aws-samples / aws-samples/transactional-outbox-pattern

How to scale horizontally?

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Description

How can I guarantee ordering by key (e.g., userId)?
For example, I must not send events for the same userId out of order.
If I have two events for user X, called A and B, I need to guarantee that A is sent first and B afterwards.
However, if I have two instances of the outbox processor, both could pick up events for the same userId. If, for any reason, one instance sends B before A, the ordering is broken.
What is the best practice or solution for this problem?

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