Throttle

This filter was added in MariaDB MaxScale 2.3

Overview

The throttle filter replaces and extends on the limit_queries functionality of the Database Firewall filter.

The throttle filter is used to limit the maximum query frequency (QPS - queries per second) of a database session to a configurable value. The main use cases are to prevent a rogue session (client side error) and a DoS attack from overloading the system.

The throttling is dynamic. The query frequency is not limited to an absolute value. Depending on the configuration the throttle will allow some amount of high frequency queries, or especially short bursts with no frequency limitation.

Configuration

Basic Configuration

[Throttle]
type = filter
module = throttlefilter
max_qps = 500
throttling_duration = 60000
...

[Routing-Service]
type = service
filters = Throttle

This configuration states that the query frequency will be throttled to around 500 qps, and that the time limit a query is allowed to stay at the maximum frequency is 60 seconds. All values involving time are configured in milliseconds. With the basic configuration the throttling will be nearly immediate, i.e. a session will only be allowed very short bursts of high frequency querying.

When a session has been continuously throttled for throttling_duration milliseconds, or 60 seconds in this example, MaxScale will disconnect the session.

Allowing high frequency bursts

The two parameters max_qps and sampling_duration together define how a session is throttled.

Suppose max qps is 400 qps and sampling duration is 10 seconds. Since QPS is not an instantaneous measure, but one could say it has a granularity of 10 seconds, we see that over the 10 seconds 10*400 = 4000 queries are allowed before throttling kicks in.

With these values, a fresh session can start off with a speed of 2000 qps, and maintain that speed for 2 seconds before throttling starts.

If the client continues to query at high speed and throttling duration is set to 10 seconds, Maxscale will disconnect the session 12 seconds after it started.

Filter Parameters

max_qps

Maximum queries per second. Required parameter.

This is the frequency to which a session will be limited over a given time period. QPS is not measured as an instantaneous value but over a configurable sampling duration (see sampling_duration).

throttling_duration

Required parameter. Time in milliseconds.

This defines how long a session is allowed to be throttled before MaxScale disconnects the session.

sampling_duration

Optional parameter. Default 250 milliseconds.

Sampling duration defines the window of time over which QPS is measured. This parameter directly affects the amount of time that high frequency queries are allowed before throttling kicks in.

The lower this value is, the more strict throttling becomes. Converserly, the longer this time is, the longer bursts of high frequency querying is allowed.

Due to the underlying granularity of time measurment (as of June 2018), it is not recommended that this value is set to less than 100 milliseconds.

continuous_duration

Optional parameter. Default 2000 milliseconds or 2 seconds.

This value defines what continuous throttling means. Continuous throttling starts as soon as the filter throttles the frequency. Continuous throttling ends when no throttling has been performed in the past continuous_duration time.