Sagas + compensação
Managing complex business transactions Not every command is able to completely execute in a single ACID transaction. A very common example that pops up quite often as an argument for transactions is the money transfer. It is often believed that an atomic and consistent transaction is absolutely required to transfer money from one account to another. Well, it's not. On the contrary, it is quite impossible to do. What if money is transferred from an account on Bank A, to another account on Bank B? Does Bank A acquire a lock in Bank B's database? If the transfer is in progress, is it strange that Bank A has deducted the amount, but Bank B hasn't deposited it yet? Not really, it's "underway". On the other hand, if something goes wrong while depositing the money on Bank B's account, Bank A's customer would want his money back. So we do expect some form of consistency, ultimately. While ACID transactions are not necessary or even impossible in some cases, some form of transaction management is still required. Typically, these transactions are referred to as BASE transactions: Basic Availability, Soft state, Eventual consistency. Contrary to ACID, BASE transactions cannot be easily rolled back. To roll back, compensating actions need to be taken to revert anything that has occurred as part of the transaction. In the money transfer example, a failure at Bank B to deposit the money, will refund the money in Bank A. In CQRS, Sagas can be used to manage these BASE transactions. They respond on Events and may dispatch Commands, invoke external applications, etc. In the context of Domain Driven Design, it is not uncommon for Sagas to be used as coordination mechanism between several bounded contexts.