This fluentd output plugin helps you to calculate messages.

This plugin is based on fluent-plugin-datacalculator written by Muddy Dixon. This plugin doesn't have a summarize function which provided by fluent-plugin-datacalculator.

$ fluent-gem install fluent-plugin-simplearithmetic

Suppose you have a message like:

{
  'apple': 7,
  'orange': 3,
  'time_start': '2001-02-03T04:05:06Z',
  'time_finish': '2001-02-03T04:06:12Z',
}

Now you can calculate with this configuration:

<match arithmetic.test>
  type simple_arithmetic
  tag calculated.test

  <formulas>
    total_price   apple * 200 + orange * 100

    # Calculation order is from up to down.
    budget   2000 - total_price

    # You can also use Time.iso8601
    time_elapsed   Time.iso8601(time_finish) - Time.iso8601(time_start)
  </formulas>
</match>

<match calculated.test>
  type stdout
</match>

Calculated results will be:

{
	"apple": 7,
	"orange": 3,
	"time_start": "2001-02-03T04:05:06Z",
	"time_finish": "2001-02-03T04:06:12Z",
	"total_price": 1700,
	"budget": 300,
	"time_elapsed": 66.0
}

If some fields are already defined before calculation, these fields will be overwritten.

The tag prefix for emitted event messages. Default is simple_arithmetic.

A message such like {'a': 20, 'x': 50} with the formulas:

<formulas>
  c   a + b
</formulas>

When undefined_variables is undefined (default) you get {'a': 20, 'x': 50}. Field 'c' will not be defined.

When nil you get {'a': 20, 'c': nil, 'x': 50}.

A message such like {'a': 20, 'b': "string"} with the formulas:

<formulas>
  c   a + b
</formulas>

In ruby, this calculation will be raise an error:

irb(main):052:0* a = 20
=> 20
irb(main):053:0> b = "String"
=> "String"
irb(main):054:0> c = a + b
TypeError: String can't be coerced into Fixnum
	from (irb):54:in `+'
	from (irb):54
	from /opt/td-agent/embedded/bin/irb:11:in `<main>'

When how_to_process_error is nil you get {'a': 20, 'b': "string", 'c': nil}.

When undefined, you get {'a': 20, 'b': "string"}. If an error is raised in the calculation, the field will not be defined.

When error_string (default), you get: {'a': 20, 'b': "String", 'c'=>"String can't be coerced into Fixnum"}. An error message will be assigned to the field.

All formulas will be evaluated as ruby sentences. Some json fields will not be fitted as ruby variables. For example,

<formulas>
   var-1  a + b
   var$2  c * d
   var@   "@@@"
</formulas>

will raise an syntax error in the initialize process of fluentd.

To get rid of this case, you can set replace_hyphen, replace_at and replace_dollar in the configuration and formulas.

replace_hyphen   __H__
replace_dollar   __D__
replace_at       __AT__

<formulas>
   var__H__1  a + b
   var__D__2  c * d
   var__AT__  "@@@"
</formulas>

Copyright:: Copyright (c) 2014- Takahiro Kamatani

License:: Apache License, Version 2.0