Pattern: Saga


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?


  • 2PC is not an option


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:

  • Choreography - each local transaction publishes domain events that trigger local transactions in other services
  • Orchestration - an orchestrator (object) tells the participants what local transactions to execute

Example: Choreography-based saga

An e-commerce application that uses this approach would create an order using a choreography-based saga that consists of the following steps:

  1. The Order Service receives the POST /orders request and creates an Order in a PENDING state
  2. It then emits an Order Created event
  3. The Customer Service’s event handler attempts to reserve credit
  4. It then emits an event indicating the outcome
  5. The OrderService’s event handler either approves or rejects the Order

Example: Orchestration-based saga

An e-commerce application that uses this approach would create an order using an orchestration-based saga that consists of the following steps:

  1. The Order Service receives the POST /orders request and creates the Create Order saga orchestrator
  2. The saga orchestrator creates an Order in the PENDING state
  3. It then sends a Reserve Credit command to the Customer Service
  4. The Customer Service attempts to reserve credit
  5. It then sends back a reply message indicating the outcome
  6. The saga orchestrator either approves or rejects the Order

Resulting context

This pattern has the following benefits:

  • It enables an application to maintain data consistency across multiple services without using distributed transactions

This solution has the following drawbacks:

  • The programming model is more complex. For example, a developer must design compensating transactions that explicitly undo changes made earlier in a saga.

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:

    • The service sends back a response once the saga completes, e.g. once it receives an OrderApproved or OrderRejected event.
    • The service sends back a response (e.g. containing the orderID) after initiating the saga and the client periodically polls (e.g. GET /orders/{orderID}) to determine the outcome
    • The service sends back a response (e.g. containing the orderID) after initiating the saga, and then sends an event (e.g. websocket, web hook, etc) to the client once the saga completes.

Learn more

Example code

The following examples implement the customers and orders example in different ways:

About is brought to you by Chris Richardson. Experienced software architect, author of POJOs in Action, the creator of the original, and the author of Microservices patterns.

Chris helps clients around the world adopt the microservice architecture through consulting engagements, and training classes and workshops.

New virtual bootcamp: Distributed data patterns in a microservice architecture

My virtual bootcamp, distributed data patterns in a microservice architecture, is now open for enrollment!

It covers the key distributed data management patterns including Saga, API Composition, and CQRS.

It consists of video lectures, code labs, and a weekly ask-me-anything video conference repeated in multiple timezones.

The regular price is $395/person but use coupon BNCDVUNB to sign up for $295 (valid until November 16th, 2021). There are deeper discounts for buying multiple seats.

Learn more

Signup for the newsletter

LEARN about microservices

Chris offers numerous resources for learning the microservice architecture.

Training classes

Chris teaches comprehensive workshops, training classes and bootcamps for executives, architects and developers to help your organization use microservices effectively.

Avoid the pitfalls of adopting microservices and learn essential topics, such as service decomposition and design and how to refactor a monolith to microservices.

Delivered in-person and remotely.

Get the book: Microservices Patterns

Read Chris Richardson's book:

Example microservices applications

Want to see an example? Check out Chris Richardson's example applications. See code

BUILD microservices

Ready to start using the microservice architecture?

Consulting services

Engage Chris to create a microservices adoption roadmap and help you define your microservice architecture,

The Eventuate platform

Use the platform to tackle distributed data management challenges in your microservices architecture.

Eventuate is Chris's latest startup. It makes it easy to use the Saga pattern to manage transactions and the CQRS pattern to implement queries.

ASSESS your architecture

Assess your application's microservice architecture and identify what needs to be improved.

Consulting services

Engage Chris to conduct an architectural assessment.

Self assessment

Alternatively, conduct a self-assessment using the Microservices Assessment Platform.

Join the microservices google group

Copyright © 2021 Chris Richardson • All rights reserved • Supported by Kong.