Monorepo

This repository is a monorepo for the Echo project. The repository will contain all applications, libraries and documentation related to the Echo project.

The reasons why I choose a monorepo are the following:

Reuse existing project and CI setup.
No need to publish versioned packages since all consumers are in the same repository.
Everything works together at every commit. There’s no such thing as a breaking change when you fix everything in the same commit.

As monorepo tool, I started with Nx by Nrwl. I chose the Nx plugin @jnxplus/nx-boot-maven to add Spring Boot and Maven multi-module project support. The generated applications and libraries look good. The only thing I adjust is how these maven modules are versioned. I applied the technique described in Maven CI Friendly Versions. That way all Maven modules have the same version that can be set by setting the revision property. For more information, read Semantic Versioning.

Using Nx and the Nx plugin @jnxplus/nx-boot-maven for a monorepo with a Maven multi-module project doesn’t feel right, and I’m close to exclude the Maven projects from the Nx workspace.

When I run nx affected:build, I see from the log output that multiple threads are building the same module concurrently. I recognized that because the build was randomly failing. While the first thread generated the .flattened-pom.xml, the second thread deleted the file. Next, the first thread tried to install that file into the local repository but failed because the file didn’t exist anymore.

Here is the corresponding log output:

[INFO] Scanning for projects...
[INFO] Scanning for projects...
[INFO]
[INFO] ---------------------< ch.harmen.echo:echo-parent >---------------------
[INFO] Building echo-parent 0.0.1-SNAPSHOT
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:3.2.0:clean (default-clean) @ echo-parent ---
[INFO]
[INFO] ---------------------< ch.harmen.echo:echo-parent >---------------------
[INFO] Building echo-parent 0.0.1-SNAPSHOT
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:3.2.0:clean (default-clean) @ echo-parent ---
[INFO] Deleting /home/runner/work/echo/echo/target
[INFO]
[INFO] --- flatten-maven-plugin:1.3.0:clean (flatten.clean) @ echo-parent ---
[INFO]
[INFO] --- flatten-maven-plugin:1.3.0:clean (flatten.clean) @ echo-parent ---
[INFO]
[INFO] --- flatten-maven-plugin:1.3.0:flatten (flatten) @ echo-parent ---
[INFO] Generating flattened POM of project ch.harmen.echo:echo-parent:pom:0.0.1-SNAPSHOT...
[INFO]
[INFO] --- maven-install-plugin:2.5.2:install (default-install) @ echo-parent ---
[INFO] Deleting /home/runner/work/echo/echo/target/.flattened-pom.xml
[INFO]
[INFO] --- flatten-maven-plugin:1.3.0:flatten (flatten) @ echo-parent ---
[INFO] Generating flattened POM of project ch.harmen.echo:echo-parent:pom:0.0.1-SNAPSHOT...
[INFO] Installing /home/runner/work/echo/echo/target/.flattened-pom.xml to /home/runner/.m2/repository/ch/harmen/echo/echo-parent/0.0.1-SNAPSHOT/echo-parent-0.0.1-SNAPSHOT.pom
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.754 s
[INFO] Finished at: 2022-11-05T20:31:34Z
[INFO] ------------------------------------------------------------------------
Error:  Failed to execute goal org.apache.maven.plugins:maven-install-plugin:2.5.2:install (default-install) on project echo-parent: Failed to install artifact ch.harmen.echo:echo-parent:pom:0.0.1-SNAPSHOT: /home/runner/work/echo/echo/target/.flattened-pom.xml (No such file or directory) -> [Help 1]

I could disable the deletion of the .flattened-pom.xml, but that wouldn’t solve the problem. I would still have multiple threads that are building the same things concurrently and therefore might interfere with each other.

I could set --parallel=1 (the default is --parallel=3) to avoid the problem, but it is sad to go all serial.

Maven’s reactor has a much better understanding of Maven multi-module projects. Therefore, the reactor can build Maven multi-module projects a lot better. If I tell Maven to build my multi-module project with multiple threads. The reactor will build every module exactly once and go parallel wherever the dependency tree allows it.

So, I’m tempted to exclude Maven projects from the Nx workspace. Then, I define a dedicated GitHub Action workflow for every deployable artifact in my Maven projects. With including and excluding paths, I ensure that these workflows only run, if relevant sources changed. Including and excluding paths are like the poor man’s nx affected. The only sad thing I see for this solution is that I would maintain the dependencies of my deployable Maven artifacts twice. Once in the pom.xml files and once in the including paths in the workflow files.