thoughts on data and backward and forward compatibility

[Adriana Kobylak]

Thanks Brad for putting this together.


> The following are guidelines and examples to help decide the location
> in which you place your data.

For additional details, refer to the FHS - 
https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.pdf


> ---
> /etc
> ---
> Further complicating the process, other applications or users may
> write data here and that data must be “merged” during any
> migration process.

Even further, /etc is mounted as an overlayfs that has its own 
restrictions on renaming/deleting files 
(https://en.wikipedia.org/wiki/OverlayFS). The overlay is either mounted 
over / or /etc depending if you want to preserve the rest of the 
filesystem as read-only.


> —--
> /var/lib/<package>
> —--
> This is dynamic, mutable data.  If your application writes data, it
> should go here (with one exception below).  Just like in /etc, data
> here is also hard to maintain forward and backward compatibility, and
> just like in /etc, applications need to support old and new data
> formats simultaneously or have data migration.  Applications other
> than the consumer or end-users should not modify data here so data
> merging is not required.

The cereal package can be used to store data and supports versioning for 
when changes are desired. Multiple apps already use it in OpenBMC.