[Q&A] %{tag} ignored inside <match>

[duncan-bayne]

What is a problem?

I'm trying to prefix my objects in S3 with the tag that was matched. However, %{tag} is always empty / nil in the <match> directive.

I suspect I'm missing something simple here as other variables such as %{path} have expected values inside <match>.

Describe the configuration of Fluentd

If I have:

<source>
  @type tail
  path /var/log/ngninx/error.log
  pos_file /tmp/error.log.pos
  tag s3.nginx.error

  <parse>
    @type none
  </parse>
</source>

<match **>
  @type s3
  path /
  s3_bucket "my-bucket"
  s3_object_key_format "%{path}%{tag}.%{time_slice}.#{hostname}_%{index}.%{file_extension}"
  s3_region us-east-2

  <buffer>
    @type file
    chunk_limit_size 256m
    path /var/log/fluent/s3
    timekey 60
    timekey_wait 1m
  </buffer>

  time_slice_format %Y%m%d%H
</match>

However, %{tag} in s3_object_key_format is always evaluated to an empty string, never to the tag that was matched (s3.nginx.error in this case).

So e.g. I get S3 object keys like 2025021206.ip-10-10-2-126_1.gz, whereas I'd expect something like s3.nginx.error.2025021206.ip-10-10-2-126_1.gz.

Environment

- Fluentd version: fluentd 1.16.6 (fa1cfbb0335e7751a2fd5547897ab611484ee000)
- Fluent Package version: fluent-package 5.0.5
- Docker image (tag): N/A
- Operating system: Ubuntu 22.04.5 LTS
- Kernel version: 6.8.0-1021-aws

Thanks in advance for any advice 🙏 Really feel like I'm missing something simple here.