My main task was to create the middle layer between web and database. This also included processing XML file and splitting it into objects and generating XML files from objects from database, and providing sales with given properties (date, category, item) for other component for further processing.

We decided, that loading any XML file must be an atomic operation - if one of the entries is somehow invalid (inserting an item that belongs to a non-existing category, creating sale/shipment for non-existing item, selling more pieces of product that are currently in stock) , the state of the database must not change. This was sometimes really challenging. I always had to move through XML, creating the objects "on the way", storing them only in some list and stopping with appropriate exception if something went wrong. Only after successfully reaching the end of file, I knew that everything was OK and I could proceed to store objects in database. It was even trickier when it came to sales. I had to check, if the summary of sales for every product is not higher than the current stock. I did this by summing sales for each product in map and than comparing value associated with each product with its current stock. Only after this all passed I stored sales in db and actually changed stock of items.

I don't recall exporting to XML to cause any bigger problems and I consider it to be the easiest part of my task.

While extracting sales of given properties from database, I could use filters that the db layer has provided me, so the only problem I encountered was with properly reformatting the date.

Quite a funny problem was when we found out (in quite a late stage of a developing), that the EAN code that we used as a business key will be to big for the integer - for testing, we used small values as 1 or 2, but it could be up to 13 number long, so much bigger than integer. Fortunately, it took only some easy refactoring to fix it.

In general, it took me some time before I learned how to work with spring (and understood how it works and what it does in our app), how to properly set everything up, how to properly use maven, etc.

This project was really interesting. I learned something about making a complete java web app, how it should be connected with database, etc. I also improved in using git to cooperate inside a team. I only used it so far for synchronizing my own work between notebook and desktop, so I didn't have to really solve any merge conflicts, use branches or anything like that. Now it was something totally different and I even learned some git functions that I had no idea they exist (eg. cherrypick). The XML part was made easier by the fact, that we validated each XML file before processing it, so I only had to process valid files in a format we agreed on.