Fluentd Yaml configuration

[gourabh09]

previously we were working with config file format now while migrating to yaml format
here i got stuck at one point

@type opensearch
host $HOSTFQDN
...

what will equivalent yaml for this $HOSTFQDN

[daipom]

Is HOSTFQDN a chunk key?
(Is it a placeholder? https://docs.fluentd.org/configuration/buffer-section#placeholders )

Just using ’$HOSTFQDN’ or ’${HOSTFQDN}’ becomes an error?

[gourabh09]

2025-07-05 14:27:34 +0000 [info]: adding match pattern="fluentbit.forwarded" type="opensearch"
2025-07-05 14:27:35 +0000 [debug]: #0 'host ${HOSTNAME}' is tested built-in placeholder(s) but there is no valid placeholder(s). error: Parameter 'host: ${HOSTNAME}' doesn't have tag placeholder
2025-07-05 14:27:35 +0000 [debug]: #0 'host ${HOSTNAME}' is tested built-in placeholder(s) but there is no valid placeholder(s). error: Parameter 'host: ${HOSTNAME}' has placeholders, but chunk keys doesn't have keys HOSTNAME
2025-07-05 14:27:35 +0000 [debug]: #0 'index_name fluentd' is tested built-in placeholder(s) but there is no valid placeholder(s). error: Parameter 'index_name: fluentd' doesn't have tag placeholder
this are the error logs
i am using host: ${HOSTNAME}

[kenhys]

I'm not sure, but it might help what you want(if it's not placeholder):
https://docs.fluentd.org/configuration/config-file-yaml#embedded-ruby-code.

[gourabh09]

this works thanks

[daipom]

Parameter 'host: ${HOSTNAME}' has placeholders, but chunk keys doesn't have keys HOSTNAME

Looks like it is a placeholder.
Did you forget to set chunk keys?

- match:
    $type: ...
    ...
    buffer:
      $type: file
      $arg: (chunk keys)
      ...