The flowfile, swapfile, provenance, and content repositories play a very important roll in NiFi's ability to be safely upgraded and rolled back. We need to have well documented behaviors, designs, and version adherence so that users can safely rely on these mechanisms.
Once this is formalized and in place we should update our versioning guidance to reflect this as well.
The following would be true from NiFi 1.2.0 onward
- No changes to how the repositories are persisted to disk can be made which will break forward/backward compatibility and specifically this means that things like the way each is serialized to disk cannot change.
- If changes are made which impact forward or backward compatibility they should be reserved for major releases only and should include a utility to help users with pre-existing data convert from some older format to the newer format. It may not be feasible to have rollback on major releases.
- The content repository should not be changed within a major release cycle in any way that will harm forward or backward compatibility.
- The flow file repository can change in that new fields can be added to existing write ahead log record types but no fields can be removed nor can any new types be added. Once a field is considered required it must remain required. Changes may only be made across minor version changes - not incremental.
- Swap File storage should follow very similar rules to the flow file repository. Adding a schema to the swap file header may allow some variation there but the variation should only be hints to optimize how they're processed and not change their behavior otherwise. Changes are only permitted during minor version releases.
- Provenance repository changes are only permitted during minor version releases. These changes may include adding or removing fields from existing event types. If a field is considered required it must always be considered required. If a field is removed then it must not be a required field and there must be a sensible default an older version could use if that value is not found in new data once rolled back. New event types may be added. Fields or event types not known to older version, if seen after a rollback, will simply be ignored.
The following also would be true:
- Apache NiFi 1.0.0 repositories should work just fine when applied to an Apache NiFi 1.1.0 installation.
- Repositories made/updated in Apache NiFi 1.1.0 onward would not work in older Apache NiFi releases (such as 1.0.0)