Packaging type -Dquarkus.package.create-appcds=true isn't documented

[jerboaa]

Describe the bug

I've just heard that there is application class data sharing support in Quarkus via -Dquarkus.package.create-appcds=true, but it isn't documented. So without anyone explicitly asking/knowing about it there is no way to discover that feature.

Expected behavior

Find a guide explaining what -Dquarkus.package.create-appcds=true does and why I should be using it.

Actual behavior

No way to find documentation other than looking through the code

$ grep -rn SharedArchiveFile
core/deployment/src/main/java/io/quarkus/deployment/pkg/steps/AppCDSBuildStep.java:108:                            "run the application jar from its directory and also add the '-XX:SharedArchiveFile=app-cds.jsa' " +
core/deployment/src/main/java/io/quarkus/deployment/pkg/steps/AppCDSBuildStep.java:239:        // We use the relative paths because at runtime 'java -XX:SharedArchiveFile=... -jar ...' expects the AppCDS and jar files
core/deployment/src/main/java/io/quarkus/deployment/pkg/steps/AppCDSBuildStep.java:242:        // The end result is that users can simply use 'java -XX:SharedArchiveFile=app-cds.jsa -jar app.jar'
core/deployment/src/main/java/io/quarkus/deployment/pkg/steps/AppCDSBuildStep.java:243:        javaArgs.add("-XX:SharedArchiveFile=" + appCDSPath.getFileName().toString());
extensions/container-image/container-image-jib/deployment/src/main/java/io/quarkus/container/image/jib/deployment/JibProcessor.java:621:                if (effectiveJvmArgument.startsWith("-XX:SharedArchiveFile")) {
extensions/container-image/container-image-jib/deployment/src/main/java/io/quarkus/container/image/jib/deployment/JibProcessor.java:627:                effectiveJvmArguments.add("-XX:SharedArchiveFile=" + appCDSResult.get().getAppCDS().getFileName().toString());
integration-tests/maven/src/test/java/io/quarkus/maven/it/JarRunnerIT.java:497:                List.of("-XX:SharedArchiveFile=app-cds.jsa", "-Xshare:on", "-Xlog:class+path=info"))

Or find the property once you know it exists:
https://quarkus.io/guides/all-config#quarkus-core_quarkus.package.appcds-use-container

How to Reproduce?

No response

Output of uname -a or ver

No response

Output of java -version

No response

GraalVM version (if different from Java)

No response

Quarkus version or git rev

No response

Build tool (ie. output of mvnw --version or gradlew --version)

No response

Additional information

No response

[quarkus-bot]

/cc @geoand (jib)

[geoand]

Thanks! I'll take care of this soon

…

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[geoand]

I am not really sure where to add the documentation. Should it be in a standalone doc, or part of the existing ones (I really don't know which would be the best fit)?

@gsmet @gastaldi WDYT?

[gastaldi]

I think a new guide about packaging in Quarkus would be nice to have

[geoand]

Packaging?

[gastaldi]

Yeah, talking about the quarkus.package.type modes and CDS as a new section

[geoand]

Hm... I am not sure there is a lot of value in describing the package types.

[gastaldi]

Sure, one about CDS only would be fine too :)

[geoand]

@maxandersen do you have any suggestions on where this should be documented?

[gsmet]

Yeah, I would go with either a Packaging your Quarkus application guide or a specific AppCDS one.

[geoand]

At long last, I conquered my laziness and actually wrote some docs 😆