Re: [9fans] replica

["Russ Cox" <rsc@plan9.bell-labs.com>]

> the advantage of keeping a hash (eg, md5 or sha1) is that it distinguishes
> contents of files, which usually is what really matters,
> whereas modification times and file length by themselves
> aren't really adequate (in general, in my experience).

> in short, i thought the use of hash values in wrap was a good idea
> and was surprised it wasn't part of replica!

i may well have dropped the ball on this one.
i'm still not sure.  replica was written only
for the distribution and as such i had little
experience with it before sending it into battle.
originally i used only mtimes and thought that
having the local metadata kept by the file system
was a feature, so you could play games if you knew
what you were doing.  then i got talked into using lengths
too (and rightfully so), and at that point i should
probably have tossed in the hash.
at this point it's hard to add fields to the database
and the log, since the current replica tools won't
like that.

i originally planned to use tra but it was clear that
tra wouldn't be solid enough by the time it was
needed.  once i understand things better i may set
up tra as an alternate way to get updates.  what i like
best about tra is that it's completely general.

with replica you have to declare a server to be the single
point of truth, and if you try to chain things (e.g.,
you try to grab updates using a different machine as
server, or vita nuova adds or tweaks some files) then
you have to be careful, because in general you can't
just grab updates from any other machine and expect everything
to work.

if we were using tra, you could install a new system
using any other plan 9 system you had available on the
network, and it would still be safe to go back to
sources for updates.  i still want to see what that
would be like, but it won't happen until the fall.

russ