════╪══════════╪═════════ + room_id │ text │ │ not null │ + event_id │ text │ │ not null │ + +matrix=> \d partial_state_rooms + Table "matrix.partial_state_rooms" + Column │ Type │ Collation │ Nullable │ Default +════════════════════════╪════════╪═══════════╪══════════╪═════════ + room_id │ text │ │ not null │ + device_lists_stream_id │ bigint │ │ not null │ 0 + join_event_id │ text │ │ │ + joined_via │ text │ │ │ + +matrix=> \d partial_state_rooms_servers + Table "matrix.partial_state_rooms_servers" + Column │ Type │ Collation │ Nullable │ Default +═════════════╪══════╪═══════════╪══════════╪═════════ + room_id │ text │ │ not null │ + server_name │ text │ │ not null │ +``` + +Indices, foreign-keys and check constraints are omitted for brevity. +</details> + +While partially joined to a room, Synapse receives events `E` from remote +homeservers as normal, and can create events at the request of its local users. +However, we run into trouble when we enforce the [checks on an event]. + +> 1. Is a valid event, otherwise it is dropped. For an event to be valid, it + must contain a room_id, and it must comply with the event format of that +> room version. +> 2. Passes signature checks, otherwise it is dropped. +> 3. Passes hash checks, otherwise it is redacted before being processed further. +> 4. Passes authorization rules based on the event’s auth events, otherwise it +> is rejected. +> 5. **Passes authorization rules based on the state before the event, otherwise +> it is rejected.** +> 6. **Passes authorization rules based on the current state of the room, +> otherwise it is “soft failed”.** + +[checks on an event]: https://spec.matrix.org/v1.5/server-server-api/#checks-performed-on-receipt-of-a-pdu + +We can enforce checks 1--4 without any problems. +But we cannot enforce checks 5 or 6 with complete certainty, since Synapse does +not know the full state before `E`, nor that of the room. + +### Partial state + +Instead, we make a best-effort approximation. +While the room is considered partially joined, Synapse tracks the "partial +state" before events. +This works in a similar way as regular state: + +- The partial state before `J` is that given to us by the partial join response. +- The partial state before an event `E` is the resolution of the partial states + after each of `E`'s `prev_event`s. +- If `E` is rejected or a message event, the partial state after `E` is the + partial state before `E`. +- Otherwise, the partial state after `E` is the partial state before `E`, plus + `E` itself. + +More concisely, partial state propagates just like full state; the only +difference is that we "seed" it with an incomplete initial state. +Synapse records that we have only calculated partial state for this event with +a row in `partial_state_events`. + +While the room remains partially stated, check 5 on incoming events to that +room becomes: + +> 5. Passes authorization rules based on **the resolution between the partial +> state before `E` and `E`'s auth events.** If the event fails to pass +> authorization rules, it is rejected. + +Additionally, check 6 is deleted: no soft-failures are enforced. + +While partially joined, the current partial state of the room is defined as the +resolution across the partial states after all forward extremities in the room. + +_Remark._ Events with partial state are _not_ considered +[outliers](../room-dag-concepts.md#outliers). + +### Approximation error + +Using partial state means the auth checks can fail in a few different ways[^2]. + +[^2]: Is this exhaustive? + +- We may erroneously accept an incoming event in check 5 based on partial state + when it would have been rejected based on full state, or vice versa. +- This means that an event could erroneously be added to the current partial + state of the room when it would not be present in the full state of the room, + or vice versa. +- Additionally, we may have skipped soft-failing an event that would have been + soft-failed based on full state. + +(Note that the discrepancies described in the last two bullets are user-visible.) + +This means that we have to be very careful when we want to lookup pieces of room +state in a partially-joined room. Our approximation of the state may be +incorrect or missing. But we can make some educated guesses. If + +- our partial state is likely to be correct, or +- the consequences of our partial state being incorrect are minor, + +then we proceed as normal, and let the resync process fix up any mistakes (see +below). + +When is our partial state likely to be correct? + +- It's more accurate the closer we are to the partial join event. (So we should + ideally complete the resync as soon as possible.) +- Non-member events: we will have received them as part of the partial join + response, if they were part of the room state at that point. We may + incorrectly accept or reject updates to that state (at first because we lack + remote membership information; later because of compounding errors), so these + can become incorrect over time. +- Local members' memberships: we are the only ones who can create join and + knock events for our users. We can't be completely confident in the + correctness of bans, invites and kicks from other homeservers, but the resync + process should correct any mistakes. +- Remote members' memberships: we did not receive these in the /send_join + response, so we have essentially no idea if these are correct or not. + +In short, we deem it acceptable to trust the partial state for non-membership +and local membership events. For remote membership events, we wait for the +resync to complete, at which point we have the full state of the room and can +proceed as normal. + +### Fixing the approximation with a resync + +The partial-state approximation is only a temporary affair. In the background, +synapse beings a "resync" process. This is a continuous loop, starting at the +partial join event and proceeding downwards through the event graph. For each +`E` seen in the room since partial join, Synapse will fetch + +- the event ids in the state of the room before `E`, via + [`/state_ids`](https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1state_idsroomid); +- the event ids in the full auth chain of `E`, included in the `/state_ids` + response; and +- any events from the previous two bullets that Synapse hasn't persisted, via + [`/state](https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1stateroomid). + +This means Synapse has (or can compute) the full state before `E`, which allows +Synapse to properly authorise or reject `E`. At this point ,the event +is considered to have "full state" rather than "partial state". We record this +by removing `E` from the `partial_state_events` table. + +\[**TODO:** Does Synapse persist a new state group for the full state +before `E`, or do we alter the (partial-)state group in-place? Are state groups +ever marked as partially-stated? \] + +This scheme means it is possible for us to have accepted and sent an event to +clients, only to reject it during the resync. From a client's perspective, the +effect is similar to a retroactive +state change due to state resolution---i.e. a "state reset".[^3] + +[^3]: Clients should refresh caches to detect such a change. Rumour has it that +sliding sync will fix this. + +When all events since the join `J` have been fully-stated, the room resync +process is complete. We record this by removing the room from +`partial_state_rooms`. + +## Faster joins on workers + +For the time being, the resync process happens on the master worker. +A new replication stream `un_partial_stated_room` is added. Whenever a resync +completes and a partial-state room becomes fully stated, a new message is sent +into that stream containing the room ID. + +## Notes on specific cases + +> **NB.** The notes below are rough. Some of them are hidden under `<details>` +disclosures because they have yet to be implemented in mainline Synapse. + +### Creating events during a partial join + +When sending out messages during a partial join, we assume our partial state is +accurate and proceed as normal. For this to have any hope of succeeding at all, +our partial state must contain an entry for each of the (type, state key) pairs +[specified by the auth rules](https://spec.matrix.org/v1.3/rooms/v10/#authorization-rules): + +- `m.room.create` +- `m.room.join_rules` +- `m.room.power_levels` +- `m.room.third_party_invite` +- `m.room.member` + +The first four of these should be present in the state before `J` that is given +to us in the partial join response; only membership events are omitted. In order +for us to consider the user joined, we must have their membership event. That +means the only possible omission is the target's membership in an invite, kick +or ban. + +The worst possibility is that we locally invite someone who is banned according to +the full state, because we lack their ban in our current partial state. The rest +of the federation---at least, those who are fully joined---should correctly +enforce the [membership transition constraints]( + https://spec.matrix.org/v1.3/client-server-api/#room-membership +). So any the erroneous invite should be ignored by fully-joined +homeservers and resolved by the resync for partially-joined homeservers. + + + +In more generality, there are two problems we're worrying about here: + +- We might create an event that is valid under our partial state, only to later + find out that is actually invalid according to the full state. +- Or: we might refuse to create an event that is invalid under our partial + state, even though it would be perfectly valid under the full state. + +However we expect such problems to be unlikely in practise, because + +- We trust that the room has sensible power levels, e.g. that bad actors with + high power levels are demoted before their ban. +- We trust that the resident server provides us up-to-date power levels, join + rules, etc. +- State changes in rooms are relatively infrequent, and the resync period is + relatively quick. + +#### Sending out the event over federation + +**TODO:** needs prose fleshing out. + +Normally: send out in a fed txn to all HSes in the room. +We only know that some HSes were in the room at some point. Wat do. +Send it out to the list of servers from the first join. +**TODO** what do we do here if we have full state? +If the prev event was created by us, we can risk sending it to the wrong HS. (Motivation: privacy concern of the content. Not such a big deal for a public room or an encrypted room. But non-encrypted invite-only...) +But don't want to send out sensitive data in other HS's events in this way. + +Suppose we discover after resync that we shouldn't have sent out one our events (not a prev_event) to a target HS. Not much we can do. +What about if we didn't send them an event but shouldn't've? +E.g. what if someone joined from a new HS shortly after you did? We wouldn't talk to them. +Could imagine sending out the "Missed" events after the resync but... painful to work out what they shuld have seen if they joined/left. +Instead, just send them the latest event (if they're still in the room after resync) and let them backfill.(?) +- Don't do this currently. +- If anyone who has received our messages sends a message to a HS we missed, they can backfill our messages +- Gap: rooms which are infrequently used and take a long time to resync. + +### Joining after a partial join + +**NB.** Not yet implemented. + +<details> + +**TODO:** needs prose fleshing out. Liase with Matthieu. Explain why /send_join +(Rich was surprised we didn't just create it locally. Answer: to try and avoid +a join which then gets rejected after resync.) + +We don't know for sure that any join we create would be accepted. +E.g. the joined user might have been banned; the join rules might have changed in a way that we didn't realise... some way in which the partial state was mistaken. +Instead, do another partial make-join/send-join handshake to confirm that the join works. +- Probably going to get a bunch of duplicate state events and auth events.... but the point of partial joins is that these should be small. Many are already persisted = good. +- What if the second send_join response includes a different list of reisdent HSes? Could ignore it. + - Could even have a special flag that says "just make me a join", i.e. don't bother giving me state or servers in room. Deffo want the auth chain tho. +- SQ: wrt device lists it's a lot safer to ignore it!!!!! +- What if the state at the second join is inconsistent with what we have? Ignore it? + +</details> + +### Leaving (and kicks and bans) after a partial join + +**NB.** Not yet implemented. + +<details> + +When you're fully joined to a room, to have `U` leave a room their homeserver +needs to + +- create a new leave event for `U` which will be accepted by other homeservers, + and +- send that event `U` out to the homeservers in the federation. + +When is a leave event accepted? See +[v10 auth rules](https://spec.matrix.org/v1.5/rooms/v10/#authorization-rules): + +> 4. If type is m.room.member: [...] + > + > 5. If membership is leave: + > + > 1. If the sender matches state_key, allow if and only if that user’s current membership state is invite, join, or knock. +> 2. [...] + +I think this means that (well-formed!) self-leaves are governed entirely by +4.5.1. This means that if we correctly calculate state which says that `U` is +invited, joined or knocked and include it in the leave's auth events, our event +is accepted by checks 4 and 5 on incoming events. + +> 4. Passes authorization rules based on the event’s auth events, otherwise + > it is rejected. +> 5. Passes authorization rules based on the state before the event, otherwise + > it is rejected. + +The only way to fail check 6 is if the receiving server's current state of the +room says that `U` is banned, has left, or has no membership event. But this is +fine: the receiving server already thinks that `U` isn't in the room. + +> 6. Passes authorization rules based on the current state of the room, + > otherwise it is “soft failed”. + +For the second point (publishing the leave event), the best thing we can do is +to is publish to all HSes we know to be currently in the room. If they miss that +event, they might send us traffic in the room that we don't care about. This is +a problem with leaving after a "full" join; we don't seek to fix this with +partial joins. + +(With that said: there's nothing machine-readable in the /send response. I don't +think we can deduce "destination has left the room" from a failure to /send an +event into that room?) + +#### Can we still do this during a partial join? + +We can create leave events and can choose what gets included in our auth events, +so we can be sure that we pass check 4 on incoming events. For check 5, we might +have an incorrect view of the state before an event. +The only way we might erroneously think a leave is valid is if + +- the partial state before the leave has `U` joined, invited or knocked, but +- the full state before the leave has `U` banned, left or not present, + +in which case the leave doesn't make anything worse: other HSes already consider +us as not in the room, and will continue to do so after seeing the leave. + +The remaining obstacle is then: can we safely broadcast the leave event? We may +miss servers or incorrectly think that a server is in the room. Or the +destination server may be offline and miss the transaction containing our leave +event.This should self-heal when they see an event whose `prev_events` descends +from our leave. + +Another option we considered was to use federation `/send_leave` to ask a +fully-joined server to send out the event on our behalf. But that introduces +complexity without much benefit. Besides, as Rich put it, + +> sending out leaves is pretty best-effort currently + +so this is probably good enough as-is. + +#### Cleanup after the last leave + +**TODO**: what cleanup is necessary? Is it all just nice-to-have to save unused +work? +</details> |