Initial batch of notes on faster joins (#14677)
Co-authored-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org> Co-authored-by: Shay <hillerys@element.io>
This commit is contained in:
David Robertson
GitHub
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Describe the ideas and the internal machinery behind faster joins.
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- [Log Contexts](log_contexts.md)
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- [Replication](replication.md)
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- [TCP Replication](tcp_replication.md)
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- [Faster remote joins](development/synapse_architecture/faster_joins.md)
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- [Internal Documentation](development/internal_documentation/README.md)
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- [Single Sign-On]()
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- [SAML](development/saml.md)
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# How do faster joins work?
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This is a work-in-progress set of notes with two goals:
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- act as a reference, explaining how Synapse implements faster joins; and
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- record the rationale behind our choices.
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See also [MSC3902](https://github.com/matrix-org/matrix-spec-proposals/pull/3902).
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The key idea is described by [MSC706](https://github.com/matrix-org/matrix-spec-proposals/pull/3902). This allows servers to
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request a lightweight response to the federation `/send_join` endpoint.
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This is called a **faster join**, also known as a **partial join**. In these
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notes we'll usually use the word "partial" as it matches the database schema.
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## Overview: processing events in a partially-joined room
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The response to a partial join consists of
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- the requested join event `J`,
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- a list of the servers in the room (according to the state before `J`),
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- a subset of the state of the room before `J`,
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- the full auth chain of that state subset.
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Synapse marks the room as partially joined by adding a row to the database table
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`partial_state_rooms`. It also marks the join event `J` as "partially stated",
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meaning that we have neither received nor computed the full state before/after
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`J`. This is done by adding a row to `partial_state_events`.
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<details><summary>DB schema</summary>
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```
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matrix=> \d partial_state_events
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Table "matrix.partial_state_events"
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Column │ Type │ Collation │ Nullable │ Default
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══════════╪══════╪═══════════╪══════════╪═════════
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room_id │ text │ │ not null │
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event_id │ text │ │ not null │
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matrix=> \d partial_state_rooms
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Table "matrix.partial_state_rooms"
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Column │ Type │ Collation │ Nullable │ Default
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════════════════════════╪════════╪═══════════╪══════════╪═════════
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room_id │ text │ │ not null │
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device_lists_stream_id │ bigint │ │ not null │ 0
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join_event_id │ text │ │ │
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joined_via │ text │ │ │
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matrix=> \d partial_state_rooms_servers
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Table "matrix.partial_state_rooms_servers"
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Column │ Type │ Collation │ Nullable │ Default
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═════════════╪══════╪═══════════╪══════════╪═════════
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room_id │ text │ │ not null │
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server_name │ text │ │ not null │
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```
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Indices, foreign-keys and check constraints are omitted for brevity.
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</details>
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While partially joined to a room, Synapse receives events `E` from remote
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homeservers as normal, and can create events at the request of its local users.
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However, we run into trouble when we enforce the [checks on an event].
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> 1. Is a valid event, otherwise it is dropped. For an event to be valid, it
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must contain a room_id, and it must comply with the event format of that
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> room version.
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> 2. Passes signature checks, otherwise it is dropped.
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> 3. Passes hash checks, otherwise it is redacted before being processed further.
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> 4. Passes authorization rules based on the event’s auth events, otherwise it
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> is rejected.
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> 5. **Passes authorization rules based on the state before the event, otherwise
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> it is rejected.**
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> 6. **Passes authorization rules based on the current state of the room,
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> otherwise it is “soft failed”.**
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[checks on an event]: https://spec.matrix.org/v1.5/server-server-api/#checks-performed-on-receipt-of-a-pdu
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We can enforce checks 1--4 without any problems.
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But we cannot enforce checks 5 or 6 with complete certainty, since Synapse does
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not know the full state before `E`, nor that of the room.
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### Partial state
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Instead, we make a best-effort approximation.
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While the room is considered partially joined, Synapse tracks the "partial
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state" before events.
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This works in a similar way as regular state:
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- The partial state before `J` is that given to us by the partial join response.
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- The partial state before an event `E` is the resolution of the partial states
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after each of `E`'s `prev_event`s.
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- If `E` is rejected or a message event, the partial state after `E` is the
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partial state before `E`.
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- Otherwise, the partial state after `E` is the partial state before `E`, plus
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`E` itself.
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More concisely, partial state propagates just like full state; the only
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difference is that we "seed" it with an incomplete initial state.
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Synapse records that we have only calculated partial state for this event with
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a row in `partial_state_events`.
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While the room remains partially stated, check 5 on incoming events to that
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room becomes:
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> 5. Passes authorization rules based on **the resolution between the partial
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> state before `E` and `E`'s auth events.** If the event fails to pass
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> authorization rules, it is rejected.
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Additionally, check 6 is deleted: no soft-failures are enforced.
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While partially joined, the current partial state of the room is defined as the
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resolution across the partial states after all forward extremities in the room.
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_Remark._ Events with partial state are _not_ considered
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[outliers](../room-dag-concepts.md#outliers).
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### Approximation error
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Using partial state means the auth checks can fail in a few different ways[^2].
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[^2]: Is this exhaustive?
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- We may erroneously accept an incoming event in check 5 based on partial state
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when it would have been rejected based on full state, or vice versa.
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- This means that an event could erroneously be added to the current partial
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state of the room when it would not be present in the full state of the room,
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or vice versa.
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- Additionally, we may have skipped soft-failing an event that would have been
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soft-failed based on full state.
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(Note that the discrepancies described in the last two bullets are user-visible.)
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This means that we have to be very careful when we want to lookup pieces of room
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state in a partially-joined room. Our approximation of the state may be
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incorrect or missing. But we can make some educated guesses. If
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- our partial state is likely to be correct, or
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- the consequences of our partial state being incorrect are minor,
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then we proceed as normal, and let the resync process fix up any mistakes (see
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below).
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When is our partial state likely to be correct?
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- It's more accurate the closer we are to the partial join event. (So we should
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ideally complete the resync as soon as possible.)
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- Non-member events: we will have received them as part of the partial join
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response, if they were part of the room state at that point. We may
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incorrectly accept or reject updates to that state (at first because we lack
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remote membership information; later because of compounding errors), so these
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can become incorrect over time.
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- Local members' memberships: we are the only ones who can create join and
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knock events for our users. We can't be completely confident in the
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correctness of bans, invites and kicks from other homeservers, but the resync
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process should correct any mistakes.
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- Remote members' memberships: we did not receive these in the /send_join
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response, so we have essentially no idea if these are correct or not.
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In short, we deem it acceptable to trust the partial state for non-membership
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and local membership events. For remote membership events, we wait for the
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resync to complete, at which point we have the full state of the room and can
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proceed as normal.
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### Fixing the approximation with a resync
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The partial-state approximation is only a temporary affair. In the background,
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synapse beings a "resync" process. This is a continuous loop, starting at the
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partial join event and proceeding downwards through the event graph. For each
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`E` seen in the room since partial join, Synapse will fetch
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- the event ids in the state of the room before `E`, via
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[`/state_ids`](https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1state_idsroomid);
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- the event ids in the full auth chain of `E`, included in the `/state_ids`
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response; and
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- any events from the previous two bullets that Synapse hasn't persisted, via
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[`/state](https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1stateroomid).
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This means Synapse has (or can compute) the full state before `E`, which allows
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Synapse to properly authorise or reject `E`. At this point ,the event
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is considered to have "full state" rather than "partial state". We record this
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by removing `E` from the `partial_state_events` table.
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\[**TODO:** Does Synapse persist a new state group for the full state
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before `E`, or do we alter the (partial-)state group in-place? Are state groups
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ever marked as partially-stated? \]
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This scheme means it is possible for us to have accepted and sent an event to
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clients, only to reject it during the resync. From a client's perspective, the
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effect is similar to a retroactive
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state change due to state resolution---i.e. a "state reset".[^3]
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[^3]: Clients should refresh caches to detect such a change. Rumour has it that
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sliding sync will fix this.
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When all events since the join `J` have been fully-stated, the room resync
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process is complete. We record this by removing the room from
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`partial_state_rooms`.
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## Faster joins on workers
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For the time being, the resync process happens on the master worker.
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A new replication stream `un_partial_stated_room` is added. Whenever a resync
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completes and a partial-state room becomes fully stated, a new message is sent
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into that stream containing the room ID.
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## Notes on specific cases
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> **NB.** The notes below are rough. Some of them are hidden under `<details>`
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disclosures because they have yet to be implemented in mainline Synapse.
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### Creating events during a partial join
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When sending out messages during a partial join, we assume our partial state is
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accurate and proceed as normal. For this to have any hope of succeeding at all,
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our partial state must contain an entry for each of the (type, state key) pairs
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[specified by the auth rules](https://spec.matrix.org/v1.3/rooms/v10/#authorization-rules):
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- `m.room.create`
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- `m.room.join_rules`
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- `m.room.power_levels`
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- `m.room.third_party_invite`
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- `m.room.member`
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The first four of these should be present in the state before `J` that is given
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to us in the partial join response; only membership events are omitted. In order
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for us to consider the user joined, we must have their membership event. That
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means the only possible omission is the target's membership in an invite, kick
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or ban.
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The worst possibility is that we locally invite someone who is banned according to
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the full state, because we lack their ban in our current partial state. The rest
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of the federation---at least, those who are fully joined---should correctly
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enforce the [membership transition constraints](
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https://spec.matrix.org/v1.3/client-server-api/#room-membership
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). So any the erroneous invite should be ignored by fully-joined
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homeservers and resolved by the resync for partially-joined homeservers.
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In more generality, there are two problems we're worrying about here:
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- We might create an event that is valid under our partial state, only to later
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find out that is actually invalid according to the full state.
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- Or: we might refuse to create an event that is invalid under our partial
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state, even though it would be perfectly valid under the full state.
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However we expect such problems to be unlikely in practise, because
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- We trust that the room has sensible power levels, e.g. that bad actors with
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high power levels are demoted before their ban.
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- We trust that the resident server provides us up-to-date power levels, join
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rules, etc.
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- State changes in rooms are relatively infrequent, and the resync period is
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relatively quick.
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#### Sending out the event over federation
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**TODO:** needs prose fleshing out.
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Normally: send out in a fed txn to all HSes in the room.
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We only know that some HSes were in the room at some point. Wat do.
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Send it out to the list of servers from the first join.
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**TODO** what do we do here if we have full state?
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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...)
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But don't want to send out sensitive data in other HS's events in this way.
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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.
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What about if we didn't send them an event but shouldn't've?
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E.g. what if someone joined from a new HS shortly after you did? We wouldn't talk to them.
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Could imagine sending out the "Missed" events after the resync but... painful to work out what they shuld have seen if they joined/left.
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Instead, just send them the latest event (if they're still in the room after resync) and let them backfill.(?)
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- Don't do this currently.
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- If anyone who has received our messages sends a message to a HS we missed, they can backfill our messages
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- Gap: rooms which are infrequently used and take a long time to resync.
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### Joining after a partial join
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**NB.** Not yet implemented.
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<details>
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**TODO:** needs prose fleshing out. Liase with Matthieu. Explain why /send_join
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(Rich was surprised we didn't just create it locally. Answer: to try and avoid
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a join which then gets rejected after resync.)
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We don't know for sure that any join we create would be accepted.
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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.
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Instead, do another partial make-join/send-join handshake to confirm that the join works.
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- 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.
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- What if the second send_join response includes a different list of reisdent HSes? Could ignore it.
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- 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.
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- SQ: wrt device lists it's a lot safer to ignore it!!!!!
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- What if the state at the second join is inconsistent with what we have? Ignore it?
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</details>
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### Leaving (and kicks and bans) after a partial join
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**NB.** Not yet implemented.
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<details>
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When you're fully joined to a room, to have `U` leave a room their homeserver
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needs to
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- create a new leave event for `U` which will be accepted by other homeservers,
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and
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- send that event `U` out to the homeservers in the federation.
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When is a leave event accepted? See
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[v10 auth rules](https://spec.matrix.org/v1.5/rooms/v10/#authorization-rules):
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> 4. If type is m.room.member: [...]
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>
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> 5. If membership is leave:
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>
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> 1. If the sender matches state_key, allow if and only if that user’s current membership state is invite, join, or knock.
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> 2. [...]
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I think this means that (well-formed!) self-leaves are governed entirely by
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4.5.1. This means that if we correctly calculate state which says that `U` is
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invited, joined or knocked and include it in the leave's auth events, our event
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is accepted by checks 4 and 5 on incoming events.
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> 4. Passes authorization rules based on the event’s auth events, otherwise
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> it is rejected.
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> 5. Passes authorization rules based on the state before the event, otherwise
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> it is rejected.
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The only way to fail check 6 is if the receiving server's current state of the
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room says that `U` is banned, has left, or has no membership event. But this is
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fine: the receiving server already thinks that `U` isn't in the room.
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> 6. Passes authorization rules based on the current state of the room,
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> otherwise it is “soft failed”.
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For the second point (publishing the leave event), the best thing we can do is
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to is publish to all HSes we know to be currently in the room. If they miss that
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event, they might send us traffic in the room that we don't care about. This is
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a problem with leaving after a "full" join; we don't seek to fix this with
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partial joins.
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(With that said: there's nothing machine-readable in the /send response. I don't
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think we can deduce "destination has left the room" from a failure to /send an
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event into that room?)
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#### Can we still do this during a partial join?
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We can create leave events and can choose what gets included in our auth events,
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so we can be sure that we pass check 4 on incoming events. For check 5, we might
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have an incorrect view of the state before an event.
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The only way we might erroneously think a leave is valid is if
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- the partial state before the leave has `U` joined, invited or knocked, but
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- the full state before the leave has `U` banned, left or not present,
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in which case the leave doesn't make anything worse: other HSes already consider
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us as not in the room, and will continue to do so after seeing the leave.
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The remaining obstacle is then: can we safely broadcast the leave event? We may
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miss servers or incorrectly think that a server is in the room. Or the
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destination server may be offline and miss the transaction containing our leave
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event.This should self-heal when they see an event whose `prev_events` descends
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from our leave.
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Another option we considered was to use federation `/send_leave` to ask a
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fully-joined server to send out the event on our behalf. But that introduces
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complexity without much benefit. Besides, as Rich put it,
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> sending out leaves is pretty best-effort currently
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so this is probably good enough as-is.
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#### Cleanup after the last leave
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**TODO**: what cleanup is necessary? Is it all just nice-to-have to save unused
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work?
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</details>
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