(Refactoring) Exclusive transaction

[andy0130tw]

The project had made a big mistake leading to deadlocks by using a single transaction to findOrCreate. I had avoid it by caching associated objects in memory and had painfully used lots of tedious Promises / async to do careful writings.

Today, by reading the document of SQLite, I noticed that the type of transactions are default to DEFERRED. This was why the deadlocks happened. The solution is quite straightforward; by changing the type to EXCLUSIVE and those operations can be done concurrently like our intuitions.

https://sqlite.org/lang_transaction.html

To use it in sequelize: http://docs.sequelizejs.com/class/lib/transaction.js~Transaction.html#static-get-TYPES

The refactoring can also be painful.