Microservices.io is brought to you by Chris Richardson. Experienced software architect, author of POJOs in Action and the creator of the original CloudFoundry.com. His latest startup is eventuate.io, a microservices application platform.
Chris offers a comprehensive set of resources for learning about microservices including articles, an O'Reilly training video, and example code. Learn more
Chris offers a comprehensive consulting services, workshops and hands on training classes to help you use microservices effectively. Get advice
Want to see an example? Check out Chris Richardson's example applications. See code
Join the microservices google group
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:
Order Serviceuses a saga orchestrator implemented using the Eventuate Tram Sagas framework
Cross cutting concerns