Have you ever sent an important email or message, only to find out later that it failed to deliver? Maybe it was a job application, a client proposal, or a simple message to a friend. Whatever the case, we've all experienced the frustration of a failed message delivery at some point. But what if there was a way to ensure that your messages were always delivered reliably, no matter what? That's where the transactional outbox pattern comes in.
In today's interconnected world, messaging is an essential part of our daily lives. From business communications to personal relationships, the ability to send and receive messages quickly and reliably is critical. But what happens when a message fails to deliver? Whether it's due to a network outage, a service disruption, or some other unforeseen error, the consequences can be significant. That's why the transactional outbox pattern is such an important concept in modern software engineering.
Metaphorical Example
Think of the transactional outbox pattern as a relay race. Just like in a relay race, where each runner passes the baton to the next with precision and care, the transactional outbox pattern ensures that messages are passed from one system to another in a reliable, consistent manner. Instead of sending messages directly, which can result in lost or failed deliveries, the transactional outbox pattern creates a buffer or queue where messages are temporarily stored until they can be processed and delivered successfully. It's like having a team of expert runners who work together to ensure that the baton always reaches the finish line.
Problem statement
In distributed systems we often come across situations where we need to update the database and send messages or events atomically. This simply means either both happens or non of it happens. This needs to be done to avoid data consistencies.
The challenge occurs when we want to send a message to a broker in the middle of a transaction. We are not sure whether the transaction will complete. The other challenge occurs when we send the message after the transaction is complete. The message broker might crash. In either of the above cases we are left with an inconsistent situation.
Out of the box solution
To solve the above problem we create a new table "outbox". So whenever a process needs to push some data in the message broker it writes that data in this table as part of the local transaction. A separate consumer will pick data from this table and publish it to the message broker. With this approach we ensure that only commited data reaches the message broker and in case of failure in message broker the data is retained in the outbox table which can be processed again.
The diagram explains the steps of imaplementation of the transactional outbox pattern.
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