What is SAGA Design Pattern in Microservices? With Examples

 Hello guys, I have been writing about Microservices and Java from long tie and in the past, I shared with you 10 essential Microservice design patterns and principles and best Microservices courses for Java developers which many of you appreciated. At the same time, a lot of you asked me to go deep down into each of those patterns and explain them with examples. So I decided to create this new series of Microservice Pattern tutorials where I will explain each of those essential Microservice pattern with example every week. So far, we have covered Database Per Microservices, CQRS, Event Sourcing, and Circuit-breaker pattern and in this article, let's have a look at SAGA Pattern, how does it work, what problem it solves and pros and cons of SAGA Pattern with examples. 

Microservices is a popular architectural style for building large and complex software systems. One of the challenges that arise when implementing microservices is how to handle data consistency across multiple services. The SAGA design pattern is an approach to address this problem. In this article, we will explain what the SAGA design pattern is, how it works, and provide some examples to illustrate its usage.

What is the SAGA design pattern? How does it work?

 The SAGA design pattern is a technique for managing data consistency in distributed transactions within a microservices architecture. It provides a way to handle transactions that span multiple services and ensure that all services involved in the transaction agree on the outcome of the transaction. The acronym SAGA stands for "Saga Pattern".

What is SAGA Design Pattern in Microservices? With Examples

In the SAGA pattern, a transaction is divided into a sequence of steps, where each step corresponds to an action that needs to be taken by a microservice. Each step is executed atomically, which means that either all the steps succeed, or the entire transaction is rolled back. Each step is also associated with a compensating action, which can be used to undo the effects of the step if the transaction needs to be rolled back.

How does the SAGA design pattern work?

The SAGA design pattern works by breaking down a transaction into a series of steps. Each step is implemented as a separate microservice. Each microservice is responsible for executing a specific action in the transaction and for ensuring that its action is either fully completed or fully rolled back in case of failure.

To coordinate the different microservices involved in the transaction, the SAGA pattern uses a coordinator. The coordinator is responsible for initiating the transaction and coordinating the different steps. 

It does this by sending messages to the different microservices and waiting for their responses. The coordinator also maintains a state machine that keeps track of the current state of the transaction and the actions that have been taken so far.

The SAGA pattern also makes use of compensating actions. A compensating action is an action that undoes the effect of a previous action. For example, if a previous action was to create a new record in a database, the compensating action would be to delete that record. Compensating actions are used to undo the effects of a step if the transaction needs to be rolled back.

Examples of the SAGA design pattern:

Let's take a look at some examples to see how the SAGA design pattern can be used in practice.

Order Processing System:

Consider an order processing system for an online retailer. The system consists of several microservices, including a service for managing inventory, a service for managing payments, and a service for managing shipping.

When a customer places an order, the order processing system needs to execute several steps to fulfill the order. These steps include checking inventory, processing payment, and shipping the order.

To ensure data consistency across these steps, the order processing system can use the SAGA design pattern. The coordinator would initiate the transaction by sending a message to the inventory service to reserve the items in the order. 

If the inventory service responds with a success message, the coordinator would then send a message to the payment service to charge the customer's credit card. If the payment service responds with a success message, the coordinator would then send a message to the shipping service to ship the order.

If any of the steps fail, the coordinator would initiate the compensating actions. For example, if the payment service fails to charge the customer's credit card, the compensating action would be to release the reserved items in the inventory service.

SAGA Pattern Example - Flight Booking System

Consider a flight booking system that allows customers to book flights from multiple airlines. The system consists of several microservices, including a service for managing airline reservations, a service for managing payment.

When a customer books a flight, the flight booking system needs to execute several steps to complete the booking. These steps include reserving a seat on the flight, charging the customer's credit card, and updating the customer's information.

To ensure data consistency across these steps, the flight booking system can use the SAGA design pattern. The coordinator would initiate the transaction by sending a message to the airline reservation service to reserve a seat on the flight. If the reservation service responds with a success message, the coordinator would then send a message to the payment service to charge the customer's credit card. If the payment service responds with a success message, the coordinator would then send a message to the customer information service to update the customer's information with the reservation details.

If any of the steps fail, the coordinator would initiate the compensating actions. For example, if the payment service fails to charge the customer's credit card, the compensating action would be to release the reserved seat in the airline reservation service.

One of the advantages of using the SAGA design pattern is that it allows for a higher degree of flexibility and scalability in microservices architecture. Each step in the transaction can be implemented as a separate microservice, allowing for better isolation of concerns and easier maintenance and scaling of the system.

Another advantage of the SAGA design pattern is that it can help to improve the fault tolerance of the system. If a step in the transaction fails, the compensating action can be used to undo the effects of the failed step and roll back the transaction. This can help to prevent data inconsistencies and maintain the integrity of the system.

However, it's important to note that the SAGA design pattern is not a silver bullet solution for all transaction management problems. It may not be suitable for all types of transactions, and there may be situations where other approaches, such as two-phase commit or event-driven architecture, may be more appropriate.

Conclusion

The SAGA design pattern is a powerful technique for managing data consistency in distributed transactions within a microservices architecture. It provides a way to handle transactions that span multiple services and ensure that all services involved in the transaction agree on the outcome of the transaction. 

By breaking down a transaction into a series of steps and using compensating actions to undo the effects of previous steps, the SAGA pattern allows for robust and reliable transaction management.

 With the examples provided in this article, we hope to have demonstrated the usefulness and versatility of the SAGA design pattern in practical scenarios.