Throttle

The Throttle filter sets the average Rate of messages per Interval, based on the leaky bucket and sliding window algorithm. In case of flooding, it will leak at a certain rate.

Configuration parameters

The plugin supports the following configuration parameters:

Key	Value Format	Description
Rate	Integer	Amount of messages for the time.
Window	Integer	Amount of intervals to calculate average over. Default: 5.
Interval	String	Time interval, expressed in sleep format. For example, 3s, 1.5m, 0.5h.
Print_Status	Bool	Whether to print status messages with current rate and the limits to information logs.

Functional description

Using the following configuration:

Rate 5
Window 5
Interval 1s

You would receive 1 message in the first second, 3 messages second, and 5 third. Disregard that Window is actually 5, because the configuration uses slow start to prevent flooding during the startup.

+-------+-+-+-+
|1|3|5| | | | |
+-------+-+-+-+
|  3  |         average = 3, and not 1.8 if you calculate 0 for last 2 panes.
+-----+

But as soon as you reach Window size * Interval, you will have true sliding window with aggregation over complete window.

+-------------+
|1|3|5|7|3|4| |
+-------------+
  |  4.4    |
  ----------+

When the average over window is more than Rate, Fluent Bit starts dropping messages, so the following:

+-------------+
|1|3|5|7|3|4|7|
+-------------+
    |   5.2   |
    +---------+

will become:

+-------------+
|1|3|5|7|3|4|6|
+-------------+
    |   5     |
    +---------+

The last pane of the window was overwritten and 1 message was dropped.

Interval versus Window size

You might notice it's possible to configure the Interval of the Window shift. It's counterintuitive, but there is a difference between the two previous examples:

Rate 60
Window 5
Interval 1m

and

Rate 1
Window 300
Interval 1s

Even though both examples will allow maximum Rate of 60 messages per minute, the first example might get all 60 messages within first second, and will drop all the rest for the entire minute:

XX        XX        XX
XX        XX        XX
XX        XX        XX
XX        XX        XX
XX        XX        XX
XX        XX        XX
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

While the second example won't allow more than 1 message per second every second, making output rate more smooth:

  X    X     X    X    X    X
XXXX XXXX  XXXX XXXX XXXX XXXX
+-+-+-+-+-+--+-+-+-+-+-+-+-+-+-+

Fluent Bit might drop some data if the rate is ragged. Use bigger intervals and rates for streams of rare but important events, while keeping Window bigger and Interval smaller for constantly intensive inputs.

Command line

It's suggested to use a configuration file.

The following command will load the Tail plugin and read the content of the lines.txt file. Then, the Throttle filter will apply a rate limit and only pass the records which are read below the rate:

fluent-bit -i tail -p 'path=lines.txt' -F throttle -p 'rate=1' -m '*' -o stdout

Configuration file

fluent-bit.yaml

pipeline:
  inputs:
    - name: tail
      path: lines.txt

  filters:
    - name: throttle
      match: '*'
      rate: 1000
      window: 300
      interval: 1s

  outputs:
    - name: stdout
      match: '*'

fluent-bit.conf

This example will pass 1000 messages per second in average over 300 seconds.