How to rewrite a stream?

[gs-jdudulski]

I have rather a common problem, that I want to upcast (or remove) some events from the stream. A recommended way for migration is copy-and-replace. On the basic level it is rather obvious how to do this:

• find a stream
• iterate over events
• upcast if needed, append to new stream (or skip if you want to delete)

The problem is how to do this if you want to preserve the stream name? (or maybe I should not look at it that way?)

Just run it twice (copy to temp stream, delete original stream, copy back to proper stream)?

How to make sure that when deleting the stream, there are no new events (maybe we should have something like delete_stream(name, expected_version:) 🤔 )?

I know there is a way to rewrite events https://railseventstore.org/docs/advanced-topics/migrating-existing-events but wonder how to approach with immutable events in mind. Plus it won't handle case where you want to merge or split events.

[mostlyobvious]

1. The delete_stream(name, expected_version:) is an interesting extension, that would be backwards compatible with appropriate default arguments.

2. @mpraglowski described how to rename a stream, which you've already mentioned.

   Check out the catch-up + pessimistic lock on the storage. This can be a basis for an online stream rewriter. I think you could skip creating the interim stream — only delete and link to the recently deleted within a transaction.

[gs-jdudulski]

I think you could skip creating the interim stream — only delete and link to the recently deleted within a transaction.

This probably could be fine for low load, but might not work where there is heavy one :D