@Hans FeldmannThe best thing to explain this is to take an example. Let's say that you want to add an attribute city with a SimpleKVLookupService. And you will be doing the lookup by an attribute id_store (look at this article to understand the logic behind this use case).
You need to configure LookupAttribute as follows:
This means that the processor will use the value of the attribute id_store as a key and query the lookupservice. the returned value will be added as an attribute city. Here's the result:
Of course, the flow file should have the attribute id_store before going in LookupAttribute.
And here's the content of the SimpleKVLookupService
I hope this helps