Saturday, 26 June 2010

GSoC Report: Week 5

In my last post I said I had complete the warm-up phase and that I will start to think about how to handle caches which are no longer available.

The caching mechanism relies in the files _darcs/prefs/sources and ~/.darcs/sources, basically the content of those file is used to generate the cache entries, each of the entries in that file indicate an alternative source to get files. If we want to specify global caches we put that in ~/.Darcs/sources but if we want an alternative repositories to pull from, we specify that in the repository sources file which is in _Darcs/prefs/sources, also each time we do a pull from an external repository it is added to the sources file
automatically.

The problem of expiring caches is given because sometimes it happens that repositories that were available, can become unavailable. For example if I had pulled from 3 different repositories and 2 of them stop being available, it could take up to 2 minutes to get each patch, because Darcs could try to fetch every patch it needs from those 2 not longer available repositories, it tries to establish a connection, an then waits for a time-out or a bad response code. After the problem I just mentioned, the idea is to design a mechanism which can help Darcs to establish which entries should be expired.


We can split the cache entries in two groups: locals and remotes. Dealing with local non-longer reachable repositories is not a big deal since if we don't find the local entries we can assume they don't exist and we can drop them from the cache and stop trying to fetch files from them. Remote repositories are more tricky, for example I can't eliminate an entry just because it gives a time-out when it tries to establish a connection with it, there are other external factors which could interfere with that particular entry in a given moment
(firewalls).

So as we seem this is not an easy task, handling remote repositories is out of our hands, we don't have control over the external sources, we don't have control over the network configuration and so on. So a first approach to this is to mark the entries which are not working and ignore them for the rest of the pulling since we don't want to try to establish a connection with an entry which we know is not available. If we try to establish a connection and fails we can mark it as a bad entry but also I think it could be awkward to wait for a 60 seconds time out, something we could implement is a default time for waiting for a connection to succeed (10-15 seconds maybe) if it doesn't happen between that time we can skip it, mark it as bad, and don't try for the rest of the patches that particular entry. Other approach suggested in the bug tracker was to try to establish a connection with all of the entries and use the one that responds first but then what if all the entries are bad entries?. I have to think more about it, I will discuss on irc and the mailing and then with a clearer idea I will start to code a patch to solve the issue.

Also I sent a first version of a failing test for the case of unreachable entries, but I need to amend, as there are some missing cases.

More of my progress can be found in the wiki.

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