[collectd] Metric ordering with collectd, the Modbus and Kafka plugins

[Felix Dreissig]

Hi,

in a test setup, we’re currently using a single collectd instance to gather data via Modbus/TCP TCP and send them to Kafka.
I’m trying to get a precise understanding of how static the ordering of metrics is in that configuration:

1. From my understanding, the "Modbus“ plugin reads registers sequentially, at least within one host. Will it always do that in the same order? Is there parallelism in place when querying multiple Modbus hosts?

2. I suppose collectd keeps an internal metric buffer of some sort. Is that buffer ordered?

3. Is the "Write Kafka“ plugin’s behavior deterministic between accessing the buffer (if I’m right that there actually is one) and sending data out? Taking a quick look at librdkafka’s API, it looks quite asynchronous. How does collectd handle errors when sending metrics to Kafka?

Thanks in advance for providing insights!

Regards,
Felix