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<h1 id="streams"><a class="header" href="#streams">Streams</a></h1>
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<p>Synapse has a concept of “streams”, which are roughly described in <a href="https://github.com/element-hq/synapse/blob/develop/synapse/storage/util/id_generators.py"><code>id_generators.py</code></a>.
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Generally speaking, streams are a series of notifications that something in Synapse’s database has changed that the application might need to respond to.
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For example:</p>
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<ul>
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<li>The events stream reports new events (PDUs) that Synapse creates, or that Synapse accepts from another homeserver.</li>
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<li>The account data stream reports changes to users’ <a href="https://spec.matrix.org/v1.7/client-server-api/#client-config">account data</a>.</li>
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<li>The to-device stream reports when a device has a new <a href="https://spec.matrix.org/v1.7/client-server-api/#send-to-device-messaging">to-device message</a>.</li>
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</ul>
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<p>See <a href="https://github.com/element-hq/synapse/blob/develop/synapse/replication/tcp/streams/__init__.py"><code>synapse.replication.tcp.streams</code></a> for the full list of streams.</p>
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<p>It is very helpful to understand the streams mechanism when working on any part of Synapse that needs to respond to changes—especially if those changes are made by different workers.
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To that end, let’s describe streams formally, paraphrasing from the docstring of <a href="https://github.com/element-hq/synapse/blob/a719b703d9bd0dade2565ddcad0e2f3a7a9d4c37/synapse/storage/util/id_generators.py#L96"><code>AbstractStreamIdGenerator</code></a>.</p>
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<h2 id="definition"><a class="header" href="#definition">Definition</a></h2>
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<p>A stream is an append-only log <code>T1, T2, ..., Tn, ...</code> of facts<sup class="footnote-reference" id="fr-1-1"><a href="#footnote-1">1</a></sup> which grows over time.
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Only “writers” can add facts to a stream, and there may be multiple writers.</p>
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<p>Each fact has an ID, called its “stream ID”.
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Readers should only process facts in ascending stream ID order.</p>
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<p>Roughly speaking, each stream is backed by a database table.
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It should have a <code>stream_id</code> (or similar) bigint column holding stream IDs, plus additional columns as necessary to describe the fact.
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Typically, a fact is expressed with a single row in its backing table.<sup class="footnote-reference" id="fr-2-1"><a href="#footnote-2">2</a></sup>
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Within a stream, no two facts may have the same stream_id.</p>
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<blockquote>
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<p><em>Aside</em>. Some additional notes on streams’ backing tables.</p>
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<ol>
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<li>Rich would like to <a href="https://github.com/matrix-org/synapse/issues/13456">ditch the backing tables</a>.</li>
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<li>The backing tables may have other uses.
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> For example, the events table serves backs the events stream, and is read when processing new events.
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> But old rows are read from the table all the time, whenever Synapse needs to lookup some facts about an event.</li>
|
||
<li>Rich suspects that sometimes the stream is backed by multiple tables, so the stream proper is the union of those tables.</li>
|
||
</ol>
|
||
</blockquote>
|
||
<p>Stream writers can “reserve” a stream ID, and then later mark it as having being completed.
|
||
Stream writers need to track the completion of each stream fact.
|
||
In the happy case, completion means a fact has been written to the stream table.
|
||
But unhappy cases (e.g. transaction rollback due to an error) also count as completion.
|
||
Once completed, the rows written with that stream ID are fixed, and no new rows
|
||
will be inserted with that ID.</p>
|
||
<h2 id="current-stream-id"><a class="header" href="#current-stream-id">Current stream ID</a></h2>
|
||
<p>For any given stream reader (including writers themselves), we may define a per-writer current stream ID:</p>
|
||
<blockquote>
|
||
<p>A current stream ID <em>for a writer W</em> is the largest stream ID such that
|
||
all transactions added by W with equal or smaller ID have completed.</p>
|
||
</blockquote>
|
||
<p>Similarly, there is a “linear” notion of current stream ID:</p>
|
||
<blockquote>
|
||
<p>A “linear” current stream ID is the largest stream ID such that
|
||
all facts (added by any writer) with equal or smaller ID have completed.</p>
|
||
</blockquote>
|
||
<p>Because different stream readers A and B learn about new facts at different times, A and B may disagree about current stream IDs.
|
||
Put differently: we should think of stream readers as being independent of each other, proceeding through a stream of facts at different rates.</p>
|
||
<p>The above definition does not give a unique current stream ID, in fact there can
|
||
be a range of current stream IDs. Synapse uses both the minimum and maximum IDs
|
||
for different purposes. Most often the maximum is used, as its generally
|
||
beneficial for workers to advance their IDs as soon as possible. However, the
|
||
minimum is used in situations where e.g. another worker is going to wait until
|
||
the stream advances past a position.</p>
|
||
<p><strong>NB.</strong> For both senses of “current”, that if a writer opens a transaction that never completes, the current stream ID will never advance beyond that writer’s last written stream ID.</p>
|
||
<p>For single-writer streams, the per-writer current ID and the linear current ID are the same.
|
||
Both senses of current ID are monotonic, but they may “skip” or jump over IDs because facts complete out of order.</p>
|
||
<p><em>Example</em>.
|
||
Consider a single-writer stream which is initially at ID 1.</p>
|
||
<div class="table-wrapper">
|
||
<table>
|
||
<thead>
|
||
<tr><th>Action</th><th>Current stream ID</th><th>Notes</th></tr>
|
||
</thead>
|
||
<tbody>
|
||
<tr><td></td><td>1</td><td></td></tr>
|
||
<tr><td>Reserve 2</td><td>1</td><td></td></tr>
|
||
<tr><td>Reserve 3</td><td>1</td><td></td></tr>
|
||
<tr><td>Complete 3</td><td>1</td><td>current ID unchanged, waiting for 2 to complete</td></tr>
|
||
<tr><td>Complete 2</td><td>3</td><td>current ID jumps from 1 -> 3</td></tr>
|
||
<tr><td>Reserve 4</td><td>3</td><td></td></tr>
|
||
<tr><td>Reserve 5</td><td>3</td><td></td></tr>
|
||
<tr><td>Reserve 6</td><td>3</td><td></td></tr>
|
||
<tr><td>Complete 5</td><td>3</td><td></td></tr>
|
||
<tr><td>Complete 4</td><td>5</td><td>current ID jumps 3->5, even though 6 is pending</td></tr>
|
||
<tr><td>Complete 6</td><td>6</td><td></td></tr>
|
||
</tbody>
|
||
</table>
|
||
</div>
|
||
<h2 id="multi-writer-streams"><a class="header" href="#multi-writer-streams">Multi-writer streams</a></h2>
|
||
<p>There are two ways to view a multi-writer stream.</p>
|
||
<ol>
|
||
<li>Treat it as a collection of distinct single-writer streams, one
|
||
for each writer.</li>
|
||
<li>Treat it as a single stream.</li>
|
||
</ol>
|
||
<p>The single stream (option 2) is conceptually simpler, and easier to represent (a single stream id).
|
||
However, it requires each reader to know about the entire set of writers, to ensures that readers don’t erroneously advance their current stream position too early and miss a fact from an unknown writer.
|
||
In contrast, multiple parallel streams (option 1) are more complex, requiring more state to represent (map from writer to stream id).
|
||
The payoff for doing so is that readers can “peek” ahead to facts that completed on one writer no matter the state of the others, reducing latency.</p>
|
||
<p>Note that a multi-writer stream can be viewed in both ways.
|
||
For example, the events stream is treated as multiple single-writer streams (option 1) by the sync handler, so that events are sent to clients as soon as possible.
|
||
But the background process that works through events treats them as a single linear stream.</p>
|
||
<p>Another useful example is the cache invalidation stream.
|
||
The facts this stream holds are instructions to “you should now invalidate these cache entries”.
|
||
We only ever treat this as a multiple single-writer streams as there is no important ordering between cache invalidations.
|
||
(Invalidations are self-contained facts; and the invalidations commute/are idempotent).</p>
|
||
<h2 id="writing-to-streams"><a class="header" href="#writing-to-streams">Writing to streams</a></h2>
|
||
<p>Writers need to track:</p>
|
||
<ul>
|
||
<li>track their current position (i.e. its own per-writer stream ID).</li>
|
||
<li>their facts currently awaiting completion.</li>
|
||
</ul>
|
||
<p>At startup,</p>
|
||
<ul>
|
||
<li>the current position of that writer can be found by querying the database (which suggests that facts need to be written to the database atomically, in a transaction); and</li>
|
||
<li>there are no facts awaiting completion.</li>
|
||
</ul>
|
||
<p>To reserve a stream ID, call <a href="https://www.postgresql.org/docs/current/functions-sequence.html"><code>nextval</code></a> on the appropriate postgres sequence.</p>
|
||
<p>To write a fact to the stream: insert the appropriate rows to the appropriate backing table.</p>
|
||
<p>To complete a fact, first remove it from your map of facts currently awaiting completion.
|
||
Then, if no earlier fact is awaiting completion, the writer can advance its current position in that stream.
|
||
Upon doing so it should emit an <code>RDATA</code> message<sup class="footnote-reference" id="fr-3-1"><a href="#footnote-3">3</a></sup>, once for every fact between the old and the new stream ID.</p>
|
||
<h2 id="subscribing-to-streams"><a class="header" href="#subscribing-to-streams">Subscribing to streams</a></h2>
|
||
<p>Readers need to track the current position of every writer.</p>
|
||
<p>At startup, they can find this by contacting each writer with a <code>REPLICATE</code> message,
|
||
requesting that all writers reply describing their current position in their streams.
|
||
Writers reply with a <code>POSITION</code> message.</p>
|
||
<p>To learn about new facts, readers should listen for <code>RDATA</code> messages and process them to respond to the new fact.
|
||
The <code>RDATA</code> itself is not a self-contained representation of the fact;
|
||
readers will have to query the stream tables for the full details.
|
||
Readers must also advance their record of the writer’s current position for that stream.</p>
|
||
<h2 id="summary"><a class="header" href="#summary">Summary</a></h2>
|
||
<p>In a nutshell: we have an append-only log with a “buffer/scratchpad” at the end where we have to wait for the sequence to be linear and contiguous.</p>
|
||
<hr>
|
||
<h2 id="cheatsheet-for-creating-a-new-stream"><a class="header" href="#cheatsheet-for-creating-a-new-stream">Cheatsheet for creating a new stream</a></h2>
|
||
<p>These rough notes and links may help you to create a new stream and add all the
|
||
necessary registration and event handling.</p>
|
||
<p><strong>Create your stream:</strong></p>
|
||
<ul>
|
||
<li><a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/replication/tcp/streams/_base.py#L728">create a stream class and stream row class</a>
|
||
<ul>
|
||
<li>will need an <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/storage/databases/main/thread_subscriptions.py#L75">ID generator</a>
|
||
<ul>
|
||
<li>may need <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/config/workers.py#L177">writer configuration</a>, if there isn’t already an obvious source of configuration for which workers should be designated as writers to your new stream.
|
||
<ul>
|
||
<li>if adding new writer configuration, add Docker-worker configuration, which lets us configure the writer worker in Complement tests: <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/docker/configure_workers_and_start.py#L331">[1]</a>, <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/docker/configure_workers_and_start.py#L440">[2]</a></li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
</li>
|
||
<li>most of the time, you will likely introduce a new datastore class for the concept represented by the new stream, unless there is already an obvious datastore that covers it.</li>
|
||
<li>consider whether it may make sense to introduce a handler</li>
|
||
</ul>
|
||
<p><strong>Register your stream in:</strong></p>
|
||
<ul>
|
||
<li><a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/replication/tcp/streams/__init__.py#L71"><code>STREAMS_MAP</code></a></li>
|
||
</ul>
|
||
<p><strong>Advance your stream in:</strong></p>
|
||
<ul>
|
||
<li><a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/storage/databases/main/thread_subscriptions.py#L111"><code>process_replication_position</code> of your appropriate datastore</a>
|
||
<ul>
|
||
<li>don’t forget the super call</li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
<p><strong>If you’re going to do any caching that needs invalidation from new rows:</strong></p>
|
||
<ul>
|
||
<li>add invalidations to <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/storage/databases/main/thread_subscriptions.py#L91"><code>process_replication_rows</code> of your appropriate datastore</a>
|
||
<ul>
|
||
<li>don’t forget the super call</li>
|
||
</ul>
|
||
</li>
|
||
<li>add local-only <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/storage/databases/main/thread_subscriptions.py#L201">invalidations to your writer transactions</a></li>
|
||
</ul>
|
||
<p><strong>For streams to be used in sync:</strong></p>
|
||
<ul>
|
||
<li>add a new field to <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/types/__init__.py#L1003"><code>StreamToken</code></a>
|
||
<ul>
|
||
<li>add a new <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/types/__init__.py#L999"><code>StreamKeyType</code></a></li>
|
||
</ul>
|
||
</li>
|
||
<li>add appropriate wake-up rules
|
||
<ul>
|
||
<li>in <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/replication/tcp/client.py#L260"><code>on_rdata</code></a></li>
|
||
<li>locally on the same worker when completing a write, <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/handlers/thread_subscriptions.py#L139">e.g. in your handler</a></li>
|
||
</ul>
|
||
</li>
|
||
<li>add the stream in <a href="https://github.com/element-hq/synapse/blob/4367fb2d078c52959aeca0fe6874539c53e8360d/synapse/streams/events.py#L127"><code>bound_future_token</code></a></li>
|
||
</ul>
|
||
<hr>
|
||
<hr>
|
||
<ol class="footnote-definition">
|
||
<li id="footnote-1">
|
||
<p>we use the word <em>fact</em> here for two reasons.
|
||
Firstly, the word “event” is already heavily overloaded (PDUs, EDUs, account data, …) and we don’t need to make that worse.
|
||
Secondly, “fact” emphasises that the things we append to a stream cannot change after the fact. <a href="#fr-1-1">↩</a></p>
|
||
</li>
|
||
<li id="footnote-2">
|
||
<p>A fact might be expressed with 0 rows, e.g. if we opened a transaction to persist an event, but failed and rolled the transaction back before marking the fact as completed.
|
||
In principle a fact might be expressed with 2 or more rows; if so, each of those rows should share the fact’s stream ID. <a href="#fr-2-1">↩</a></p>
|
||
</li>
|
||
<li id="footnote-3">
|
||
<p>This communication used to happen directly with the writers <a href="../../tcp_replication.html">over TCP</a>;
|
||
nowadays it’s done via Redis’s Pubsub. <a href="#fr-3-1">↩</a></p>
|
||
</li>
|
||
</ol>
|
||
|
||
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|
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