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https://issues.apache.org/jira/browse/OAK-4810?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15492777#comment-15492777
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Chetan Mehrotra commented on OAK-4810:
--------------------------------------

bq. I think default for writing (if not configured explicitly) could still be 
SHA-1.

The change can be made anytime. It should not affect any other part much. So 
default value can be simply switched to SHA-256

Once a binary is added by any digest method we do not need the method details 
while doing a read as that would be purely on the basis of id. Still it would 
be good to encode the algo in the id which is passed back to NodeStore

> FileDataStore: support SHA-2
> ----------------------------
>
>                 Key: OAK-4810
>                 URL: https://issues.apache.org/jira/browse/OAK-4810
>             Project: Jackrabbit Oak
>          Issue Type: New Feature
>          Components: blob
>            Reporter: Thomas Mueller
>
> The FileDataStore currently uses SHA-1, but that algorithm is deprecated. We 
> should support other algorithms as well (mainly SHA-256).
> Migration should be painless (no long downtime). I think default for writing 
> (if not configured explicitly) could still be SHA-1. But when reading, 
> SHA-256 should also be supported (depending on the identifier). That way, the 
> new Oak version for all repositories (in a cluster + shared datastore) can be 
> installed "slowly".
> After all repositories are running with the new Oak version, the 
> configuration for SHA-256 can be enabled. That way, SHA-256 is used for new 
> binaries. Both SHA-1 and SHA-256 are supported for reading.
> One potential downside is deduplication would suffer a bit if a new Blob with 
> same content is added again as digest based match would fail. That can be 
> mitigated by computing 2 types of digest if need arises. The downsides are 
> some additional file operations and CPU, and slower migration to SHA-256.
> Some other open questions: 
> * While we are at it, it might makes senses to additionally support SHA-3 and 
> other algorithms (make it configurable). But the length of the identifier 
> alone might then not be enough information to know what algorithm is used, so 
> maybe add a prefix.
> * The number of subdirectory levels: should we keep it as is, or should we 
> reduce it (for example one level less).



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