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 implement transactions that span services. For example, let’s 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 owned by different services the application cannot simply use a local ACID transaction.
How to implement transactions that span services?
Implement each business transaction that spans multiple services is 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 Servicereceives the
POST /ordersrequest and creates an
Customer Service’s event handler attempts to reserve credit
OrderService’s event handler either approves or rejects the
An e-commerce application that uses this approach would create an order using an orchestration-based saga that consists of the following steps:
Order Servicereceives the
POST /ordersrequest and creates the
Create Ordersaga orchestrator
Reserve Creditcommand to the
Customer Serviceattempts to reserve credit
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
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