Filebeat rename fields example - filebeat

Question

Using filebeat 6.8 open source version, I'm trying to use the field rename feature. I'm not seeing any errors in startup or processing, but the field isn't getting renamed. The logs are JSON formatted. Am I missing something in my config, or is this combination not supported yet?
filebeat.yml
processors:
- rename:
fields:
- from: "a"
to: "b"
filebeat.inputs:
- type: log
enabled: true
json.keys_under_root: true
fields_under_root: true
sample log
{
"a": "blah"
}

Answer 1

Edit:
According to the official documentation, processors can be placed at top level or under an input.
So, perhaps what your configuration is missing is the file paths to prospect.
I'm running filebeat 7.6.2 and I use this feature without any issues, but I'm pretty sure it works on version 6.8 just as well since these fields are documented for this version. See here.
Have you tried:
filebeat.inputs:
- type: log
paths:
- /path/to/your/logs
enabled: true
json.keys_under_root: true
fields_under_root: true
processors:
- rename:
fields:
- from: "a"
to: "b"
Processors should be declared after inputs AFAIK. As for not seeing any errors, I'm not sure how far beyond simple yaml linting filebeat goes when it comes to validating your configuration file.