Hi Matt,
In the wiki there is no precise IMHO statement about write:
> The overlay blob store fulfills write requests by attempting to write to each
> delegate in priority order. Once a write is successfully satisfied by a
> delegate, the result of the delegate write is returned as the result of the
> overlay blob store write and no subsequent writes are attempted for that
> request.
Does this mean that only one write to one delegate blob store must
succeed (in order of priority) before returning a result for a primary
blob store (Overlay in this case) ?
Besides this I imagine such Overlay blob store might be helpful during
no downtime migrations. Imagine the following migration case:
FileBlobStore ---> S3DataStore
or
S3DataStore ---> AmazonDataStore
If you could configure priority for read in the following order: {FBS,
S3DS} but for writes in inverse order {S3DS, FBS} then this will
almost satisfy any DataStore migration scenarios (except the need to
transfer content stored in initial blob store) but it can be done in
the background and it can be done by any other mechanism (offline copy
process).
Regarding CompositeBlobStore -- what if customer changes the storage
rules in the meantime (refers also to Curation section). This will
result in the new layout for writes but binaries won't be read
correctly, am I right?
I guess this could be resolved by nesting: CompositeBlobStore with old
rules and CompositeBlobStore with new rules in OverlayBlobStore, but I
see that "curation" is for now deferred to other topic but still I
think this is quite important IMO to think about it a little upfront
at least for this blob store type.
Thanks,
Arek
2017-07-26 2:20 GMT+02:00 Matt Ryan <[email protected]>:
> There are two concepts. One I’m currently calling the Overlay blob store
> (we haven’t voted on this name yet). In this case, delegates are
> configured with a preferred order of lookup. When a read is issued, the
> overlay blob store will attempt to satisfy the read by going through the
> delegates in order until one can satisfy the read. [0]
