I've slightly "abused" the front-end "version" concept in App Engine (java), to implement modules before they were introduced. I have a configuration consisting of: module1-dot-myapp.appspot.com, module2-dot-myapp.appspot.com, module3-dot-myapp.appspot.com, etc., based on the version concept (more commonly used with numbers: 1-dot-myapp, etc.).
Specifically, the code in all versions is identical, but each is practically used for different purposes. This separation allows different clients to use different api versions, separate deployment schedule, staging versions, logs separation, etc.
My question is, under theses conditions, what is the best way to convert my application to "real" modules? such that "module1" is an actual module (still mapped to the same url - module1-dot-appspot.com)?
Note: my answer comes from a somehow similar exercise but in the python GAE runtime, there is likely aditional Java-specific stuff to look at as well.
First things to look at (possible show stoppers) are the app-level configs - those will need to be merged in from your different old app versions (if they exist) and will be shared by all your modules (or directed to the default module only), so they might not work as before, best to revisit the latest documentation on these configs:
- dispatch file
- queue
- cron
- DB indexes
Note: in multi-module python apps these configs might not be updated automatically at app upload, each of them may need to be uploaded explicitly, using the respective app configuration utility options.
The separate deployment schedule is almost free (each module can be deployed independently). But there may be some impact due to the app-level configs (multiple CLI invocations instead of a single one, for example)
The logs separation comes for free.
The staging story might need to be revisited, depending on what exactly you mean by that.
Other than that - you'd bring the different old versions of your app in separate module sub-directories in your new app. Check if your version control system supports this easier. The old app config file(s) would need to be "translated" into the respective module's config file(s) and some of the info would go into the new app's top dir config file.
The module URL routing should allow transparent URL mapping, but note that the URLs will actually be <module>-dot-<appname>.appspot.com and the only way to get exactly the same URLs would be to delete all older app versions before deploying the new one (due to conflicting URLs: <module>-dot-<appname> vs <appversion>-dot-<appname>, not sure if you'll get the old or the new code serving or if it's even possible to deploy the new code without error). You could use a new appname at first, just to get all ducks lined up before the switchover (possibly a new staging story you might consider going forward).
You might find helpful complementing URL routing with a dispatch file if you didn't have one before.
Finally, if you have identical files shared across modules you may consider a single per-app copy of the file, symlinked into the respective modules, if that's easier or makes sense from your source code management prospective.
