SOLID in MuleSoft — runnable examples

Before-and-after examples of the five SOLID principles applied to Mule 4 flows. Each principle has a before endpoint that breaks the principle and an after endpoint that follows it, and MUnit tests show where the two behave the same and where the "before" version causes problems.

This repository accompanies the article series SOLID in MuleSoft (also available in Portuguese).

The app needs no external systems: ERPs, databases and payment providers are simulated with synthetic data, so it runs anywhere.

The examples

Principle	Scenario	Before	After	Article
Single Responsibility	Process an order	One flow validates, prices and calls the ERP	The flow orchestrates; each step is a focused sub-flow	SRP
Open/Closed	Take a payment	A choice router that must be edited for every new method	The router looks up the implementation in config; pix was added without touching it	OCP
Liskov Substitution	Read an order from the legacy or the new ERP	The new ERP adapter looks compatible but changes values and error behaviour, so the consumer silently takes wrong decisions	Both adapters honour the whole contract (shape, values, errors) and can replace each other	LSP
Interface Segregation	Read an order	One endpoint returns everything to everyone, including internal cost data	Mobile and finance each get an interface shaped for their need	ISP
Dependency Inversion	Read an order and decide if it can be cancelled	The business rule knows the legacy ERP's field names and codes	The rule depends on an "order repository"; in-memory and ERP adapters are swapped in config	DIP

Layout

src/main/mule/
  global.xml              HTTP listener, properties, error handler (400/404 as JSON)
  srp/ ocp/ lsp/ isp/ dip/
    <principle>-before.xml
    <principle>-after.xml
src/main/resources/config/app.yaml   port, OCP payment routing, DIP repository choice
src/test/munit/<principle>-test-suite.xml
docs/contracts/          RAML types for the LSP and ISP contracts
requests.http            sample requests for every endpoint

Run

Requirements: Java 17, Maven 3.9+, Mule runtime 4.9 (Anypoint Studio or a standalone runtime).

• Anypoint Studio: import the project (File → Import → Anypoint Studio project from file system) and run it. The app listens on port 8081.
• Try it: open requests.http in VS Code (REST Client) or IntelliJ and send the requests, or use curl:

curl -s -X POST localhost:8081/lsp/before/quotes -H 'Content-Type: application/json' \
  -d '{"orderId":"ORD-1","orderType":"express","price":50,"quantity":2}'

Test

mvn clean test

MUnit runs on the Mule Enterprise runtime, so Maven needs access to the MuleSoft Enterprise repository (credentials in ~/.m2/settings.xml, as in any MuleSoft project). The build fails below 80% application coverage; the current suites cover 36 cases at about 95%.

Notes

• The OCP router uses a dynamic flow-ref. MUnit only loads flows that are referenced statically, so the OCP suite references the payment implementations in a before-suite. Real applications do not need this.
• Everything here is intentionally small. The point is the shape of the flows, not the business logic.

Licence

MIT © Bruno Pinto de Souza