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Document room complexity guard for constrained Synapse setups

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Slavi Pantaleev
2026-02-21 17:47:11 +02:00
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docs/configuring-playbook-synapse.md
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@@ -80,6 +80,29 @@ A separate Ansible role (`matrix-synapse-reverse-proxy-companion`) and component
In case any problems occur, make sure to have a look at the [list of synapse issues about workers](https://github.com/element-hq/synapse/issues?q=workers+in%3Atitle) and your `journalctl --unit 'matrix-*'`.
### Limit joining heavy rooms on constrained hosts
If your server is underpowered, joining heavy rooms can cause Synapse to consume a lot of resources and be unavailable for long (while it catches up).
To avoid this, Synapse can be configured to reject joins for remote rooms that are too complex before users enter them.
Complexity is computed as `current_state_events / 500` (Synapse state event count for current room state). When the resulting value is higher than `matrix_synapse_limit_remote_rooms_complexity` and `matrix_synapse_limit_remote_rooms_enabled` is `true`, Synapse blocks joining the room.
We recommend using this as a guardrail on low-resource servers:
```yaml
matrix_synapse_limit_remote_rooms_enabled: true
# Tweak as necessary
matrix_synapse_limit_remote_rooms_complexity: 1.0
# Uncomment and tweak if necessary
# matrix_synapse_limit_remote_rooms_complexity_error: "Your homeserver is unable to join rooms this large or complex. Please speak to your server administrator, or upgrade your instance to join this room."
# If you'd like your admins to be exempt from this limit, uncomment the line below
# matrix_synapse_limit_remote_rooms_admins_can_join: true
```
### Synapse + OpenID Connect for Single-Sign-On
💡 An alternative to setting up OIDC in Synapse is to use [Matrix Authentication Service](./configuring-playbook-matrix-authentication-service.md) (MAS). Newer clients (like Element X) only support SSO-based authentication via MAS and not via the legacy Synapse OIDC setup described below. That said, MAS is still a new experimental service which comes with its own downsides. Consult its documentation to learn if it will be a good fit for your deployment.