[ESDB-106-5] Bump up the chunk format version to 4 (Transformed)#4289
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timothycoleman merged 1 commit intomasterfrom Jun 13, 2024
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[ESDB-106-5] Bump up the chunk format version to 4 (Transformed)#4289timothycoleman merged 1 commit intomasterfrom
Transformed)#4289timothycoleman merged 1 commit intomasterfrom
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v4 basically represents the addition of the `TransformType` field. We want to use v4 for all new chunks, whether a transformation is applied or not (i.e if we're using the `Identity` transform). However, the existing chunk version verification code throws an exception on startup whenever a chunk with an unknown version is opened. This means that old versions of the database will not be able to read/write to v4 chunks even if no transformation is applied. To work around this, we implement forward compatibility in the following way: - the current `Version` field is renamed to `MinCompatibleVersion` - the next free slot of the chunk header is used to store the actual version (`Version`) `MinCompatibleVersion` represents the minimum chunk version that the db needs to understand to be able to read/write to this chunk. If a new chunk format is forward compatible, then `MinCompatibleVersion` can still point to an older version. However, if a change is not forward compatible (e.g when a transform is applied), then `MinCompatibleVersion` must also be bumped up to a higher value.
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This PR adds a second version field to the Chunk header. Now there are two:
This facilitates the creation of forwards-compatible chunk versions and therefore facilitates rolling upgrades when the chunk version is changing. When a new chunk has no transformation, it will have Version 4 (because it is) and MinVersion 3 (because old versions of ESDB will open it fine) When a new chunk has a transformation, both versions will be 4. Old versions of ESDB will (correctly) refuse to open it. The MinimumCompatibleVersion can be thought of as a promise of compatibility from a newer version of ESDB to an older version. In other circumstances this role could be played by a schema registry but that isn't applicable here. |
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Added: Chunk version 4 (
Transformed) file formatAdded: Support for forward compatibility in chunks
Thanks to @ylorph for the initial idea to bump up the chunk version and @timothycoleman and @sakno for their feedback.