A system may save an approved request and then fail before notifying the next system. Reversing the order creates the opposite risk: a notification about a change that never saved. When these two actions must stay connected, the agency should explain the failure boundary.
Related service: Custom software development- 01Validate the business change.
- 02Save the change and pending event together.
- 03Deliver the recorded event asynchronously.
- 04Handle repeated delivery safely.
- 05Monitor pending and failed events.
SECTION 01
Ask where the promise begins and ends
A database transaction groups related local changes so that they commit together or not at all. PostgreSQL's transaction tutorial explains that boundary. A request to another system does not automatically participate in the same transaction. Saving locally and sending remotely as unrelated steps leaves a gap where one succeeds and the other fails.
AWS describes the transactional outbox as a way to address this dual-write problem. The pending event is stored with the business change, then relayed later. Delivery can still repeat, and operational monitoring is still necessary. Ask which records share the transaction, how events are identified and how a receiver recognizes that it already applied a particular change.
SECTION 02
Illustrative example: an approved service request
Imagine a portal approves a service request and must create a scheduling task in another system. Approval and its pending event are saved together. If the delivery worker stops, the portal can show approved with scheduling pending. Restarting the worker discovers the recorded intention rather than relying on a message that existed only in memory.
In this hypothetical sequence, the scheduling system creates its task but the reply is lost. The worker sends the event again. A stable event or business reference lets the receiver return or recognize the existing task. Without that protection, the outbox can reliably deliver the same intention twice and still create duplicate work.
SECTION 03
Use the pattern where the missing event matters
A small application may only need an ordinary transaction when all relevant work stays inside one database. An established platform may already provide change-event delivery. A custom outbox is most useful when a committed business change must lead to a separately recoverable external action; it is not a requirement for every analytics click.
For acceptance, interrupt execution before commit, after commit, after delivery and before delivery acknowledgement. Confirm no event escapes a rolled-back change, pending work survives restart and repeated delivery does not duplicate the downstream effect. Define ordering where several changes affect one record, retention of delivered events and an owner for stuck items. Keep the first release focused on a few important event types rather than building an unrestricted event platform.
SECTION 04
Frequently asked questions
Does an outbox guarantee exactly-once delivery?
No. Repeated delivery remains possible. The receiver needs an appropriate duplicate-handling strategy, and the integration needs visible recovery.
Does it undo an external action if a later step fails?
Not by itself. External corrections or compensating actions are separate workflow decisions and need their own authorization and failure handling.
Is this necessary for a simple internal tool?
Only when its consistency requirements justify it. If the important changes remain within one database, a normal transaction may be sufficient.
PRIMARY REFERENCES
Sources and further reading
These references cover the standards, platforms, or published prices discussed in the guide. Worked examples and checklists are our editorial guidance.
- Transactional outbox patternAWS Prescriptive Guidance
- Database transactionsPostgreSQL
EDITORIAL METHOD
About this guide
We use AI to assist with drafting and editing. Catapult AI Work is responsible for the published content. Examples illustrate possible approaches; they are not client case studies unless identified as such.
Budget examples are not Catapult package prices. Check linked provider pages for current fees and plan limits before making a purchase.
Read the editorial policy