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<h1 id="how-do-faster-joins-work"><a class="header" href="#how-do-faster-joins-work">How do faster joins work?</a></h1>
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<p>This is a work-in-progress set of notes with two goals:</p>
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<ul>
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<li>act as a reference, explaining how Synapse implements faster joins; and</li>
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<li>record the rationale behind our choices.</li>
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</ul>
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<p>See also <a href="https://github.com/matrix-org/matrix-spec-proposals/pull/3902">MSC3902</a>.</p>
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<p>The key idea is described by <a href="https://github.com/matrix-org/matrix-spec-proposals/pull/3706">MSC3706</a>. This allows servers to
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request a lightweight response to the federation <code>/send_join</code> endpoint.
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This is called a <strong>faster join</strong>, also known as a <strong>partial join</strong>. In these
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notes we'll usually use the word "partial" as it matches the database schema.</p>
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<h2 id="overview-processing-events-in-a-partially-joined-room"><a class="header" href="#overview-processing-events-in-a-partially-joined-room">Overview: processing events in a partially-joined room</a></h2>
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<p>The response to a partial join consists of</p>
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<ul>
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<li>the requested join event <code>J</code>,</li>
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<li>a list of the servers in the room (according to the state before <code>J</code>),</li>
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<li>a subset of the state of the room before <code>J</code>,</li>
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<li>the full auth chain of that state subset.</li>
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</ul>
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<p>Synapse marks the room as partially joined by adding a row to the database table
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<code>partial_state_rooms</code>. It also marks the join event <code>J</code> as "partially stated",
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meaning that we have neither received nor computed the full state before/after
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<code>J</code>. This is done by adding a row to <code>partial_state_events</code>.</p>
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<details><summary>DB schema</summary>
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<pre><code>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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</code></pre>
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<p>Indices, foreign-keys and check constraints are omitted for brevity.</p>
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</details>
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<p>While partially joined to a room, Synapse receives events <code>E</code> 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 <a href="https://spec.matrix.org/v1.5/server-server-api/#checks-performed-on-receipt-of-a-pdu">checks on an event</a>.</p>
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<blockquote>
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<ol>
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<li>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.</li>
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<li>Passes signature checks, otherwise it is dropped.</li>
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<li>Passes hash checks, otherwise it is redacted before being processed further.</li>
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<li>Passes authorization rules based on the event’s auth events, otherwise it
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is rejected.</li>
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<li><strong>Passes authorization rules based on the state before the event, otherwise
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it is rejected.</strong></li>
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||
<li><strong>Passes authorization rules based on the current state of the room,
|
||
otherwise it is “soft failed”.</strong></li>
|
||
</ol>
|
||
</blockquote>
|
||
<p>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 <code>E</code>, nor that of the room.</p>
|
||
<h3 id="partial-state"><a class="header" href="#partial-state">Partial state</a></h3>
|
||
<p>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:</p>
|
||
<ul>
|
||
<li>The partial state before <code>J</code> is that given to us by the partial join response.</li>
|
||
<li>The partial state before an event <code>E</code> is the resolution of the partial states
|
||
after each of <code>E</code>'s <code>prev_event</code>s.</li>
|
||
<li>If <code>E</code> is rejected or a message event, the partial state after <code>E</code> is the
|
||
partial state before <code>E</code>.</li>
|
||
<li>Otherwise, the partial state after <code>E</code> is the partial state before <code>E</code>, plus
|
||
<code>E</code> itself.</li>
|
||
</ul>
|
||
<p>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 <code>partial_state_events</code>.</p>
|
||
<p>While the room remains partially stated, check 5 on incoming events to that
|
||
room becomes:</p>
|
||
<blockquote>
|
||
<ol start="5">
|
||
<li>Passes authorization rules based on <strong>the resolution between the partial
|
||
state before <code>E</code> and <code>E</code>'s auth events.</strong> If the event fails to pass
|
||
authorization rules, it is rejected.</li>
|
||
</ol>
|
||
</blockquote>
|
||
<p>Additionally, check 6 is deleted: no soft-failures are enforced.</p>
|
||
<p>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.</p>
|
||
<p><em>Remark.</em> Events with partial state are <em>not</em> considered
|
||
<a href="../room-dag-concepts.html#outliers">outliers</a>.</p>
|
||
<h3 id="approximation-error"><a class="header" href="#approximation-error">Approximation error</a></h3>
|
||
<p>Using partial state means the auth checks can fail in a few different ways<sup class="footnote-reference"><a href="#2">1</a></sup>.</p>
|
||
<div class="footnote-definition" id="2"><sup class="footnote-definition-label">1</sup>
|
||
<p>Is this exhaustive?</p>
|
||
</div>
|
||
<ul>
|
||
<li>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.</li>
|
||
<li>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.</li>
|
||
<li>Additionally, we may have skipped soft-failing an event that would have been
|
||
soft-failed based on full state.</li>
|
||
</ul>
|
||
<p>(Note that the discrepancies described in the last two bullets are user-visible.)</p>
|
||
<p>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</p>
|
||
<ul>
|
||
<li>our partial state is likely to be correct, or</li>
|
||
<li>the consequences of our partial state being incorrect are minor,</li>
|
||
</ul>
|
||
<p>then we proceed as normal, and let the resync process fix up any mistakes (see
|
||
below).</p>
|
||
<p>When is our partial state likely to be correct?</p>
|
||
<ul>
|
||
<li>It's more accurate the closer we are to the partial join event. (So we should
|
||
ideally complete the resync as soon as possible.)</li>
|
||
<li>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.</li>
|
||
<li>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.</li>
|
||
<li>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.</li>
|
||
</ul>
|
||
<p>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.</p>
|
||
<h3 id="fixing-the-approximation-with-a-resync"><a class="header" href="#fixing-the-approximation-with-a-resync">Fixing the approximation with a resync</a></h3>
|
||
<p>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
|
||
<code>E</code> seen in the room since partial join, Synapse will fetch </p>
|
||
<ul>
|
||
<li>the event ids in the state of the room before <code>E</code>, via
|
||
<a href="https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1state_idsroomid"><code>/state_ids</code></a>;</li>
|
||
<li>the event ids in the full auth chain of <code>E</code>, included in the <code>/state_ids</code>
|
||
response; and</li>
|
||
<li>any events from the previous two bullets that Synapse hasn't persisted, via
|
||
<a href="https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1stateroomid">`/state</a>.</li>
|
||
</ul>
|
||
<p>This means Synapse has (or can compute) the full state before <code>E</code>, which allows
|
||
Synapse to properly authorise or reject <code>E</code>. At this point ,the event
|
||
is considered to have "full state" rather than "partial state". We record this
|
||
by removing <code>E</code> from the <code>partial_state_events</code> table.</p>
|
||
<p>[<strong>TODO:</strong> Does Synapse persist a new state group for the full state
|
||
before <code>E</code>, or do we alter the (partial-)state group in-place? Are state groups
|
||
ever marked as partially-stated? ]</p>
|
||
<p>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".<sup class="footnote-reference"><a href="#3">2</a></sup></p>
|
||
<div class="footnote-definition" id="3"><sup class="footnote-definition-label">2</sup>
|
||
<p>Clients should refresh caches to detect such a change. Rumour has it that
|
||
sliding sync will fix this.</p>
|
||
</div>
|
||
<p>When all events since the join <code>J</code> have been fully-stated, the room resync
|
||
process is complete. We record this by removing the room from
|
||
<code>partial_state_rooms</code>.</p>
|
||
<h2 id="faster-joins-on-workers"><a class="header" href="#faster-joins-on-workers">Faster joins on workers</a></h2>
|
||
<p>For the time being, the resync process happens on the master worker.
|
||
A new replication stream <code>un_partial_stated_room</code> 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.</p>
|
||
<h2 id="notes-on-specific-cases"><a class="header" href="#notes-on-specific-cases">Notes on specific cases</a></h2>
|
||
<blockquote>
|
||
<p><strong>NB.</strong> The notes below are rough. Some of them are hidden under <code><details></code>
|
||
disclosures because they have yet to be implemented in mainline Synapse.</p>
|
||
</blockquote>
|
||
<h3 id="creating-events-during-a-partial-join"><a class="header" href="#creating-events-during-a-partial-join">Creating events during a partial join</a></h3>
|
||
<p>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
|
||
<a href="https://spec.matrix.org/v1.3/rooms/v10/#authorization-rules">specified by the auth rules</a>:</p>
|
||
<ul>
|
||
<li><code>m.room.create</code></li>
|
||
<li><code>m.room.join_rules</code></li>
|
||
<li><code>m.room.power_levels</code></li>
|
||
<li><code>m.room.third_party_invite</code></li>
|
||
<li><code>m.room.member</code></li>
|
||
</ul>
|
||
<p>The first four of these should be present in the state before <code>J</code> 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.</p>
|
||
<p>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 <a href="https://spec.matrix.org/v1.3/client-server-api/#room-membership">membership transition constraints</a>. So any the erroneous invite should be ignored by fully-joined
|
||
homeservers and resolved by the resync for partially-joined homeservers.</p>
|
||
<p>In more generality, there are two problems we're worrying about here:</p>
|
||
<ul>
|
||
<li>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.</li>
|
||
<li>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.</li>
|
||
</ul>
|
||
<p>However we expect such problems to be unlikely in practise, because</p>
|
||
<ul>
|
||
<li>We trust that the room has sensible power levels, e.g. that bad actors with
|
||
high power levels are demoted before their ban.</li>
|
||
<li>We trust that the resident server provides us up-to-date power levels, join
|
||
rules, etc.</li>
|
||
<li>State changes in rooms are relatively infrequent, and the resync period is
|
||
relatively quick.</li>
|
||
</ul>
|
||
<h4 id="sending-out-the-event-over-federation"><a class="header" href="#sending-out-the-event-over-federation">Sending out the event over federation</a></h4>
|
||
<p><strong>TODO:</strong> needs prose fleshing out.</p>
|
||
<p>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.
|
||
<strong>TODO</strong> 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.</p>
|
||
<p>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 should 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.(?)</p>
|
||
<ul>
|
||
<li>Don't do this currently.</li>
|
||
<li>If anyone who has received our messages sends a message to a HS we missed, they can backfill our messages</li>
|
||
<li>Gap: rooms which are infrequently used and take a long time to resync.</li>
|
||
</ul>
|
||
<h3 id="joining-after-a-partial-join"><a class="header" href="#joining-after-a-partial-join">Joining after a partial join</a></h3>
|
||
<p><strong>NB.</strong> Not yet implemented.</p>
|
||
<details>
|
||
<p><strong>TODO:</strong> 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.)</p>
|
||
<p>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.</p>
|
||
<ul>
|
||
<li>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.</li>
|
||
<li>What if the second send_join response includes a different list of reisdent HSes? Could ignore it.
|
||
<ul>
|
||
<li>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.</li>
|
||
</ul>
|
||
</li>
|
||
<li>SQ: wrt device lists it's a lot safer to ignore it!!!!!</li>
|
||
<li>What if the state at the second join is inconsistent with what we have? Ignore it?</li>
|
||
</ul>
|
||
</details>
|
||
<h3 id="leaving-and-kicks-and-bans-after-a-partial-join"><a class="header" href="#leaving-and-kicks-and-bans-after-a-partial-join">Leaving (and kicks and bans) after a partial join</a></h3>
|
||
<p><strong>NB.</strong> Not yet implemented.</p>
|
||
<details>
|
||
<p>When you're fully joined to a room, to have <code>U</code> leave a room their homeserver
|
||
needs to</p>
|
||
<ul>
|
||
<li>create a new leave event for <code>U</code> which will be accepted by other homeservers,
|
||
and</li>
|
||
<li>send that event <code>U</code> out to the homeservers in the federation.</li>
|
||
</ul>
|
||
<p>When is a leave event accepted? See
|
||
<a href="https://spec.matrix.org/v1.5/rooms/v10/#authorization-rules">v10 auth rules</a>:</p>
|
||
<blockquote>
|
||
<ol start="4">
|
||
<li>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. [...]</li>
|
||
</ol>
|
||
</blockquote>
|
||
<p>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 <code>U</code> 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.</p>
|
||
<blockquote>
|
||
<ol start="4">
|
||
<li>Passes authorization rules based on the event’s auth events, otherwise
|
||
> it is rejected.</li>
|
||
<li>Passes authorization rules based on the state before the event, otherwise
|
||
> it is rejected.</li>
|
||
</ol>
|
||
</blockquote>
|
||
<p>The only way to fail check 6 is if the receiving server's current state of the
|
||
room says that <code>U</code> is banned, has left, or has no membership event. But this is
|
||
fine: the receiving server already thinks that <code>U</code> isn't in the room.</p>
|
||
<blockquote>
|
||
<ol start="6">
|
||
<li>Passes authorization rules based on the current state of the room,
|
||
> otherwise it is “soft failed”.</li>
|
||
</ol>
|
||
</blockquote>
|
||
<p>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.</p>
|
||
<p>(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?)</p>
|
||
<h4 id="can-we-still-do-this-during-a-partial-join"><a class="header" href="#can-we-still-do-this-during-a-partial-join">Can we still do this during a partial join?</a></h4>
|
||
<p>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</p>
|
||
<ul>
|
||
<li>the partial state before the leave has <code>U</code> joined, invited or knocked, but</li>
|
||
<li>the full state before the leave has <code>U</code> banned, left or not present,</li>
|
||
</ul>
|
||
<p>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.</p>
|
||
<p>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 <code>prev_events</code> descends
|
||
from our leave.</p>
|
||
<p>Another option we considered was to use federation <code>/send_leave</code> 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,</p>
|
||
<blockquote>
|
||
<p>sending out leaves is pretty best-effort currently</p>
|
||
</blockquote>
|
||
<p>so this is probably good enough as-is.</p>
|
||
<h4 id="cleanup-after-the-last-leave"><a class="header" href="#cleanup-after-the-last-leave">Cleanup after the last leave</a></h4>
|
||
<p><strong>TODO</strong>: what cleanup is necessary? Is it all just nice-to-have to save unused
|
||
work?</p>
|
||
</details>
|
||
|
||
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|
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