In our deployment we have 40 external tasks clients executing the fetch and locks for the same topic.That topic name and max tasks are the only filters used when doing the fetch and lock.
There are around 1 million external tasks pending to be fetched for the topic in the DB.
We have observed that the response to most of the external task fetch and lock requests is a 200 OK with an empty task list even though the fetch and lock handler debug logs shows the sql queries being executed for both the select to find the available
tasks and the update to lock them.
Based on the observed behavior we have reached the conclusion that for optimal performance we should only have 1 external task client executing the fetch and lock for a given topic.
We even tested this theory by reducing the number of external tasks clients per topic from 40 to 4 which resulted in a very significant improvement in the number of fetch and lock requests that did not return an empty list.
We would like to confirm our conclusion with the subject matter experts in this area since it seems like a limitation like this would have been highlighted in the Camunda documentation.