Saga Design Pattern in C#.Net

In the realm of microservices and distributed systems, maintaining data consistency across multiple services can be a daunting challenge. Traditional ACID transactions are often inadequate in such environments due to their inherent limitations when it comes to scalability and network latency. Enter the Saga Pattern, a design pattern that provides a means to manage distributed transactions effectively while preserving data consistency. In this article, we'll delve into the Saga Pattern, explore its principles, and understand how it facilitates complex operations across microservices.

Introduction to the Saga Pattern

The Saga Pattern is an architectural pattern used to manage long-lived and complex transactions that involve multiple services in a distributed system. It provides an alternative approach to the traditional two-phase commit (2PC) protocol, which tends to introduce scalability and performance bottlenecks in microservices environments. In contrast, the Saga Pattern breaks a transaction into a series of smaller, self-contained steps or activities, each encapsulated within a service. These steps are orchestrated in a way that ensures eventual consistency across the entire transaction.

Key Concepts and Principles

To understand the Saga Pattern better, let's explore its fundamental concepts and principles:

1. Saga:

• A Saga represents a sequence of related and coordinated activities, each implemented as a service operation.
• Sagas are designed to be idempotent, meaning that they can be safely retried without causing unintended side effects. I'll talk about idempotent saga later here.

2. Activity:

• An Activity is a single step or operation within a Saga.
• Activities are responsible for executing a specific part of the overall transaction and making the necessary changes to the data.

3. Compensation:

• Compensation is the inverse operation of an Activity.
• It is used to undo or compensate for the changes made by a previous activity in case of failure.
• Compensation ensures that the system can rollback to a consistent state when necessary.

4. Saga Orchestrator:

• The Saga Orchestrator is responsible for coordinating the execution of Saga activities.
• It defines the order of activities and handles any compensating actions when a failure occurs.
• The Orchestrator is a central component that manages the Saga's state and progress.

Types of Sagas

Sagas can be categorized into two primary types based on their coordination and management approaches: orchestrator sagas and choreography (choreographer) sagas. These two types differ in how they handle the sequencing and coordination of individual steps or activities within a distributed transaction. Let's explore both types in detail:

1. Orchestrator Saga:Definition: Orchestrator sagas are characterized by having a centralized component known as the "orchestrator." The orchestrator is responsible for coordinating and sequencing the various activities that make up the saga. It dictates the order in which activities are executed and manages their interactions.Coordination: In an orchestrator saga, the orchestrator actively controls the execution of activities. It initiates each activity, monitors their progress, handles any compensating actions in case of failures, and ensures that the saga reaches a consistent state.Benefits:Centralized control makes it easier to manage the overall flow of the saga.The orchestrator can implement complex business logic and decision-making processes.Drawbacks:A single point of control can become a bottleneck.Orchestrator failures can disrupt the entire saga.Complexity increases as the number of activities and dependencies grows.
2. Choreography (Choreographer) Saga:Definition: Choreography sagas, also known as choreographer sagas, take a decentralized approach. Instead of a central orchestrator, interactions between activities are determined by the activities themselves. Each activity knows how to initiate its own execution and respond to events from other activities.Coordination: In choreography sagas, activities communicate directly with each other or through a message broker. They emit events when they complete their work, and other activities respond to these events as triggers for their actions.Benefits:Decentralized coordination can lead to more scalable and loosely coupled systems.Failures in one activity are less likely to affect the entire saga.Drawbacks:Complex choreographies may be challenging to design and maintain.Coordinating complex business processes may require additional effort.

In this article I talk mostly about Orchestrator Saga but in general the concepts are not so different.

Does All Saga Need To Be Idempotent?

In the context of the Saga Pattern, not all sagas need to be strictly idempotent, but idempotence can be a desirable property in certain scenarios. Let's explore this in more detail:

Idempotence is a property that ensures that performing an action multiple times has the same effect as performing it once. In the context of the Saga Pattern, an idempotent saga means that executing the same saga activity multiple times (perhaps due to retries or failures) does not have unintended or different consequences.

Here are some considerations regarding idempotence in sagas:

1. Saga Activities Can Vary:Not all saga activities are the same. Some activities may naturally be idempotent, while others might not be. For example, a "send email" activity could be idempotent, but a "charge a credit card" activity may not be due to the financial implications.
2. Retry and Compensation:Sagas often include mechanisms for retrying activities in case of transient failures. If an activity is idempotent, retries become less complex because you can safely execute the same activity multiple times. Non-idempotent activities require more careful handling, often involving compensation actions to revert or adjust the effects of the failed activity.
3. Data Consistency:In a distributed system, ensuring data consistency is essential. Idempotent activities can help maintain consistency when retries or failures occur, as they won't introduce unexpected side effects. Non-idempotent activities may require additional safeguards to maintain data integrity.
4. Practical Considerations:Achieving perfect idempotence for all activities may not always be practical. Some activities, like processing a financial transaction, inherently have non-idempotent characteristics. In such cases, it's crucial to design compensation or error-handling mechanisms to handle potential issues.
5. Design Decisions:The decision to make an activity idempotent or not often depends on the specific requirements and constraints of your application. It's a trade-off between system complexity, data consistency, and the practicality of handling failures.

How the Saga Pattern Works

Let's walk through an example scenario to see how the Saga Pattern (Orchestrator) works in practice:

Scenario: Consider an e-commerce platform where a customer places an order that involves multiple microservices: Order Service, Payment Service, and Inventory Service.

• Order Creation (Saga Start):The customer initiates an order, triggering the Order Service.The Order Service creates an Order Saga to manage the entire order process.
• Payment Authorization (Activity 1):The Order Service invokes the Payment Service to authorize the payment.If successful, the Payment Service records the payment. If not, it initiates compensation by canceling the payment authorization.
• Inventory Reservation (Activity 2):The Order Service reserves the items in the inventory.If successful, the Inventory Service updates the inventory. If not, it initiates compensation by releasing the reserved items.
• Order Confirmation (Saga End):If both previous activities succeed, the Order Service confirms the order.If any activity fails, the Orchestrator triggers compensating actions to maintain consistency.

Benefits of the Saga Pattern

The Saga Pattern offers several advantages for managing distributed transactions:

• Scalability: Sagas distribute transactional load across multiple services, improving scalability compared to traditional monolithic transactions.
• Resilience: Failures in one part of a Saga can be isolated and handled independently, enhancing fault tolerance.
• Performance: Sagas minimize the blocking nature of distributed transactions, reducing contention and improving system performance.
• Flexibility: Sagas are flexible and can accommodate various transactional scenarios, including long-running processes.

Challenges and Considerations

While the Saga Pattern is powerful, it also comes with its set of challenges and considerations:

• Compensation Complexity: Designing compensating actions can be complex, and ensuring idempotent is crucial to avoid inconsistencies.
• Sagas State Management: Managing the state of ongoing Sagas and handling retries or timeouts requires careful implementation.
• Eventual Consistency: The Saga Pattern guarantees eventual consistency but not immediate consistency. Applications must be designed to handle temporary inconsistencies.

Implementing Sagas

To implement Sagas, you can use libraries, frameworks, or build custom solutions tailored to your specific use case. Popular tools like Apache Kafka, RabbitMQ, and event sourcing architectures can assist in Saga orchestration and state management.

You’ll find the complete source in this link:

https://github.com/amirdoosti6060/SagaPattern

There are several .NET libraries that implement the saga pattern, such as:

• OpenSleigh: A saga management library for .NET Core that supports different persistence and transport mechanisms, such as SQL Server, MongoDB, RabbitMQ, and Azure Service Bus.
• Orangeloop.Sagas: A simple C# implementation of the unit of work pattern using sagas. It allows you to define sagas as a sequence of steps, each one with its own rollback logic.
• MassTransit: A free, open-source distributed application framework for .NET that supports various messaging systems, such as RabbitMQ, Azure Service Bus, Amazon SQS, and Kafka. It also provides saga orchestration and state machine capabilities.

In order to have a better understanding of the Saga pattern, I created a project in GitHub that shows a simple implementation of the Saga pattern.

Note that the project needs a lot of changes to be able to use in the production environment.

To Implement a complete Saga pattern library you need to implement these steps:

• Define Saga Steps (Activities): Identify the individual steps or activities that make up your Saga. Each step should correspond to a specific operation within a service.
• Implement Sagas and Activities: Create Sagas as classes or components that orchestrate the sequence of activities.Implement Activities as methods or functions within your services. Each Activity should perform a specific operation and have corresponding compensation logic.
• Use a Reliable Messaging System: Utilize a reliable messaging system like Apache Kafka, RabbitMQ, or Azure Service Bus to facilitate communication and coordination between Sagas and Activities. Note that we don't need always a messaging system to implement saga pattern and we can implement it for example with API calls (like what I did in the sample project)
• Implement Compensation Logic: Ensure that each Activity has a compensating action that can revert the changes made if needed. These compensating actions should be idempotent to handle retries.
• Saga Orchestrator: Create a Saga Orchestrator component that manages the execution of Saga instances, tracks their state, and handles retries and compensations. As I mentioned before, Saga can be Choreogrphy and if so, we don't need a Saga Orchestrator for sure. Also note that in production, we can you circuit breaker, bucketing and retry patterns to control our transaction behavior.
• Persistence: Implement state persistence for your Sagas. This could involve storing Saga state in a database or using event sourcing techniques.
• Handle Timeouts and Retries: Implement timeout mechanisms to handle situations where an Activity takes too long to complete.Handle retries for activities that encounter transient failures.
• Testing: Thoroughly test your Saga implementations, including scenarios involving failures, compensations, and retries.
• Monitoring and Logging: Implement logging and monitoring to track the progress of Sagas, detect failures, and troubleshoot issues.

Saga class

In my implementation Saga class has a Run() method which orchestrates execution of several activities to do all of them or none of them.?

Using GetActivities() of this class you can add any number of Acticities to it. The order of added activities matter.

It also has some properties to get Status and CurrentActivity.

IActivity interface

This interface provides an abstraction for Activity. Each Activity has two methods:

• ExecuteAsync
• CompensationsAsync

The first one is run when an activity needs to do its job and the second one is run when an activity needs to be undone.

namespace SagaPattern
{
    public interface IActivity
    {
         Task<ActivityStatus> ExecuteAsync();
         Task<ActivityStatus> CompensateAsync();
    }
}        

SagaContext class

This class is keeping all necessary contextual information for Saga to orchestrate the microservices. It contains an Id and a list of all activities and current status of the Saga. It also keeps the current index and the last successful activity run before reverting.

public class SagaContext
{
    public Guid SagaId { get; set; }
    public IList<IActivity>? Activities { get; set; }
    public SagaStatus Status { get; set; }
    public int CurrentActivity { get; set; }
    public int LastActivity { get; set; }
}        

Conclusion

The Saga Pattern is a valuable addition to the toolkit of architects and developers building microservices and distributed systems. It enables the management of complex, long-running transactions while addressing the challenges of scalability, resilience, and performance. By breaking down transactions into manageable steps, each with its compensation logic, the Saga Pattern promotes robust and reliable distributed systems, facilitating business processes that span multiple services. However, it also demands careful planning, especially in terms of compensating actions and state management, to ensure the desired level of consistency and reliability.

#microservice #design_pattern #saga #saga_pattern