Transactions across REST microservices?

Question

Let\'s say we have a User, Wallet REST microservices and an API gateway that glues things together. When Bob registers on our website, our API gateway needs to create a user

Answer 1

Eventual consistency is the key here.

• One of the services is chosen to become primary handler of the event.
• This service will handle the original event with single commit.
• Primary handler will take responsibility for asynchronously communicating the secondary effects to other services.
• The primary handler will do the orchestration of other services calls.

The commander is in charge of the distributed transaction and takes control. It knows the instruction to be executed and will coordinate executing them. In most scenarios there will just be two instructions, but it can handle multiple instructions.

The commander takes responsibility of guaranteeing the execution of all instructions, and that means retires. When the commander tries to effect the remote update and doesn’t get a response, it has no retry. This way the system can be configured to be less prone to failure and it heals itself.

As we have retries we have idempotence. Idempotence is the property of being able to do something twice such a way that the end results be the same as if it had been done once only. We need idempotence at the remote service or data source so that, in the case where it receives the instruction more than once, it only processes it once.

Eventual consistency This solves most of distributed transaction challenges, however we need to consider couple of points here. Every failed transaction will be followed by a retry, the amount of attempted retries depends on the context.

Consistency is eventual i.e., while the system is out of consistent state during a retry, for example if a customer has ordered a book, and made a payment and then updates the stock quantity. If the stock update operations fail and assuming that was the last stock available, the book will still be available till the retry operation for the stock updating has succeeded. After the retry is successful your system will be consistent.