Migrating from DataWeave 1.0 to 2.0: A Practical Guide

If you're upgrading from Mule 3 to Mule 4, every DataWeave file needs to be rewritten. DW 2.0 is not backwards-compatible with 1.0 — the syntax changed, type names changed, flow control changed, and the module system is completely new. I've migrated dozens of Mule projects and compiled the most common changes, gotchas, and anti-patterns into a comprehensive migration guide . Here's the practical version. The Big Syntax Changes 1. Header Declarations Lost the % DW 1.0 used % prefixes for everything in the header. DW 2.0 dropped them. What DW 1.0 DW 2.0 Version %dw 1.0 %dw 2.0 Output %output application/json output application/json Variables %var x = 42 var x = 42 Functions %function name(args) fun name(args) = Namespaces %namespace ns http://... ns ns http://... DW 1.0: %dw 1.0 %output application/json %var threshold = 100 %function isActive(emp) emp.status == "ACTIVE" --- payload filter isActive($) filter $.salary > threshold DW 2.0: %dw 2.0 output application/json var threshold = 100