Can you explain the concept of Transactional Inbox and its purpose in systems?

Transactional Inbox is a pattern used to ensure reliable delivery of messages or events in distributed systems with transaction support.

The idea is that when performing a business operation that involves sending a message (for example, to a queue or another service), this message is first saved in a local database in a special table (Inbox). This operation occurs within the same transaction as the main business operation.

After the transaction is successfully committed, a separate process or service reads messages from the Inbox and sends them to the target system. This guarantees that the message will not be lost and will not be sent twice, even if the system crashes or a failure occurs.

Thus, Transactional Inbox helps implement reliable integration and data consistency between services, avoiding issues with distributed transactions.

An example of a simplified model:

// Within a transaction, save data and message to Inbox
using (var transaction = dbContext.Database.BeginTransaction())
{
    dbContext.Orders.Add(order);
    dbContext.InboxMessages.Add(new InboxMessage { Payload = messagePayload });
    dbContext.SaveChanges();
    transaction.Commit();
}

// A separate process reads InboxMessages and sends them