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A database transaction makes a group of changes atomic within the database participating in that transaction. It does not automatically include a payment provider, message broker, email service, or other external system. To coordinate a database update with a message, store both the business change and an outbox event in one database transaction, then publish the event separately and make consumers safe to receive duplicates.

What does a database transaction actually guarantee?

A transaction gives the participating database a defined unit of work. If it commits, its changes become part of the database state; if it rolls back, those changes are discarded. PostgreSQL’s documentation describes the core property this way: “A transaction is said to be atomic: from the point of view of other transactions, it either happens completely or not at all.” PostgreSQL 19 documentation on transactions also explains that intermediate states are not visible to other concurrent transactions.

Consider a bank transfer with a debit from one account and a credit to another. Both changes belong in one transaction: a committed transfer should not leave only the debit or only the credit. The guarantee applies to the database changes made within that transaction, not automatically to every action the application performs while processing the transfer.

ACID is useful, but it does not enforce every business rule

ACID describes properties of transaction processing, but it is not a promise that every application invariant is automatically correct. The application must express its rules and use constraints, transaction logic, and appropriate isolation behavior to enforce them. Database engines offer different isolation mechanisms, and their configuration affects which concurrent behaviors are possible. For example, Microsoft’s SQL Server guide describes locking and row-versioning options, along with their effects on resource use and contention.

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Transaction duration matters operationally. SQL Server documentation advises keeping transactions short because they can hold locks and other resources; long-running transactions can increase contention. Distributed transactions can also have costs: MongoDB’s transaction manual notes that distributed transactions often cost more than single-document writes and should not replace effective schema design.

A commit’s durability depends on engine settings

“Commit succeeded” has meaning within the database’s configured durability behavior. In SQL Server, full durability waits for the transaction log to be persisted before reporting a successful commit. With delayed durability, commit can return before the log is flushed, leaving a window in which a failure can lose the acknowledged transaction. That is a SQL Server configuration example, not a universal description of every database. See Microsoft’s SQL Server durability documentation.

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Are external API calls part of a database transaction?

Usually, no. An HTTP request to a payment provider, a message published to a broker, or an email sent to a customer is outside a local database transaction unless a distributed transaction mechanism explicitly coordinates the participating systems and they support the required semantics. A database rollback cannot ordinarily retract an email already sent, reverse an independent provider’s charge, or make a broker forget a published event. The transactional outbox pattern describes the resulting gap between database state and broker publication.

Why “send, then commit” and “commit, then send” both have a gap

  • Send before commit: the broker or external service may receive the event, then the database transaction may fail or roll back. The outside world has been told about a change that never committed.
  • Commit before send: the database can commit successfully, then the application can crash before publishing. The business state exists, but the event is missing.

These are different failure windows, not problems that a longer database transaction automatically solves. If a transaction is retried, external effects can also repeat. Google Cloud’s Spanner transaction documentation warns that side effects outside Spanner may happen more than once when the transaction is retried. Keep retryable transaction work free of non-idempotent external actions where possible.

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How do I atomically update a database and publish a message?

For the common database-to-broker case, use a transactional outbox. In one local database transaction, write the business-state changes and an event record to an outbox table (or equivalent database structure). A separate relay reads committed outbox records and publishes them. The commit atomically records both the business change and the intent to publish; it does not include the broker in that commit.

  1. Start a database transaction. Apply the application’s business changes, such as creating an order or reserving inventory.
  2. Insert an outbox record in the same transaction. Include the information the relay needs to publish the event, such as a stable message identifier and event payload.
  3. Commit both writes together. If the transaction rolls back, neither the business change nor its outbox record is committed.
  4. Run a separate relay. Publish committed outbox records to the broker, using polling or database-log-based capture.
  5. Make consumers idempotent. Assume a record might be published more than once. A consumer can record the message identifier in the same transaction as its own business update, and reject an identifier it has already processed.

The outbox closes the gap where a business update commits but no durable record of the intent to publish exists. It does not guarantee exactly-once delivery: the relay may publish successfully and crash before recording progress, so it can publish the same event again. The idempotent consumer pattern explains how to make duplicate processing safe.

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Choose a relay that fits the database and ordering needs

Relay approach How it works Trade-offs to account for
Polling publisher Repeatedly queries the database for pending outbox rows and publishes them. The pattern reference says it works with any SQL database; event ordering can be difficult to manage. Polling publisher.
Transaction-log tailing / change data capture Reads changes from the database log or an equivalent change stream. The pattern reference gives PostgreSQL WAL, MySQL binlog, and DynamoDB streams as examples. Mechanisms are database-specific, and duplicate publishing remains a concern. Transaction log tailing.

Neither relay choice makes delivery exactly once by itself. Design for duplicate events, and define ordering requirements explicitly: if a consumer must process events in sequence for a particular entity, the relay and consumer need a way to preserve or validate that sequence.

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What to decide before implementing the pattern

  • Atomic boundary: identify exactly which writes share one database transaction. Treat the broker or external API as outside it unless a supported coordination protocol is deliberately in use.
  • Business invariants: specify what must remain true and enforce it with appropriate application logic, constraints, and isolation settings for the selected engine.
  • Durability expectations: check the database’s configured commit behavior rather than assuming every successful commit has identical persistence semantics.
  • Retry behavior: determine whether transaction bodies can be repeated, and keep irreversible external side effects out of retryable work where possible.
  • Duplicate and order handling: give events stable identifiers, make consumers idempotent, and decide whether ordering is required and how it will be enforced.
  • Operational ownership: plan how the relay tracks progress and how operators will identify and recover from events that remain unpublished or repeatedly fail.

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