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<h1 id="message-retention-policies"><a class="header" href="#message-retention-policies">Message retention policies</a></h1>
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<p>Synapse admins can enable support for message retention policies on
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their homeserver. Message retention policies exist at a room level,
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follow the semantics described in
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<a href="https://github.com/matrix-org/matrix-doc/blob/matthew/msc1763/proposals/1763-configurable-retention-periods.md">MSC1763</a>,
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and allow server and room admins to configure how long messages should
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be kept in a homeserver's database before being purged from it.
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<strong>Please note that, as this feature isn't part of the Matrix
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specification yet, this implementation is to be considered as
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experimental.</strong> </p>
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<p>A message retention policy is mainly defined by its <code>max_lifetime</code>
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parameter, which defines how long a message can be kept around after
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it was sent to the room. If a room doesn't have a message retention
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policy, and there's no default one for a given server, then no message
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sent in that room is ever purged on that server.</p>
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<p>MSC1763 also specifies semantics for a <code>min_lifetime</code> parameter which
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defines the amount of time after which an event <em>can</em> get purged (after
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it was sent to the room), but Synapse doesn't currently support it
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beyond registering it.</p>
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<p>Both <code>max_lifetime</code> and <code>min_lifetime</code> are optional parameters.</p>
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<p>Note that message retention policies don't apply to state events.</p>
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<p>Once an event reaches its expiry date (defined as the time it was sent
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plus the value for <code>max_lifetime</code> in the room), two things happen:</p>
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<ul>
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<li>Synapse stops serving the event to clients via any endpoint.</li>
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<li>The message gets picked up by the next purge job (see the "Purge jobs"
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section) and is removed from Synapse's database.</li>
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</ul>
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<p>Since purge jobs don't run continuously, this means that an event might
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stay in a server's database for longer than the value for <code>max_lifetime</code>
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in the room would allow, though hidden from clients.</p>
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<p>Similarly, if a server (with support for message retention policies
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enabled) receives from another server an event that should have been
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purged according to its room's policy, then the receiving server will
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process and store that event until it's picked up by the next purge job,
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though it will always hide it from clients.</p>
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<p>Synapse requires at least one message in each room, so it will never
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delete the last message in a room. It will, however, hide it from
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clients.</p>
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<h2 id="server-configuration"><a class="header" href="#server-configuration">Server configuration</a></h2>
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<p>Support for this feature can be enabled and configured in the
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<code>retention</code> section of the Synapse configuration file (see the
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<a href="https://github.com/matrix-org/synapse/blob/v1.36.0/docs/sample_config.yaml#L451-L518">sample file</a>).</p>
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<p>To enable support for message retention policies, set the setting
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<code>enabled</code> in this section to <code>true</code>.</p>
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<h3 id="default-policy"><a class="header" href="#default-policy">Default policy</a></h3>
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<p>A default message retention policy is a policy defined in Synapse's
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configuration that is used by Synapse for every room that doesn't have a
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message retention policy configured in its state. This allows server
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admins to ensure that messages are never kept indefinitely in a server's
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database. </p>
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<p>A default policy can be defined as such, in the <code>retention</code> section of
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the configuration file:</p>
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<pre><code class="language-yaml">default_policy:
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min_lifetime: 1d
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max_lifetime: 1y
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</code></pre>
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<p>Here, <code>min_lifetime</code> and <code>max_lifetime</code> have the same meaning and level
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of support as previously described. They can be expressed either as a
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duration (using the units <code>s</code> (seconds), <code>m</code> (minutes), <code>h</code> (hours),
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<code>d</code> (days), <code>w</code> (weeks) and <code>y</code> (years)) or as a number of milliseconds.</p>
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<h3 id="purge-jobs"><a class="header" href="#purge-jobs">Purge jobs</a></h3>
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<p>Purge jobs are the jobs that Synapse runs in the background to purge
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expired events from the database. They are only run if support for
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message retention policies is enabled in the server's configuration. If
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no configuration for purge jobs is configured by the server admin,
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Synapse will use a default configuration, which is described in the
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<a href="https://github.com/matrix-org/synapse/blob/v1.36.0/docs/sample_config.yaml#L451-L518">sample configuration file</a>.</p>
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<p>Some server admins might want a finer control on when events are removed
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depending on an event's room's policy. This can be done by setting the
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<code>purge_jobs</code> sub-section in the <code>retention</code> section of the configuration
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file. An example of such configuration could be:</p>
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<pre><code class="language-yaml">purge_jobs:
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- longest_max_lifetime: 3d
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interval: 12h
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- shortest_max_lifetime: 3d
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longest_max_lifetime: 1w
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interval: 1d
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- shortest_max_lifetime: 1w
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interval: 2d
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</code></pre>
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<p>In this example, we define three jobs:</p>
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<ul>
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<li>one that runs twice a day (every 12 hours) and purges events in rooms
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which policy's <code>max_lifetime</code> is lower or equal to 3 days.</li>
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<li>one that runs once a day and purges events in rooms which policy's
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<code>max_lifetime</code> is between 3 days and a week.</li>
|
|
<li>one that runs once every 2 days and purges events in rooms which
|
|
policy's <code>max_lifetime</code> is greater than a week.</li>
|
|
</ul>
|
|
<p>Note that this example is tailored to show different configurations and
|
|
features slightly more jobs than it's probably necessary (in practice, a
|
|
server admin would probably consider it better to replace the two last
|
|
jobs with one that runs once a day and handles rooms which which
|
|
policy's <code>max_lifetime</code> is greater than 3 days).</p>
|
|
<p>Keep in mind, when configuring these jobs, that a purge job can become
|
|
quite heavy on the server if it targets many rooms, therefore prefer
|
|
having jobs with a low interval that target a limited set of rooms. Also
|
|
make sure to include a job with no minimum and one with no maximum to
|
|
make sure your configuration handles every policy.</p>
|
|
<p>As previously mentioned in this documentation, while a purge job that
|
|
runs e.g. every day means that an expired event might stay in the
|
|
database for up to a day after its expiry, Synapse hides expired events
|
|
from clients as soon as they expire, so the event is not visible to
|
|
local users between its expiry date and the moment it gets purged from
|
|
the server's database.</p>
|
|
<h3 id="lifetime-limits"><a class="header" href="#lifetime-limits">Lifetime limits</a></h3>
|
|
<p>Server admins can set limits on the values of <code>max_lifetime</code> to use when
|
|
purging old events in a room. These limits can be defined as such in the
|
|
<code>retention</code> section of the configuration file:</p>
|
|
<pre><code class="language-yaml">allowed_lifetime_min: 1d
|
|
allowed_lifetime_max: 1y
|
|
</code></pre>
|
|
<p>The limits are considered when running purge jobs. If necessary, the
|
|
effective value of <code>max_lifetime</code> will be brought between
|
|
<code>allowed_lifetime_min</code> and <code>allowed_lifetime_max</code> (inclusive).
|
|
This means that, if the value of <code>max_lifetime</code> defined in the room's state
|
|
is lower than <code>allowed_lifetime_min</code>, the value of <code>allowed_lifetime_min</code>
|
|
will be used instead. Likewise, if the value of <code>max_lifetime</code> is higher
|
|
than <code>allowed_lifetime_max</code>, the value of <code>allowed_lifetime_max</code> will be
|
|
used instead.</p>
|
|
<p>In the example above, we ensure Synapse never deletes events that are less
|
|
than one day old, and that it always deletes events that are over a year
|
|
old.</p>
|
|
<p>If a default policy is set, and its <code>max_lifetime</code> value is lower than
|
|
<code>allowed_lifetime_min</code> or higher than <code>allowed_lifetime_max</code>, the same
|
|
process applies.</p>
|
|
<p>Both parameters are optional; if one is omitted Synapse won't use it to
|
|
adjust the effective value of <code>max_lifetime</code>.</p>
|
|
<p>Like other settings in this section, these parameters can be expressed
|
|
either as a duration or as a number of milliseconds.</p>
|
|
<h2 id="room-configuration"><a class="header" href="#room-configuration">Room configuration</a></h2>
|
|
<p>To configure a room's message retention policy, a room's admin or
|
|
moderator needs to send a state event in that room with the type
|
|
<code>m.room.retention</code> and the following content:</p>
|
|
<pre><code class="language-json">{
|
|
"max_lifetime": ...
|
|
}
|
|
</code></pre>
|
|
<p>In this event's content, the <code>max_lifetime</code> parameter has the same
|
|
meaning as previously described, and needs to be expressed in
|
|
milliseconds. The event's content can also include a <code>min_lifetime</code>
|
|
parameter, which has the same meaning and limited support as previously
|
|
described.</p>
|
|
<p>Note that over every server in the room, only the ones with support for
|
|
message retention policies will actually remove expired events. This
|
|
support is currently not enabled by default in Synapse.</p>
|
|
<h2 id="note-on-reclaiming-disk-space"><a class="header" href="#note-on-reclaiming-disk-space">Note on reclaiming disk space</a></h2>
|
|
<p>While purge jobs actually delete data from the database, the disk space
|
|
used by the database might not decrease immediately on the database's
|
|
host. However, even though the database engine won't free up the disk
|
|
space, it will start writing new data into where the purged data was.</p>
|
|
<p>If you want to reclaim the freed disk space anyway and return it to the
|
|
operating system, the server admin needs to run <code>VACUUM FULL;</code> (or
|
|
<code>VACUUM;</code> for SQLite databases) on Synapse's database (see the related
|
|
<a href="https://www.postgresql.org/docs/current/sql-vacuum.html">PostgreSQL documentation</a>).</p>
|
|
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