[BUG] Logstash pushing logs to opensearch with uneven batch size

[NamanChandra1]

Describe the bug
Logstash pushing logs to opensearch with uneven batch size.
I am using default pipeline.batch.size and pipeline.worker.
output configuration:

output {
      if "_jsonparsefailure" not in [tags] {  
            opensearch {
                hosts => ["https://admin:[email protected]:9200"]
                index => "uat-app-log-%{+YYYY.MM.dd}"
                user => "xxx"
                password => "xxx"
                ssl => true
                ssl_certificate_verification => false
                sniffing  => true
                sniffing_delay => 30
                sniffing_path  => "/_nodes/data:true"
                target_bulk_bytes => 20000000
        }
    }
}

Debug Logs:

[2024-10-21T05:31:37,063][DEBUG][logstash.outputs.opensearch][main][e67881b8b7c301e7d63b09960cb80d7322ea5a5020e63ce9780f628394358289] Sending final bulk request for batch. {:action_count=>2, :payload_size=>2410, :content_length=>2410, :batch_offset=>0}
[2024-10-21T05:31:37,070][DEBUG][logstash.outputs.opensearch][main][b929b8d03a3787f0ae738eff4a4e20aa5b328a50c85b4b975d4f0449a0e44bdb] Sending final bulk request for batch. {:action_count=>98, :payload_size=>136636, :content_length=>136636, :batch_offset=>0}
[2024-10-21T05:31:37,057][DEBUG][logstash.outputs.opensearch][main][b929b8d03a3787f0ae738eff4a4e20aa5b328a50c85b4b975d4f0449a0e44bdb] Sending final bulk request for batch. {:action_count=>74, :payload_size=>103739, :content_length=>103739, :batch_offset=>0}
[2024-10-21T05:31:37,061][DEBUG][logstash.outputs.opensearch][main][b929b8d03a3787f0ae738eff4a4e20aa5b328a50c85b4b975d4f0449a0e44bdb] Sending final bulk request for batch. {:action_count=>67, :payload_size=>93933, :content_length=>93933, :batch_offset=>0}
[2024-10-21T05:31:37,098][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>14, :payload_size=>14581, :content_length=>14581, :batch_offset=>0}
[2024-10-21T05:31:37,074][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>93, :payload_size=>138313, :content_length=>138313, :batch_offset=>0}
[2024-10-21T05:31:37,071][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>7, :payload_size=>8302, :content_length=>8302, :batch_offset=>0}
[2024-10-21T05:31:37,071][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>8, :payload_size=>9847, :content_length=>9847, :batch_offset=>0}
[2024-10-21T05:31:37,074][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>125, :payload_size=>165378, :content_length=>165378, :batch_offset=>0}
[2024-10-21T05:31:37,093][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>51, :payload_size=>68384, :content_length=>68384, :batch_offset=>0}
[2024-10-21T05:31:37,098][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>58, :payload_size=>77009, :content_length=>77009, :batch_offset=>0}
[2024-10-21T05:31:37,103][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>125, :payload_size=>165540, :content_length=>165540, :batch_offset=>0}
[2024-10-21T05:31:37,116][DEBUG][logstash.outputs.opensearch][main][b929b8d03a3787f0ae738eff4a4e20aa5b328a50c85b4b975d4f0449a0e44bdb] Sending final bulk request for batch. {:action_count=>125, :payload_size=>175130, :content_length=>175130, :batch_offset=>0}
[2024-10-21T05:31:37,063][DEBUG][logstash.outputs.opensearch][main][b929b8d03a3787f0ae738eff4a4e20aa5b328a50c85b4b975d4f0449a0e44bdb] Sending final bulk request for batch. {:action_count=>44, :payload_size=>61732, :content_length=>61732, :batch_offset=>0}
[2024-10-21T05:31:37,113][DEBUG][logstash.outputs.opensearch][main][480ab6bfbcda240d793d62512345df39027250bde58c8527358f92c58c6a4afb] Sending final bulk request for batch. {:action_count=>125, :payload_size=>165220, :content_length=>165220, :batch_offset=>0} 

Host/Environment (please complete the following information):

• EKS version 1.30

[dblock]

I bet there's some time-based criteria in addition to batch size. Care to dig into the implementation to see how it's deciding to send the data? Documenting it would be a great start.