Rate limit joins per-room (#13276)

2 files changed, 29 insertions, 0 deletions

diff --git a/docs/upgrade.md b/docs/upgrade.md
index 3aaeb499ce..2c7c258909 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -104,6 +104,16 @@ minimum, a `notif_from` setting.)
 Specifying an `email` setting under `account_threepid_delegates` will now cause
 an error at startup.
 
+## Changes to the event replication streams
+
+Synapse now includes a flag indicating if an event is an outlier when
+replicating it to other workers. This is a forwards- and backwards-incompatible
+change: v1.63 and workers cannot process events replicated by v1.64 workers, and
+vice versa.
+
+Once all workers are upgraded to v1.64 (or downgraded to v1.63), event
+replication will resume as normal.
+
 # Upgrading to v1.62.0
 
 ## New signatures for spam checker callbacks
diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md
index 5fe502e33a..be272a400c 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -1472,6 +1472,25 @@ rc_joins:
     burst_count: 12
 ```
 ---
+### `rc_joins_per_room`
+
+This option allows admins to ratelimit joins to a room based on the number of recent
+joins (local or remote) to that room. It is intended to mitigate mass-join spam
+waves which target multiple homeservers.
+
+By default, one join is permitted to a room every second, with an accumulating
+buffer of up to ten instantaneous joins.
+
+Example configuration (default values):
+```yaml
+rc_joins_per_room:
+  per_second: 1
+  burst_count: 10
+```
+
+_Added in Synapse 1.64.0._
+
+---
 ### `rc_3pid_validation`
 
 This option ratelimits how often a user or IP can attempt to validate a 3PID.