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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 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 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 Service
receives the POST /orders
request and creates an Order
in a PENDING
stateOrder Created
eventCustomer Service
’s event handler attempts to reserve creditOrderService
’s event handler either approves or rejects the Order
An e-commerce application that uses this approach would create an order using an orchestration-based saga that consists of the following steps:
Order Service
receives the POST /orders
request and creates the Create Order
saga orchestratorOrder
in the PENDING
stateReserve Credit
command to the Customer Service
Customer Service
attempts to reserve creditOrder
This pattern has the following benefits:
This solution has the following drawbacks:
There are also the following issues to address:
In order to be reliable, a service must atomically update its database and publish a message/event. It cannot use the traditional mechanism of a distributed transaction that spans the database and the message broker. Instead, it must use one of the patterns listed below.
A client that initiates the saga, which an asynchronous flow, using a synchronous request (e.g. HTTP POST /orders
) needs to be able to determine its outcome.
There are several options, each with different trade-offs:
OrderApproved
or OrderRejected
event.orderID
) after initiating the saga and the client periodically polls (e.g. GET /orders/{orderID}
) to determine the outcomeorderID
) after initiating the saga, and then sends an event (e.g. websocket, web hook, etc) to the client once the saga completes.The following examples implement the customers and orders example in different ways:
Order Service
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