Logstash is a program that collects json documents from various inputs, transform them according to a configuration file (a filter), and sends them to various outputs. This script helps you make sure that your filter does what you expect by writing a test suite.

This is similar to logstash-filter-verifier. After being frustrated with it for a few days I decided to write my own. I find it to be much simpler and easier. Try it and decide for yourself!

You can install using pip:

pip install logstash-filter-test

or just copy logstash_filter_run.py and logstash_filter_test.py. There are no dependencies. Python 3.5+ and 2.7 are supported.

Here is a simple test suite file:

testcases.js:

[
  // Test a message with a value
  [
    {
      "message": "<<2018-06-11 13:45:39,127+0300>> TAU = 6.2831853"
    },
    {
      "date": "2018-06-11 13:45:39,127+0300",
      "@timestamp": "2018-06-11T10:45:39.127Z",
      "msg_text": "TAU = 6.2831853",
      "metric": "TAU",
      "value": 6.2831853,
    }
  ],
]

filter.conf:

filter {
  grok {
    match => {
      "message" => [
        "^<<%{TIMESTAMP_ISO8601:date}>> %{GREEDYDATA:msg_text}$"
      ]
    }
  }
  date {
    match => ["date", "ISO8601"]
  }
  grok {
    match => {
      "msg_text" => [
        "^%{WORD:metric} = %{NUMBER:value:float}"
      ]
    }
    tag_on_failure => []
  }
}

Put them in a directory, run ./logstash_filter_test.py and it will inform you if the testcases passed, and if not, why.

testcases.js can be a simple json file. It has the extension .js because javascript comments and trailing commas are allowed, thanks to jstyleson. It is a list of test cases. Each test case is a list of size 2. The first item is the input json document. The second item is compared to the output json document. All fields that are defined must be equal to the fields in the output document. The output document may include other fields. To test that an output field doesn't exist, use "field": null.

If you don't like the testcase file format, it's easy to test by yourself:

>>> from logstash_filter_run import logstash_filter_run
>>> logstash_filter_run([{"a": 3}], 'filter { mutate { copy => { "a" => "b" } } }')
[{'@timestamp': '2018-07-10T18:26:58.411Z',
  '@version': '1',
  'a': 3,
  'b': 3,
  'host': 'myhost'}]