Shelves and Shoves

Today is the day to hit bug #105, another of the leftover bugs from last year. The story goes something like this: there's a lot of data on nysenate.gov that is nice to have, but asking for that info on every call is a little cumbersome. We want to cache as much of the data as we can, particularly the stuff that's not going to change in the next week or longer. Previously, I had implemented a simple pylons cache which was fast, but had no persistent storage, so every time the server went down it pulled all the info again. And due to the way the scrape was written, it pulled all the info for all the senators at once, creating quite a bit of lag before the first page showed up. This clearly wasn't going to be something we could continue to develop with. Now, I know nothing about caching data, so I did some poking around in CIVX to see how it is done elsewhere. Most of the other caches I found in CIVX code were related to caching text feeds, which were not as explanatory as I was hoping. Once I felt I had a handle on how things were done there, I began to try to implement some of it, only to be shown this post on shelve, a Python library for storing arbitrary data. Combined with the decorator, this seemed to do exactly what I wanted, namely provide a permanent storage area for a bunch of data with a configurable expire time. I dumped the code into the dashboard, hooked the proper inputs up and let it run. The results were... promising, but not astonishing. The file storage worked, once the data was cached, we stopped looking to nysenate.gov for data and instead used our own data, even after server restarts. The problem was that the file storage seemed to be slower than the previous memory cache. This is all perfectly reasonable, since disk access is much slower than memory, and a lot of data has to get pulled for each senator. The first obvious thing I could do is to re-enable the memory cache, but this did not seem to help as much as I wanted it to. At this point, Luke popped up in chat to sat that Moksha had a Shove cache it uses for feeds. Sure enough, back in the files I had been poking through earlier, there were some references to Shove. Back to the net, I started to explore what Shove was and how it could help me. It turns out Shove is mostly drop in compatible with shelve, and aims to be a more extensible replacement for it. Once I got a handle on how Shove works differently from shelve (answer, not very), I made a few tiny tweaks and got a version successfully working with Shove and a sqlite backend. This didn't make the end result any faster (well maybe a little, but not much), but there is a lot of room for improvement, particularly if I can hook into Moksha's own stores. Further, Shove has its own abilities to cache items in memory in addition to storing them, which I would like to look into. The best route for efficiencies, I think is to change how the data gets stored in the cache. Currently all the data gets pulled at once, which was done to pacify the pylons cache. However, if I can get individual caches for each senator, then I can pull smaller volumes of data at a time, hopefully speeding up the process. We'll see where I get tomorrow, but so far I'm feeling pretty good about all this.


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