Dev services documentation

docs/src/main/asciidoc/extension-writing-dev-service.adoc

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+////
+This document is maintained in the main Quarkus repository
+and pull requests should be submitted there:
+https://github.com/quarkusio/quarkus/tree/main/docs/src/main/asciidoc
+////
+[id="extension-writing-dev-service"]
+= Writing a dev service
+include::_attributes.adoc[]
+:categories: writing-extensions
+:diataxis-type: howto
+:topics: extensions
+////
+////
+
+
+== Prerequisites
+
+- You should already have an xref:building-my-first-extension.adoc[extension structure] in place
+- You should have a containerised version of your external service (not all dev services rely on containers, but most do)
+
+== Creating a dev service
+
+If your extension provides APIs for connecting to an external service, it's a good idea to provide a xref:dev-services.adoc[dev service] implementation.
+
+To create a dev service, add a new build step into the extension processor class that returns a `DevServicesResultBuildItem`.
+Here, the link:https://hub.docker.com/_/hello-world`hello-world` image is used, but you should set up the right image for your service.
+
+[source%nowrap,java]
+----
+    @BuildStep(onlyIfNot = IsNormal.class, onlyIf = GlobalDevServicesConfig.Enabled.class) {
+    public DevServicesResultBuildItem createContainer() {
+        DockerImageName dockerImageName = DockerImageName.parse("hello-world");
+        GenericContainer container = new GenericContainer<>(dockerImageName)
+                                         .withExposedPorts(SERVICE_PORT, OTHER_SERVICE_PORT)
+                                         .waitingFor(Wait.forLogMessage(".*" + "Started" + ".*", 1))
+                                         .withReuse(true);
+
+        container.start();
+
+        String newUrl = "http://" + container.getHost() + ":" + container.getMappedPort(SERVICE_PORT);
+        Map<String, String> configOverrides = Map.of("some-service.base-url", newUrl);
+
+        return new DevServicesResultBuildItem.RunningDevService(FEATURE, container.getContainerId(),
+            container::close, configOverrides)
+            .toBuildItem();
+    }
+----
+
+With this code, you should be able to see your container starting if you add your extension to a test application and run `quarkus dev`.
+However, the application will not be able to connect to it, because no ports are exposed. To expose ports, add `withExposedPorts` to the container construction.
+For example,
+
+[source%nowrap,java]
+----
+GenericContainer container = new GenericContainer<>(dockerImageName)
+                                 .withExposedPorts(SERVICE_PORT, OTHER_SERVICE_PORT);
+----
+
+Testcontainers will map these ports to random ports on the host. This avoids port conflicts, but presents a new problem – how do applications connect to the service in the container?
+
+To allow applications to connect, the extension should override the default configuration for the service with the mapped ports.
+This must be done after starting the container.
+For example,
+
+[source%nowrap,java]
+----
+    container.start();
+    Map<String, String> configOverrides = Map.of("some-service.base-url",
+         "http://" + container.getHost() + ":" + container.getMappedPort(SERVICE_PORT));
+----
+
+Other configuration overrides may be included in the same map.
+
+== Waiting for the container to start
+
+You should add a `.waitingFor` call to the container construction, to wait for the container to start. For example
+
+[source%nowrap,java]
+----
+            .waitingFor(Wait.forLogMessage(".*" + "Started" + ".*", 1))
+----
+
+Waiting for a port to be open is another option. See the link:https://java.testcontainers.org/features/startup_and_waits/[Testcontainers documentation] for a full discussion of wait strategies.
+
+== Configuring the dev service
+
+To configure the dev service launch process, your build step can accept a `ConfigPhase.BUILD_TIME` config class in its constructor.
+For example,
+
+[source%nowrap,java]
+----
+    @BuildStep(onlyIfNot = IsNormal.class, onlyIf = GlobalDevServicesConfig.Enabled.class) {
+    public DevServicesResultBuildItem createContainer(MyConfig config) {
+----
+
+You may wish to use this config to set a fixed port, or set an image name, for example.
+
+[source%nowrap,java]
+----
+    if (config.port.isPresent()) {
+        container.setPortBindings(List.of(config.port.get() + ":" + SERVICE_PORT));
+    }
+----
+
+== Controlling re-use
+
+In dev mode, with hot reload, Quarkus may restart frequently. By default, this will also restart test containers.
+Quarkus restarts are usually very fast, but containers may take much longer to restart.
+To prevent containers restarting on every code change, you can mark the container as reusable:
+
+[source%nowrap,java]
+----
+         .withReuse(true)
+----
+
+Some dev services implement sophisticated reuse logic in which they track the state of the container in the processor itself.
+You may need this if your service has more complex requirements, or needs sharing across instances.
+
+
+== References
+
+- xref:dev-services.adoc[Dev services overview]
+- xref:writing-extensions.adoc[Guide to writing extensions]

docs/src/main/asciidoc/writing-extensions.adoc

@@ -1978,6 +1978,12 @@ in your runtime module, and add a `META-INF/services/io.quarkus.dev.spi.HotRepla
 On startup the `setupHotDeployment` method will be called, and you can use the provided `io.quarkus.dev.spi.HotReplacementContext`
 to initiate a scan for changed files.
 
+==== Dev services
+
+Where extensions use an external service, adding dev service can improve the user experience in development and test modes.
+See xref:extension-writing-dev-service.adoc[how to write a dev service] for more details.
+
+
 === Testing Extensions
 
 Testing of Quarkus extensions should be done with the `io.quarkus.test.QuarkusUnitTest` JUnit 5 extension.