Pluggable persistence interfaces for SubmitQueue entities (requests, batches, dependents, logs, etc.). Implementations live under extension/storage/<impl>/.

Entities that support concurrent mutation carry an int32 Version field. Updates are conditional on the version: the write only succeeds if the persisted version matches the caller's expected version. On mismatch, the implementation returns storage.ErrVersionMismatch.

Version arithmetic is owned by the controller, not the store. Update methods take both oldVersion (the where-clause guard) and newVersion (the value to write):

UpdateState(ctx, id, oldVersion, newVersion int32, newState entity.RequestState) error

The store performs a pure conditional write — it does not compute oldVersion + 1 internally. This keeps the in-memory entity and the persisted row in sync without the storage layer mutating values the caller didn't supply.

newVersion := entity.Version + 1
if err := store.UpdateState(ctx, entity.ID, entity.Version, newVersion, newState); err != nil {
    return err // entity.Version unchanged on failure — safe to retry
}
entity.Version = newVersion // only after the write succeeded

The post-success assignment matters whenever the entity is read again later in the same flow. Pre-incrementing in memory before the call is a bug pattern: if the call fails and the caller swallows the error, the in-memory version is now ahead of the database and subsequent updates will fail with ErrVersionMismatch for non-obvious reasons.