Rate limiting is a crucial defense method against service misuse. We have implemented a global rate controlling service called rated. It handles rate limiting for all processes, based on an "items per time interval" algorithm. Our implementation features a sliding rate limit (in contrast to leaking bucket; which has a fixed output rate regardless of exactly when the rate was exceeded).
Rate limits may be "hit" using the HSL rate() function, and sometimes using the graphical blocks in the flows. It returns true as long as the rate() call interval is less than its rate limit, but false when exceeded. If the count parameter is zero, the current rate is returned instead.
// get the rate limit number, without increasing
if (rate("failed-login:saslusername", $saslusername, 0, 60) >= 3)
Reject("Too many failed logins");
// increase, and reject if exceeded
if (rate("spammers", $senderip, 100, 86400) == false)
Reject("You may only send 100 suspicious messages per day");
Rate limits are synchronized in the cluster if a HMAC-SHA1 key is specified on the system settings page. The protocol is based on UDP and uses port 13131. In terms of security it may be wise to only activate this feature on local networks, as the HMAC-SHA1 only serves as a packet-authenticity firewall. It does not provide encryption nor protect against reply attacks. It should probably be protected by other means such as a VPN tunnel or psychical security (DMZ).
The rate limits performance is roughly O(log N + log M) and lookups are currently implemented as a std::multi map (C++).