Message retention policies

Synapse admins can enable support for message retention policies on their homeserver. Message retention policies exist at a room level, follow the semantics described in MSC1763, and allow server and room admins to configure how long messages should be kept in a homeserver's database before being purged from it. Please note that, as this feature isn't part of the Matrix specification yet, this implementation is to be considered as experimental. There are known bugs which may cause database corruption. Proceed with caution.

A message retention policy is mainly defined by its max_lifetime parameter, which defines how long a message can be kept around after it was sent to the room. If a room doesn't have a message retention policy, and there's no default one for a given server, then no message sent in that room is ever purged on that server.

MSC1763 also specifies semantics for a min_lifetime parameter which defines the amount of time after which an event can get purged (after it was sent to the room), but Synapse doesn't currently support it beyond registering it.

Both max_lifetime and min_lifetime are optional parameters.

Note that message retention policies don't apply to state events.

Once an event reaches its expiry date (defined as the time it was sent plus the value for max_lifetime in the room), two things happen:

Synapse stops serving the event to clients via any endpoint.
The message gets picked up by the next purge job (see the "Purge jobs" section) and is removed from Synapse's database.

Since purge jobs don't run continuously, this means that an event might stay in a server's database for longer than the value for max_lifetime in the room would allow, though hidden from clients.

Similarly, if a server (with support for message retention policies enabled) receives from another server an event that should have been purged according to its room's policy, then the receiving server will process and store that event until it's picked up by the next purge job, though it will always hide it from clients.

Synapse requires at least one message in each room, so it will never delete the last message in a room. It will, however, hide it from clients.

Server configuration

Support for this feature can be enabled and configured by adding a the retention in the Synapse configuration file (see configuration manual).

To enable support for message retention policies, set the setting enabled in this section to true.

Default policy

A default message retention policy is a policy defined in Synapse's configuration that is used by Synapse for every room that doesn't have a message retention policy configured in its state. This allows server admins to ensure that messages are never kept indefinitely in a server's database.

A default policy can be defined as such, by adding the retention option in the configuration file and adding these sub-options:

default_policy:
  min_lifetime: 1d
  max_lifetime: 1y

Here, min_lifetime and max_lifetime have the same meaning and level of support as previously described. They can be expressed either as a duration (using the units s (seconds), m (minutes), h (hours), d (days), w (weeks) and y (years)) or as a number of milliseconds.

Purge jobs

Purge jobs are the jobs that Synapse runs in the background to purge expired events from the database. They are only run if support for message retention policies is enabled in the server's configuration. If no configuration for purge jobs is configured by the server admin, Synapse will use a default configuration, which is described here in the configuration manual.

Some server admins might want a finer control on when events are removed depending on an event's room's policy. This can be done by setting the purge_jobs sub-section in the retention section of the configuration file. An example of such configuration could be:

purge_jobs:
  - longest_max_lifetime: 3d
    interval: 12h
  - shortest_max_lifetime: 3d
    longest_max_lifetime: 1w
    interval: 1d
  - shortest_max_lifetime: 1w
    interval: 2d

In this example, we define three jobs:

one that runs twice a day (every 12 hours) and purges events in rooms which policy's max_lifetime is lower or equal to 3 days.
one that runs once a day and purges events in rooms which policy's max_lifetime is between 3 days and a week.
one that runs once every 2 days and purges events in rooms which policy's max_lifetime is greater than a week.

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 policy's max_lifetime is greater than 3 days).

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.

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.

Lifetime limits

Server admins can set limits on the values of max_lifetime to use when purging old events in a room. These limits can be defined under the retention option in the configuration file:

allowed_lifetime_min: 1d
allowed_lifetime_max: 1y

The limits are considered when running purge jobs. If necessary, the effective value of max_lifetime will be brought between allowed_lifetime_min and allowed_lifetime_max (inclusive). This means that, if the value of max_lifetime defined in the room's state is lower than allowed_lifetime_min, the value of allowed_lifetime_min will be used instead. Likewise, if the value of max_lifetime is higher than allowed_lifetime_max, the value of allowed_lifetime_max will be used instead.

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.

If a default policy is set, and its max_lifetime value is lower than allowed_lifetime_min or higher than allowed_lifetime_max, the same process applies.

Both parameters are optional; if one is omitted Synapse won't use it to adjust the effective value of max_lifetime.

Like other settings in this section, these parameters can be expressed either as a duration or as a number of milliseconds.

Room configuration

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 m.room.retention and the following content:

{
    "max_lifetime": ...
}

In this event's content, the max_lifetime parameter has the same meaning as previously described, and needs to be expressed in milliseconds. The event's content can also include a min_lifetime parameter, which has the same meaning and limited support as previously described.

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.

Note on reclaiming disk space

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.

If you want to reclaim the freed disk space anyway and return it to the operating system, the server admin needs to run VACUUM FULL; (or VACUUM; for SQLite databases) on Synapse's database (see the related PostgreSQL documentation).

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