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· 6 min read

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A recent migration of functionality to a new service went wrong. When ships cross the exit line of the harbor, we check if currently an open visit exists for that ship. If so, we close the visit and delete it from certain projections.

Then something a ship did I never imagine it would; it turned around right before entering the harbor, sailing back over the exit point line. When it wanted to enter, the Harbor Control Center could not find the visit, since it was closed.

Besides fixing the validations in our aggregate, we needed a way to restore the data. We could replay the entire projections. However, over the past 6 months we have accumulated around 2.5M events and even though we tuned it very well, a replay would take an estimated two hours. This replay would make the data for all other ships invalid with the Harbor Control Center having to fall back to painful methods. Not a good option.

We needed a way to replay events for a single aggregate. There was nothing on the topic to be found on the web, except questions a bout how to do it and some vague "I don't understand why you would do such a thing" or "It's not possible" replies.