Following fluentd SDK at https://docs.fluentd.org/plugin-development

reverse_key is a filter plugin that takes a record and reverses one of its values.

To configure the plugin at fluent.conf:

<filter **>
  @type reverse_key
  key level
</filter>

This filter takes a record with key named level, and reverse its value. By default, key equals to "message"

For example,

# Input record
{"datetime":"2024-06-01T22:32:14.774291","level":"ERROR","file":{"name":"main.py","line":44},"message":"Hello Error log","content":[]}

# Output record
{"datetime":"2024-06-01T22:32:14.774291","level":"RORRE","file":{"name":"main.py","line":44},"message":"Hello Error log","content":[]}

To add the plugin to fluentd, add the file filter_reverse_key.rb to /fluentd/plugins folder

Using docker-compose with a FastAPI app:

docker compose up --build -d