Adopting a microservices architecture is a powerful strategic move. It allows different teams to develop and scale parts of your application independently, accelerating innovation. But this very strength creates a significant business challenge: how do you reliably complete a business process that spans multiple, separate services?
Consider a customer placing an order. This single action needs to coordinate the Order Service, the Payment Service, and the Inventory Service. In a traditional system, this would be one single transaction. In a microservices world, each service has its own database, making that simple approach impossible.
The key is to shift from a mindset of immediate perfection to one of managed reliability. Instead of demanding that all services update at the exact same second, we design systems that guarantee they will all reach the correct state, even if it takes a moment.
Here are the most practical patterns to achieve this.
The Saga pattern
Think of a Saga as a documented business process for your services. It breaks a large transaction into a series of smaller, local steps. If any step fails, the Saga executes a pre-defined “undo” action to reverse the previous steps.
How it works in practice:
Let’s use the order placement example:
- The Order Service creates a “PENDING” order.
- The Payment Service attempts to charge the customer’s card. If this fails, the order is marked as “FAILED.” The process stops.
- The Inventory Service reserves the items. If this fails, the Saga triggers a compensating action: the Payment Service issues a refund, and the order is “CANCELLED.”
The business benefit:
Sagas model real-world business processes, including the necessary steps for handling exceptions. This makes complex workflows transparent, reliable, and auditable.
The Orchestrator vs. Choreography Decision
When implementing a Saga, you have a key architectural choice that impacts how your teams work:
Orchestration (a central conductor):
You create a central service whose sole job is to tell the other services what to do and when. This is like a project manager directing a team.
- Pros: Simple to understand and manage. The entire workflow logic is in one place.
- Cons: This central point can become complex and a single point of logic, though not of failure.
Choreography (a coordinated dance):
Each service listens for events and knows what to do next. When the Order Service creates an order, it publishes an “OrderCreated” event. The Payment Service sees this and automatically tries to process the payment.
- Pros: Highly decoupled; teams have more autonomy.
- Cons: The overall workflow can be hard to see, and it’s more difficult to change a process once it’s distributed.
For business leaders, this choice often comes down to control versus team autonomy. Orchestration offers more centralized control, while choreography empowers individual teams.
The Outbox pattern
A critical risk in these systems is a service successfully updating its own database but failing to send the message to the next service. This creates silent, hard-to-find errors.
The Outbox Pattern is a simple but brilliant solution.
When a service updates its data (e.g., creating an order), it does not immediately send a message. Instead, it stores the “message” in a special database table (the “outbox”) as part of the same single database update. A separate, reliable process then scans this table and delivers the messages.
The business benefit:
This eliminates a major source of data inconsistency, ensuring that every completed action is reliably communicated. It makes your system far more trustworthy.
Making the strategic choice
Choosing the right pattern is a business decision, not just a technical one.
- For complex, multi-step processes that update data (like order fulfillment, customer onboarding), the Saga pattern is your default choice.
- For simple data retrieval (like showing an order summary with customer info), use API Composition, where a service fetches data from multiple sources and combines it.
- For all event-driven communication, the Outbox pattern should be considered a foundational best practice to ensure reliability.
Conclusion: Reliability over perfection
The goal of implementing distributed transactions is not to replicate the simplicity of a monolith. It is to build a system that is both agile and fundamentally reliable. By embracing patterns like Sagas and the Outbox, you acknowledge the distributed nature of your architecture and design your business processes to be resilient, transparent, and ultimately, more trustworthy for your customers.
