Make occurrence import process atomic

[niconoe]

It would be great to have a robust mechanism to ensure the data update process happens atomically: either the Django command is successful, either the data is left unchanged (and the data update will be tried again the next day).

Possible approaches for this:

• Use database transactions
• Perform the data update on a copy of the database, and replace the web app's database once everything is successful

I currently favour the second option for its simplicity. Might be related/entangled with riparias/early-alert-webapp#14.

remark: even if we work on a copy of the database, don't forget to put the website in maintenance mode (#22) right before we copy the database so we are sure no changes are done (new comments added to occurrences, for example).

[damianooldoni]

Or both? By using transactions towards a copy of the db?

[niconoe]

Update: after going through the documentation, using Django's transaction support (via context manager or decorator) doesn't seem too complex/scary. So I might try this route first.

[niconoe]

I've now implemented a simple atomic block wrapping all the database changes during data import.

On paper it looks perfect for our purpose, but we'll need to aggressively test the robustness (it looks to good to be true :) )

[niconoe]

Now implemented, we're lacking:

• proper testing (part of Test suite for the occurrence import mechanism #40)
• mail send to the admins in case the transaction is aborted