There are two clean ways to enrich, and the difference matters. For data that is static or derivable — a country name from a code, a check digit, a category from a table baked into the platform — use a pure custom function inside the map. For live data that must be fetched from a system of record, do the lookup on a port via a caller adapter, not as a hidden call buried in a map.

Keeping live lookups on a port preserves everything the bus gives you: the call is visible, retryable and resilient, and a slow reference system does not freeze a map mid-transform. The opposite — a function quietly calling an API for every record — is exactly what keep custom functions pure warns against.

In Art2link, where the missing value is derivable, compute it in the map with a custom function — but call it the right way. A function that derives a value from the message has to read its inputs from the current node, so invoke it from the XSLT with art:invoke and pass the loop-context nodes as arguments — art:invoke('lineTotal', Quantity, UnitPrice) inside the template that matches each line. Do not feed it binding tokens like {{Promoted.Order.Qty}}: bindings are resolved by a preprocessor before the map runs, so they are not loop-aware — every iteration receives the same single pre-resolved value, not the value of the line being processed. Reserve the binding form for genuinely scalar, message-level inputs. Where the value is live reference data — a current price, a credit status — do not hide a fetch inside a map. Make it explicit: a two-way send port with a SQL Caller looks the value up and republishes the enriched message to the bus, where the rest of the flow picks it up.