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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.
An e-commerce application that uses this approach would create an order using a saga that consists of the following local transactions:
Order Servicecreates an Order in a pending state
Customer Serviceattempts to reserve credit for that Order.
Order Servicechanges the state of the order to either approved or cancelled
Each local transaction publishes an event or message that triggers the next step.
For example, in the
CreateOrder saga, the
OrderService publishes an
OrderCreated event and the
Customer Service publishes either a
Credit Reserved event or a
This pattern has the following benefits:
This solution has the following drawbacks:
There are also the following issues to address:
Cross cutting concerns