Add instructions for manually generating and testing the spring compatibility project (#33)

Author: holly-cummins

README.md

@@ -7,7 +7,7 @@ This project contains the following modules:
 - [quarkus3](quarkus3)
     - A Quarkus 3.x version of the application
 - [quarkus3-spring-compatibility](quarkus3-spring-compatibility)
-    - A Quarkus 3.x version of the application using the Spring compatibility layer
+    - A Quarkus 3.x version of the application using the Spring compatibility layer. You can also recreate this application from the spring application using [a few manual steps](spring-conversion.md).
 
 ## Building
 

spring-conversion.md

@@ -0,0 +1,26 @@
+# Converting the Spring application to a Quarkus one with compatibility libraries
+
+It is possible to recreate the application parts of the `quarkus3-spring-compatibility` project with a few steps.
+This is a great way of seeing that the Quarkus performance gains are because of the runtime, not the programming model or code of the application.
+
+## Doing the conversion for dev mode
+
+1. Save the `pom.xml`, which we don't want to write by hand: `cp quarkus3-spring-compatibility/pom.xml spring-pom.xml` 
+2. Delete the compatibility project (we're about to recreate it from scratch!): `rm -rf quarkus3-spring-compatibility`
+3. Replace the spring pom: `mv spring-pom.xml springboot3/pom.xml`
+4. Delete the tests, which are hard to convert ([for now](https://github.com/orgs/quarkusio/projects/60)): `rm -rf springboot3/src/test`
+5. Start the app, with `(cd springboot3; quarkus dev)`. You'll see a failure and a crash.
+6. What's the problem? The `SpringBootApplication` class doesn't compile. The good news is it's not even needed with Quarkus. Delete it.
+7. Try `quarkus dev` again, and everything should work. Visiting the endpoint in a browser should work.
+
+## Prod mode and stress testing
+
+Although everything is working in dev mode, if you look at the `application.yml`, you can see it's filled with spring config.
+There's no Quarkus database configured. In dev mode, that's fine, because Quarkus auto-starts one as a dev service, but that won't work in prod mode.
+So we need to fill in config.
+
+1. Copy the Quarkus config over to the Spring app: `cp ./quarkus3/src/main/resources/application.yml ./springboot3/src/main/resources/application.yml`
+2. Build the app `(cd springboot3; ./mvnw clean package)` (don't forget the clean)
+3. Run the stress tests: `./scripts/stress.sh springboot3/target/quarkus-app/quarkus-run.jar`
+
+You should see that the throughput almost identical to the throughput of the 'normal' Quarkus app, and more than double that of the Quarkus-free Spring app.