Saga: Orchestration, Choreography and Compensation

Objectives

By the end of the lesson, you can explain what the saga solves, how orchestration and choreography differ, and why compensation is not a distributed rollback.

Why is it needed?

If a business operation touches several data owners, there is no shared transaction that either commits everything or nothing (5.1). The saga addresses how a coherent end result can still be reached when the steps happen in separate local transactions.

What is a saga?

According to the definition of microservices.io, a saga is a sequence of local transactions. Each step updates the database of its own service and publishes a message or event that triggers the next step. If a step violates a business rule, the saga runs compensating transactions that offset the earlier changes in business terms.

flowchart TD
  A["Step 1:<br/>local transaction"] --> B["Step 2:<br/>local transaction"]
  B --> C{"Succeeded?"}
  C -->|"Yes"| D["Step 3"]
  C -->|"No"| K["Compensation:<br/>offsetting step 1"]

Two modes of coordination

Mode	How does it work?	Advantage	Cost
Choreography	Each local transaction publishes an event that triggers a local transaction in another service; there is no central controller	Little central dependence with few steps	Hard to follow with many branches and cycles; a new consumer may create new feedback
Orchestration	An orchestrator tells the participants which local transaction to execute	A transparent flow, the rule described in one place	The orchestrator's contract and availability; its durable state must be resumable after a restart

The orchestrator should not take over the services' own internal rules.

Compensation is a business operation

Compensation is not time travel. Refunding a charge is a new financial operation, not making the original record disappear. After a reservation is released, someone else may take the resource. A compensating step can also fail and be repeated, so it needs its own idempotency, retry and final reconciliation path (6.5). If automatic recovery is not possible, the system must provide a recognisable, traceable state.

What does a saga not give?

microservices.io mentions three drawbacks:

• No automatic rollback: the compensating transactions must be designed by the developer.
• No isolation: concurrent sagas and transactions may see anomalies, so countermeasures are needed (5.4).
• The outcome must be reported: the result of an asynchronous saga must be learned by the client through a separate channel (delayed response, polling, notification).

In addition, a service cannot use a distributed transaction between its database and the message broker, so a separate pattern (outbox or event sourcing) is needed to link data saving and message sending atomically (5.3).

By aspect

Aspect	Saga
Coupling	The contracts of the steps and the meaning of the events are a shared dependency
Consistency	Intermediate states are visible; eventual agreement
Failure handling	Compensation, retry, handling an unknown outcome
Cost	Keeping track of the process state and observing it

Common misconceptions

Statement	Clarification
"A saga is the same as a distributed transaction."	There is no isolation and no automatic undo.
"Compensation restores the previous state."	It performs a new business operation that offsets the earlier one.
"Choreography is simpler because there is no orchestrator."	The process still exists, just harder to follow.
"A saga guarantees that it always succeeds."	It guarantees that a failure is handled and ends up in a traceable state.

In short

• A saga is a sequence of local transactions with compensation.
• Two coordination modes: choreography and orchestration.
• Compensation is a new business operation, not an undo.
• There is no isolation and no automatic rollback; the outcome must be reported separately.

Review questions

1. Why is compensation not a rollback?
2. When is it better to use an orchestrator instead of choreography?
3. What happens if the compensating operation fails too?

Learning resources

• Saga pattern (microservices.io) — definition, two modes, drawbacks.