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You have applied the Database per Service pattern. Each service has its own database. Some business transactions, however, span multiple service so you need a mechanism to ensure data consistency across services. For example, lets imagine that you are building an e-commerce store where customers have a credit limit. The application must ensure that a new order will not exceed the customer’s credit limit. Since Orders and Customers are in different databases the application cannot simply use a local ACID transaction.
How to maintain data consistency across services?
Implement each business transaction that spans multiple services as a saga. A saga is a sequence of local transactions. Each local transaction updates the database and publishes a message or event to trigger the next local transaction in the saga. If a local transaction fails because it violates a business rule then the saga executes a series of compensating transactions that undo the changes that were made by the preceding local transactions.
There are two ways of coordination sagas:
An e-commerce application that uses this approach would create an order using a choreography-based saga that consists of the following steps:
Order Servicecreates an Order in a pending state and publishes an
Customer Servicereceives the event attempts to reserve credit for that Order. It publishes either a
Credit Reservedevent or a
Order Servicereceives the event and changes the state of the order to either approved or cancelled
An e-commerce application that uses this approach would create an order using an orchestration-based saga that consists of the following steps:
Order Servicecreates an Order in a pending state and creates a
ReserveCreditcommand to the
Customer Serviceattempts to reserve credit for that Order and sends back a reply
CreateOrderSagareceives the reply and sends either an
RejectOrdercommand to the
Order Servicechanges the state of the order to either approved or cancelled
This pattern has the following benefits:
This solution has the following drawbacks:
There are also the following issues to address:
The following examples implement the customers and orders example in different ways:
Order Serviceuses a saga orchestrator implemented using the Eventuate Tram Sagas framework
Application architecture patterns
Cross cutting concerns