How to Run One Mule 4 App Across Dev, UAT, and Prod Without Code Changes

A developer building a Mule 4 integration between a bank's core-banking system and Salesforce CRM faced the challenge of deploying a single application across three environments with entirely different endpoints, credentials, and timeouts. To avoid hardcoding values — a practice that once caused a team to accidentally generate over a thousand bogus transactions in production — the solution was to externalize all configuration using Mule 4's configuration properties feature. Environment-specific values are stored in separate YAML files (dev.yaml, uat.yaml, prod.yaml), with a single system property like -Dmule.env=prod determining which file loads at startup. This approach keeps the deployed artifact identical across all environments, with connectors and DataWeave scripts referencing placeholder keys that resolve at runtime. A key detail is file loading order: the environment-specific file must be declared after the shared common.yaml so that per-environment values correctly override any shared defaults.