mojohaus#725: Migrate Update Child Modules to Markdown

src/site/markdown/examples/update-child-modules.md

@@ -0,0 +1,123 @@
+title: Fixing a multi-module build
+author: Stephen Connolly
+date: 2008-09-25
+
+<!---
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+  https://www.apache.org/licenses/LICENSE-2.0
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# Fixing a multi-module build
+
+If you have a multi-module build where the aggregator pom (i.e. the one with packaging of `pom` and the
+`modules` section) is also the parent referenced by its child modules, and the aggregator version does not
+match the version specified in the parent section of the child modules, Maven will not let you build the project.
+
+The `update-child-modules` is designed to try and help you fix these kind of build failures.
+
+You know you have this kind of problem when you see output from Maven like this:
+
+```log
+[INFO] Scanning for projects...
+[INFO] ------------------------------------------------------------------------
+[ERROR] FATAL ERROR
+[INFO] ------------------------------------------------------------------------
+[INFO] Error building POM (may not be this project's POM).
+
+
+Project ID: com.foo.bar:bar-child:jar:null
+
+Reason: Cannot find parent: com.foo.bar:bar for project: com.foo.bar:bar-child:jar:null
+```
+
+The above example was for a project structured like this:
+
+- /pom.xml
+    - /bar-child/pom.xml
+
+With `/pom.xml` looking like
+
+```xml
+<project>
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>com.foo.bar</groupId>
+  <artifactId>bar</artifactId>
+  <version>2.0</version>
+  <packaging>pom</packaging>
+
+  <modules>
+    <module>bar-child</module>
+  </modules>
+
+  ...
+
+</project>
+```
+
+And with `/bar-child/pom.xml` referencing a previous version of the parent:
+
+```xml
+<project>
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+    <groupId>com.foo.bar</groupId>
+    <artifactId>bar</artifactId>
+    <version>1.0</version>
+  </parent>
+
+  <artifactId>bar-child</artifactId>
+
+  ...
+
+</project>
+```
+
+In order to fix the project, you need to either update the version of the child-module, or roll-back the version of
+the parent module. When there is only one child module, this is easiest to do by hand. When there are many modules,
+some referencing the new version and some referencing old versions of the parent, this is easiest to do using the
+[versions:update-child-modules](../update-child-modules-mojo.html) goal.
+
+Note: it is essential in this situation to run Maven with the `-N` option, as otherwise Maven will be unable
+to run the goal.
+
+```shell
+mvn -N versions:update-child-modules
+```
+
+  Which produces the following output:
+
+```log
+[INFO] Scanning for projects...
+[INFO] ------------------------------------------------------------------------
+[INFO] Building update-child-modules
+[INFO]    task-segment: [versions:update-child-modules] (aggregator-style)
+[INFO] ------------------------------------------------------------------------
+[INFO] [versions:update-child-modules]
+[INFO] Module: bar-child
+[INFO]   Parent was com.foo.bar:bar:1.0, now com.foo.bar:bar:2.0
+[INFO] ------------------------------------------------------------------------
+[INFO] BUILD SUCCESSFUL
+[INFO] ------------------------------------------------------------------------
+[INFO] Total time: 3 seconds
+[INFO] Finished at: Thu Nov 20 15:37:40 GMT 2008
+[INFO] Final Memory: 12M/288M
+[INFO] ------------------------------------------------------------------------
+```
+
+If you have several layers of projects, and the versions are really out of whack, you may have to change directory
+into the child aggregator projects and re-run the [versions:update-child-modules](../update-child-modules-mojo.html)
+goal in order to finish fixing the project.