Initial batch of notes on faster joins (#14677)
Co-authored-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org> Co-authored-by: Shay <hillerys@element.io>release-v1.76
David Robertson
GitHub
parent
fca5617a0d
commit
cbb0ee43cc
|
|
@ -0,0 +1 @@
|
|||
Describe the ideas and the internal machinery behind faster joins.
|
||||
|
|
@ -97,6 +97,7 @@
|
|||
- [Log Contexts](log_contexts.md)
|
||||
- [Replication](replication.md)
|
||||
- [TCP Replication](tcp_replication.md)
|
||||
- [Faster remote joins](development/synapse_architecture/faster_joins.md)
|
||||
- [Internal Documentation](development/internal_documentation/README.md)
|
||||
- [Single Sign-On]()
|
||||
- [SAML](development/saml.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>
|
||||
Loading…
Reference in New Issue