redundant cloud servers

[Dirk Hohndel]

Over the past couple of weeks I quietly moved the cloud server backend and the authentication database out of my home network and back into the public cloud.
The fact that apparently no one noticed and nothing broke was rather encouraging :-)

I then setup a second cloud server in the public cloud, this one in Europe (actually, Frankfurt, Germany).

A few minutes ago I merged a pull request that adds a couple of interesting new features to both Subsurface and Subsurface-mobile:

- geo based choice of cloud server (those in the Americas should end up using the US server, everyone else should get directed to the European server), which is intended to be completely transparent to the user.
 I.e., unless you explicitly look for it, you shouldn't notice (or care) which server you are using
- hot failover between the two servers (so if the server that you have used before is unavailable, Subsurface and Subsurface-mobile should try the other one - and the implementation makes it easy to add more servers if that appears useful)

The two servers are automatically kept in sync in the background. I can create one extremely artificial case where there is a fairly short (generally sub one second in my tests) window for a race condition. A user would have to have two different devices that are synced to different servers (for example because they took their phone with them on an overseas dive trip), that have made conflicting changes to the dive log, and the user would then have to cause a save to the cloud on both devices within the roughly one second window. Forcing that race turned out to be fairly hard because the mobile app does a few other things before storing the data to the cloud - so for testing I ended up using two instances of Subsurface and synchronized clicks... really -- not a realistic scenario.

If this were to happen, I will get an automated email alert and fixing the problem isn't really hard - but I wanted to give a full disclosure of the issues that I am aware of.

What would help right now if people tested all this.
So far I have not had the time to make new beta versions of Subsurface-mobile, but as I said, it's actually easier to test on the desktop.

/D