Monday, December 7, 2020

Working with the Google Datastore emulator

 I do a good chunk of my business in Google App Engine; you package up your web application, send it to GCP, and then it takes care of scaling and uptime and all that stuff.

When I started out in 2014, I created my main application in Java because that was the least-crappy language that was supported.  However, in 2020, there are a whole lot more languages (in particular: Go).  I've slowly been working on porting my application from Java 8 to Go 1.14.  Along the way, I've run into some really annoying issues.

For today, I'm going to be focusing on the Datastore emulator.  In "old" App Engine (Java 8, Go 1.11, Python 2, etc.), they gave you a whole emulator suite.  Your application ran inside of that suite, and you had fake Google-based App Engine authentication, inbound e-mail, and a Datastore emulator that also had a web UI that you could use to see your entities and manipulate them.  The Datastore emulator's web UI wasn't as good as the current one that you get in production, but it was good enough to use for development.

Well, in "new" App Engine, the emulator suite is gone, and now you have to emulate or mock every aspect of App Engine that you plan on using.  It's not a huge deal, but it is a bit inconvenient.  In particular, you now have to start your own Datastore emulator.

It's easy to start:

gcloud config set project <your-project-id>;
gcloud beta emulators datastore start;

There are some environment variables that you'll need to export for the various libraries to detect and use instead of the production instance; run this to see them:

gcloud beta emulators datastore env-init;

That part is fine.

There are also two halfway-decent third party web UIs for the Datastore emulator:


I fought for hours trying to figure out why either of those two web UIs didn't work.  Neither would show any namespaces (and thus, neither would show any entities).

The short answer is that despite what the Datastore emulator claims it's using for the project ID, the only thing that it actually uses is "dummy-emulator-datastore-project".

I got a hint about it by poking around in the emulator's data file, and I got some confirmation in this file, which is the only thing on the Internet at the time of this writing that references that string:

So, if you start the Datastore emulator according to the instructions and either of those two web UIs aren't working, try setting the project ID to "dummy-emulator-datastore-project".

  1. In "google-cloud-gui", you set the project ID in the UI when you hit the "+" button to create a new project.
  2. In "dsui", you set the project ID using the "--projectId" flag.

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