Can you explain the concept of the Transactional Outbox pattern and its application?

The Transactional Outbox pattern is used to solve the problem of atomicity between writing to a database and sending messages (e.g., to a queue or event) in distributed systems. The main idea is to, instead of directly sending a message within a transaction, save the event in a special table (outbox) within the same transaction as the business data. Then, a separate process or service asynchronously reads these events from the table and sends them to a queue or message broker.

This guarantees that data and events are not out of sync, as the outbox entry and the business operation occur atomically.

Example:

// Within a transaction, save data and event to outbox
using var transaction = dbContext.Database.BeginTransaction();

// Save business object
dbContext.Orders.Add(order);

// Add event to outbox
dbContext.OutboxEvents.Add(new OutboxEvent {
    EventType = "OrderCreated",
    Payload = JsonSerializer.Serialize(order),
    CreatedAt = DateTime.UtcNow
});

await dbContext.SaveChangesAsync();
transaction.Commit();

// A separate service reads from OutboxEvents and publishes to a queue