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Any modern DB with a WAL (write ahead log) is an immutable event system, where the events are the DB primitives (insert, update, delete...).

When you construct your own event system you are constructing a DB with your own primitives (deposit, withdraw, transfer, apply monthly interest...).

You have to figure out your transaction semantics. For example, how to reject invalid events.

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> Any modern DB with a WAL (write ahead log) is an immutable event system, where the events are the DB primitives (insert, update, delete...).

Agreed, I just wish apart from WAL they also had versioning as first class and their update api required clients to pass the version they have "last seen" to prevent inconsistencies.

On most SQL databases, you can put CHECK constraints on columns so that the database rejects events. But this is controversial, as people don't like putting logic on the DB.

CosmosDB has etags on every document

DBs only work because the events are artificial and nobody cares about what's written in them.

And DBs are not really CQRS because the events are artificial and don't have business data that people are interested in keeping.

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