Reactive Choreography vs Orchestrated Declarative. Choosing the Optimal Approach for Effective Service Communication in Distributed Systems

Introduction

In distributed systems, effective communication between services is crucial for building scalable, resilient, and maintainable architectures. Two common approaches to service communication are choreography and declarative communication. Each approach offers distinct benefits and considerations, and understanding their differences is essential for making informed architectural decisions. In this article, we will delve into choreography and declarative communication, explore their characteristics, and discuss suitable use cases and scenarios.

Choreography: Autonomous Collaboration

Choreography refers to a communication pattern where each service in a distributed system makes decisions and communicates with other services directly. There is no centralized orchestration or control component that coordinates the interactions between services. Each service acts autonomously based on events it receives or triggers. Let’s explore some characteristics and use cases of choreography:

1. Loose Coupling and Scalability: Choreography promotes loose coupling between services, enabling them to evolve independently. Each service handles its own business logic based on the events it receives or triggers. This loose coupling facilitates scalability, as services can be added or modified without impacting the entire system.
2. Event-Driven Architecture: Choreography is often implemented using an event-driven architecture. Services publish events to notify other services of specific occurrences, and interested services subscribe to relevant events to react accordingly. This approach enables asynchronous communication and decouples services.

Use Case: E-commerce Order Processing

In an e-commerce system, multiple services collaborate through choreography for order processing. Each service acts autonomously based on events they receive or trigger:

• The Order Service receives a new order request and publishes an OrderPlaced event.
• The Inventory Service subscribes to the OrderPlaced event, checks product availability, and publishes an InventoryUpdated event.
• The Payment Service subscribes to the OrderPlaced event, processes the payment, and publishes a PaymentProcessed event.
• The Shipping Service subscribes to the InventoryUpdated event, prepares the shipment, and publishes a ShipmentPrepared event.
• The Notification Service subscribes to the PaymentProcessed and ShipmentPrepared events, sends notifications to the customer, and updates the order status.

Each service acts autonomously, reacting to events and updating its own state accordingly. The collaboration between services is achieved through event-driven choreography.

Orchestrated Declarative Communication: Centralized Coordination

Declarative communication involves a central orchestration or coordination component that defines and controls the interactions between services. The coordination component defines a higher-level specification or policy that describes how services should communicate and coordinate their actions. Let’s explore some characteristics and use cases of declarative communication:

1. Centralized Control and Visibility: Declarative communication provides centralized control over the system’s behavior. A central orchestration component defines the desired workflow or behavior, and services follow the prescribed flow. This centralized control enables better visibility, monitoring, and management of the system’s behavior.

Use Case: Approval Workflow

Consider an approval workflow where multiple services collaborate to approve a request:

• A Request Service receives a request and triggers the approval workflow.
• The Approval Service, acting as the central orchestrator, defines the workflow and guides the services through the process.
• The Review Service, responsible for reviewing the request, receives instructions from the Approval Service, performs the review, and reports the outcome to the Approval Service.
• The Verification Service, responsible for verifying the request, receives instructions from the Approval Service, performs the verification, and reports the outcome.
• The Decision Service, controlled by the Approval Service, evaluates the review and verification results and makes the final approval decision.

In this declarative communication model, the central orchestrator defines the workflow, specifies the sequence of actions, and coordinates the interactions between services. The services rely on the orchestration component to guide their behavior and enforce the defined approval process.

Choosing the Right Approach

Selecting the appropriate approach depends on various factors, including the system’s complexity, service autonomy requirements, and the nature of business processes. Here are some considerations for making the right choice:

1. Degree of Service Autonomy: Choreography empowers individual services with autonomy and flexibility. If services need to make independent decisions based on their local state or specific business rules, choreography can be suitable.
2. Complexity and Evolution: If the system’s requirements are subject to frequent changes or if services need to evolve independently, choreography’s loose coupling enables more flexibility and scalability.
3. Centralized Control and Auditing: If centralized control, visibility, and auditing of the system’s behavior are critical, declarative communication provides a better fit. It enables a clear overview of the entire process, ensuring compliance and facilitating process optimization.

Conclusion

In distributed systems, choreography and declarative communication offer distinct approaches to service communication. Choreography promotes autonomous collaboration between services, allowing them to make decisions and communicate directly. Declarative communication emphasizes centralized orchestration, controlling and coordinating service interactions. By understanding their characteristics and suitable use cases, architects can make informed decisions when designing distributed systems.

Consider the degree of service autonomy, system complexity, need for centralized control, and workflow structure when choosing between choreography and declarative communication. Striking the right balance between autonomy and control enables the construction of scalable, resilient, and maintainable distributed systems.

By carefully considering the requirements and constraints of the system, architects can choose the most appropriate approach to facilitate effective communication between services and build robust distributed systems.

References

• Fowler, M. (2014). Choreography. martinfowler.com/articles/choreography.html
• Alur, D., Khalaf