Is max_send_limit_bytes Evaluated Before Compression in Fluentd rdkafka2 Plugin?

[anupg336]

Describe the bug

We are using the Fluentd Kafka output plugin (rdkafka2) to send data to Kafka, and we noticed some behavior regarding max_send_limit_bytes when compression is enabled. We would like to confirm if this is expected or a known limitation.

Our configuration enables gzip compression with a 1MB send limit:

<match **>
  @type rdkafka2
  brokers <broker-list>
  compression_codec gzip
  max_send_limit_bytes 1m
</match>

We observed that max_send_limit_bytes is enforced based on the uncompressed payload size. Even when the compressed batch size is below 1MB, Fluentd does not send the data if the uncompressed size exceeds 1MB. This indicates that size validation is occurring before compression is applied by rdkafka.

Can you please confirm if this behavior is expected?

Additionally, if this is the intended behavior, is there any Fluentd or rdkafka configuration that allows enforcing the 1MB limit after compression is applied?

To Reproduce

NA

Expected behavior

NA

Your Environment

- Fluentd version: 1.19.0
- TD Agent version: 6.0.0
- fluent-plugin-kafka version:0.19.5
- ruby-kafka version: 1.5.0
- rdkafka version: 0.21.0
- Operating system: rocky8
- Kernel version: 5.14

Your Configuration

NA

Your Error Log

NA

Additional context

No response

[Watson1978]

1. Why the size check happens before compression

In the current implementation of rdkafka2, the plugin checks the size of the serialized message before passing it to librdkafka.

The ompression happens inside librdkafka afterwards, so Fluentd cannot know the compressed size in advance.

Therefore, the behavior you observed is the expected behavior given the current design.

2. Can Fluentd enforce “compressed size must be < 1MB”?

Unfortunately, Fluentd cannot calculate or predict the compressed size because:

• Compression is performed by librdkafka after the plugin hands over the payload
• The plugin only has access to the pre-compressed bytes

So Fluentd cannot reliably enforce a threshold based on post-compression size.