Out-box Pattern

Definition: The Outbox pattern solves the problem of needing to update a database and publish a message reliably without using a distributed transaction.
The application writes both the business change and an outbox message in the same database transaction, then a separate process publishes the message.

The problem

Imagine:

Create Order
    ↓
Update Database ✓
    ↓
Publish Event ❌

The database says:

Order = CREATED

but other services never receive:

OrderCreated

Now your system is inconsistent.

Outbox solution

Instead:

Database Transaction
┌───────────────────────────┐
│ Create Order               │
│                            │
│ Insert OrderCreated event  │
│ into OUTBOX table          │
└───────────────────────────┘
             ↓
          COMMIT ✓
             ↓
      Outbox Publisher
             ↓
        Message Broker
             ↓
       Other Services

The important part is that the order and the outbox message are committed together.

So:

Outbox = “Save my database change and the message I need to publish in the same transaction, then publish the message afterward.”

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