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(Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> + <i class="fa fa-search"></i> + </button> + </div> + + <h1 class="menu-title">Synapse</h1> + + <div class="right-buttons"> + <a href="../../print.html" title="Print this book" aria-label="Print this book"> + <i id="print-button" class="fa fa-print"></i> + </a> + <a href="https://github.com/matrix-org/synapse" title="Git repository" aria-label="Git repository"> + <i id="git-repository-button" class="fa fa-github"></i> + </a> + <a href="https://github.com/matrix-org/synapse/edit/develop/docs/development/synapse_architecture/faster_joins.md" title="Suggest an edit" aria-label="Suggest an edit"> + <i id="git-edit-button" class="fa fa-edit"></i> + </a> + </div> + </div> + + <div id="search-wrapper" class="hidden"> + <form id="searchbar-outer" class="searchbar-outer"> + <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> + </form> + <div id="searchresults-outer" class="searchresults-outer hidden"> + <div id="searchresults-header" class="searchresults-header"></div> + <ul id="searchresults"> + </ul> + </div> + </div> + <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> + <script type="text/javascript"> + document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); + document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); + Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { + link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); + }); + </script> + + <div id="content" class="content"> + <main> + <!-- Page table of contents --> + <div class="sidetoc"> + <nav class="pagetoc"></nav> + </div> + + <h1 id="how-do-faster-joins-work"><a class="header" href="#how-do-faster-joins-work">How do faster joins work?</a></h1> +<p>This is a work-in-progress set of notes with two goals:</p> +<ul> +<li>act as a reference, explaining how Synapse implements faster joins; and</li> +<li>record the rationale behind our choices.</li> +</ul> +<p>See also <a href="https://github.com/matrix-org/matrix-spec-proposals/pull/3902">MSC3902</a>.</p> +<p>The key idea is described by <a href="https://github.com/matrix-org/matrix-spec-proposals/pull/3902">MSC706</a>. This allows servers to +request a lightweight response to the federation <code>/send_join</code> endpoint. +This is called a <strong>faster join</strong>, also known as a <strong>partial join</strong>. In these +notes we'll usually use the word "partial" as it matches the database schema.</p> +<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> +<p>The response to a partial join consists of</p> +<ul> +<li>the requested join event <code>J</code>,</li> +<li>a list of the servers in the room (according to the state before <code>J</code>),</li> +<li>a subset of the state of the room before <code>J</code>,</li> +<li>the full auth chain of that state subset.</li> +</ul> +<p>Synapse marks the room as partially joined by adding a row to the database table +<code>partial_state_rooms</code>. It also marks the join event <code>J</code> as "partially stated", +meaning that we have neither received nor computed the full state before/after +<code>J</code>. This is done by adding a row to <code>partial_state_events</code>.</p> +<details><summary>DB schema</summary> +<pre><code>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 │ +</code></pre> +<p>Indices, foreign-keys and check constraints are omitted for brevity.</p> +</details> +<p>While partially joined to a room, Synapse receives events <code>E</code> 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 <a href="https://spec.matrix.org/v1.5/server-server-api/#checks-performed-on-receipt-of-a-pdu">checks on an event</a>.</p> +<blockquote> +<ol> +<li>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.</li> +<li>Passes signature checks, otherwise it is dropped.</li> +<li>Passes hash checks, otherwise it is redacted before being processed further.</li> +<li>Passes authorization rules based on the event’s auth events, otherwise it +is rejected.</li> +<li><strong>Passes authorization rules based on the state before the event, otherwise +it is rejected.</strong></li> +<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 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.(?)</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> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + <a rel="prev" href="../../tcp_replication.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + <a rel="next" href="../../development/internal_documentation/index.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + <div style="clear: both"></div> + </nav> + </div> + </div> + + <nav class="nav-wide-wrapper" aria-label="Page navigation"> + <a rel="prev" href="../../tcp_replication.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + <a rel="next" href="../../development/internal_documentation/index.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + </nav> + + </div> + + <script type="text/javascript"> + window.playground_copyable = true; + </script> + <script src="../../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../../mark.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../../searcher.js" type="text/javascript" charset="utf-8"></script> + <script src="../../clipboard.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../../highlight.js" type="text/javascript" charset="utf-8"></script> + <script src="../../book.js" type="text/javascript" charset="utf-8"></script> + + <!-- Custom JS scripts --> + <script type="text/javascript" src="../../docs/website_files/table-of-contents.js"></script> + </body> +</html> \ No newline at end of file |