Mark Payne commented on NIFI-2854:

[~joewitt] - thanks. I think this outlines the intent quite well.

> Enable repositories to support upgrades and rollback in well defined scenarios
> ------------------------------------------------------------------------------
>                 Key: NIFI-2854
>                 URL: https://issues.apache.org/jira/browse/NIFI-2854
>             Project: Apache NiFi
>          Issue Type: Improvement
>          Components: Core Framework
>            Reporter: Mark Payne
>            Assignee: Mark Payne
>             Fix For: 1.2.0
> 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.

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