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Compensating Transaction — Middle

At middle level, focus on this question:

How do you design a compensating action for a specific, already-committed step?

Prerequisite: junior.md.

The design checklist per step

For each step in a multi-step process, design its compensation by asking:

  1. What did this step change, concretely?
  2. What action returns the system to an equivalent business state?
  3. Is the compensation itself idempotent and retryable (per Retries & Idempotency)?
# Forward step
def reserve_inventory(order_id, sku, qty):
    db.execute(
        "UPDATE inventory SET reserved = reserved + %s WHERE sku = %s",
        qty, sku
    )

# Compensating action - the inverse EFFECT, not a magic "undo"
def release_inventory_reservation(order_id, sku, qty):
    db.execute(
        "UPDATE inventory SET reserved = reserved - %s WHERE sku = %s",
        qty, sku
    )
flowchart LR Forward["Forward step:\nreserved += qty"] --> Compensation["Compensation:\nreserved -= qty"] Compensation --> Idempotent["Must be safe to run\nmore than once\n(e.g. via a dedup key\nper order_id)"]

Compensation order: reverse of execution order

sequenceDiagram participant Saga participant Inventory participant Payment participant Shipping Saga->>Inventory: reserve (succeeds) Saga->>Payment: charge (succeeds) Saga->>Shipping: create shipment (FAILS) Note over Saga: Compensate in REVERSE order Saga->>Payment: refund Saga->>Inventory: release reservation

Compensations run in the reverse order of the forward steps — undoing the most recently completed step first, then working backward, mirroring how a stack unwinds. This matters because later steps can sometimes depend on earlier ones' effects still being in place until they're specifically compensated.

🎓 Takeaway: designing a compensation is a per-step exercise: identify the concrete state change, design the inverse-effect action, and make that action idempotent — then chain compensations in reverse execution order when a multi-step process needs to unwind.

Test yourself

  1. Why must the release_inventory_reservation compensation also be idempotent, not just the forward reserve_inventory step?
  2. Why do compensations run in reverse order rather than forward order or some other order?
  3. Design the compensation for a "send a push notification" step — is this step even compensatable, or does it belong to a different category entirely?

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