[Monotone-devel] Re: Broken database?

[graydon hoare]

Peter Simons wrote:
> Hi,
>
> I'm having trouble with Monotone. Apparently, there is some
> broken certificate in my database. Either that, or the depot
> stuff isn't working as expected:

the phenomenon you're seeing is a working ancestry graph (in certs) and 
a bunch of missing versions in your storage system.

I can reproduce this with my own fetch from your depot. during the 
fetch, I see a number of "manifest delta pre-image '...' not found in 
database" sort of messages. unfortunately this is exactly the problem 
I'm trying to address with the netsync code: the part of depots which 
plays with deltas is time-sensitive, and that's simply not robust. the 
depot is replaying a log to you of things it received, including deltas. 
if you fetch deltas out of order from the way they were composed -- 
which can happen either due to bugs, misconfiguration, or possibly even 
races between posters -- it can't apply deltas, so drops them on the 
floor. I was, earlier, going to build some sort of input queue to 
reorder stuff. now that I have netsync beginning to work, I'm strongly 
considering just dropping the log-based stuff altogether. ironically, 
due to its "protocol level" simplicity, its systemic complexity is way 
too high. too many ways to mis-order logs.

the certs, being more "atomic" objects, don't suffer from this as much; 
there's not really any temporal coupling between them (though you can 
still get out of sync with your server, hence miss some).

there's not much I can recommend at the moment, aside from waiting for 
the netsync code. it's a design error on my part; I made the first 
networking layer of monotone log-based, and that's turned out to be a 
mistake. the majority of breakage I've seen reported has had to do with 
temporal inconsistency in transmitted logs. so I'm replacing it.

-graydon