+1. I think that even with these caveats (read-only, some data may require 
re-interpretation) it would still be a great help for accessing legacy data, 
for which the original source may no longer exist.

> On 23 Jan 2019, at 15:11, Simon Willnauer <[email protected]> wrote:
> 
> Hey folks,
> 
> tl;dr; I want to be able to open an indexreader on an old index if the
> SegmentInfo version is supported and all segment codecs are available.
> Today that's not possible even if I port old formats to current
> versions.
> 
> Our BWC policy for quite a while has been N-1 major versions. That's
> good and I think we should keep it that way. Only recently, caused by
> changes how we encode/decode norms we also hard-enforce a the
> index-version-created in several places and the version a segment was
> written with. These are great enforcements and I understand why. My
> request here is if we can find consensus on allowing somehow (a
> special DirectoryReader for instance) to open such an index for
> reading only that doesn't provide the guarantees that our high level
> APIs decode norms correctly for instance. This would be enough to for
> instance consume stored fields etc. for reindexing or if a users are
> aware do they norms decoding in the codec. I am happy to work on a
> proposal how this would work. It would still enforce no writing or
> anything like this. I am also all for putting such a reader into misc
> and being experimental.
> 
> simon
> 
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