Microservices Design Patterns - Saga

Problem Statement

Microservice architecture structures an application as a set of loosely coupled microservices and each service can be developed independently in agile manner to enable continous delivery/deployment and if we've used a database per service design pattern then how to implement a transaction which spans multiple services.

Solution

We can use Saga Pattern. A saga is a sequence of local transactions. In this pattern, each transaction updates the database and triggers an event or publishes a message for next transaction in saga. In case, any local transaction fails, saga will trigger series of transactions to undo the changes done so far by the local transactions.

Consider an example of order service and customer service. Order service can make an order and then ask customer service if credit limit is crossed or not. In case credit is crossed, the customer service will raise an event to order service to cancel the order otherwise to place the order successfully.

In order to implement this pattern, we often need to Choreography based saga or Orchestrator based saga.

In choreography based saga, services handles the domain events during local transactions and either complete the transaction or undo the same while in orchestrator based saga, an orchestrator object handles events during local transactions and then tell coordinate which local transaction is to be executed.