[tahoe-dev] on backward-compatibility and new features

[Zooko Wilcox-O'Hearn]

Folks:

This is a general lesson that I've learned over the years, from  
following the darcs project among other things.

Consider this use case:

There is a person who uses Tahoe-LAFS vX, and she shares files with  
other people who use Tahoe-LAFS vX, such as friends, family, or  
members of her organization or company.  She wants to upgrade her  
Tahoe-LAFS to vX+1 even though the people that she shares with aren't  
ready to upgrade theirs.  *And* she doesn't want to change anything  
about her work-flow or configuration, nor does she want to read the  
NEWS file carefully and think about compatibility issues.

This implies that any new backwards-incompatible feature that is  
added in Tahoe-LAFS vX+1, such as immutable directories, has to be  
disabled by default, and to be turned on only by explicit action on  
the part of the user.

So concretely, we should make sure that in Tahoe-LAFS v1.6  
directories are created mutable by default, just like in Tahoe-LAFS  
v1.5.  This is easy because the way that directories were created in  
Tahoe-LAFS v1.5 was to create a new empty mutable directory, and  
there is no point in creating a new empty immutable directory.  :-)

It would probably be wise to make the "tahoe backup" command continue  
to create mutable directories by default, because there might be  
people who use "tahoe backup" to create directories which they then  
share with their friends.  I know that immutable directories in  
"tahoe backup" are sweet (#606), and I want people to use them, but  
for Tahoe-LAFS v1.6 the way to get people to use them should be to  
announce them in the NEWS and explain them in the docs and tell  
people to turn them on in their configuration.

Anyway, part of my reason to post this is to inform developers and to  
reassure users that the use case sketched above is important to me.   
If this use case isn't taken care of, then what happens is people  
just put off upgrading until all of the people they share with have  
upgraded, which can sometimes mean forever.

I would like people to get used to the idea that they can always  
upgrade to the new version of Tahoe-LAFS and continue sharing with  
their more conservative friends without having to even think about  
these issues.

This is also why "forward-compatibility" features are so important to  
me.  They can sometimes ease these transitions.  Here is the list of  
tickets tagged "forward-compatibility":

http://allmydata.org/trac/tahoe/query?status=%21closed&keywords=% 
7Eforward-compatibility

Regards,

Zooko

http://allmydata.org/trac/tahoe/ticket/606# backupdb: add directory  
cache