Saga (Choreography)

The Saga pattern is a distributed transaction management pattern for coordinating a sequence of local transactions across multiple microservices. Unlike traditional distributed transactions (like two-phase commit), Sagas don’t rely on centralized locking or coordination. Instead, each local transaction updates data within a single service, and the Saga coordinates the overall process by defining a series of steps where each step publishes events that trigger the next. If a step fails, the Saga executes a series of compensating transactions to undo the changes made by the preceding steps, ensuring eventual consistency.

Saga choreography moves the coordination logic into each service. Each service listens for events from other services and decides when to execute its local transaction. This avoids a central orchestrator but can make the overall flow harder to understand and debug, as the logic is spread across multiple services. It’s best suited for simpler, well-defined workflows where dependencies aren’t heavily interwoven.

Usage

The Saga pattern is commonly used in microservices architectures to manage long-lived transactions that span multiple services, particularly in scenarios where traditional ACID transactions are not feasible or desirable. Key use cases include:

• E-commerce Order Management: Creating an order involves inventory reservation, payment authorization, and shipping scheduling. Each of these can be a separate service, and a Saga ensures that if any step fails, the entire order is rolled back consistently.
• Travel Booking: Booking a flight, hotel, and rental car often involves interacting with different external services. A Saga can orchestrate thesebookings, ensuring that if one booking fails, the others are cancelled.
• Financial Transactions: Splitting payments across multiple accounts or services (e.g., a bank transfer involving a fraud check).
• Complex Data Pipelines: Coordinating updates across various data stores or processing stages.

Examples

• Axon Framework (Java): Axon Framework provides built-in support for Sagas, allowing developers to define Saga logic using annotations or configuration. Axon supports both orchestration and choreography-based Sagas. The framework handles event dispatching and compensating transaction execution, simplifying the implementation of distributed transactions. axon-framework.org

• Netflix Dastur (Java/Python): Although Dastur is a more general-purpose workflow automation tool, it effectively implements the Saga pattern by allowing the creation of composable, event-driven workflows. Netflix used Dastur extensively to manage complex operational tasks and dependencies between various internal services, effectively handling failures and ensuring eventual consistency. github.com/Netflix/Dastur

• Eventualize (Go): Eventualize is a lightweight Go library that provides tools for building event-driven applications, including support for the Saga pattern. It allows developers to define Sagas as a series of event handlers and compensators, simplifying the development and management of distributed transactions, particularly in Kubernetes environments. github.com/Bit-Monk/Eventualize