Hello, adding more random thoughts below on LAMP sucking, and spitting. Glyph Lefkowitz wrote:
Nathan Seven wrote:
Hmmmm I would qualify that- I dont think the filesystem is the place to be handling dynamic data.
The filesystem is a fine place.
For example, in the Prevayler persistence model, you just write logfiles to disk, and synchronize your state at a checkpoint. For highly dynamic applications, especially ones which require failover, (You can re-play the transaction log live, after all) this works quite well.
Databases were created *specifically* for this purpose.
I think that databases were specifically designed to store accounting information, actually.
Sure, storing all your static blobs in your database is a really quick way to grind shit to a halt, but locking and concurrency? If you're doing things properly, and your http server is just serving static objects, then these are non-issues.
Databases can be amazingly slow, especially if you have a lot of updates to do. (Even a very fast database can be made slow by I/O bottlenecks if you are trying to make it remote for scalability reasons.) This has an easy solution: you can cache everything! Of course, then you need to be able to easily access the cache from all of the machines, because it may have been updated. Now you have problems with coherency. Then you need to lock the cache, because it could have been updated, and then you need to read from it.
Pretty soon you're talking to your caching server as if it were a database. This is _great_ if you are Livejournal:
Yeah- through my line of work I deal with a *lot* of different infrastructures. Everything from "Joe's BBQ Sauce Garage" to Amazon. Literally the only organization I can think of that can keep anything coherent with MySQL is Livejournal- and then I believe only because Brad seems to be a cache-god with memcached and such.
because then you don't have to worry about computation, mutable data, etc - you're basically just storing data and then spitting it back out, and you don't care if the timestamps are a little off.
This is the important point about LAMP and Twisted:
There are applications which can connect to HTTP which are not blogs.
If you are writing a multiplayer game which wants to support lots of concurrent users, you can't afford to spawn a thread and do a database request every time a player picks something up.
New versions of linux have very quick threads. Also different apps have different speeds for threads. Small programs (say less than 100 kilobytes) which are entirely static are lots faster than multi megabyte processes which dynamically load things. Threads and processes also can easily use multiple cpus. Of course there are other reasons people don't like threads. http for games? If your game is sensitive to latency and you can help it, avoid http for games. Http gives you much more latency than a db. So does a centralised server for that matter. In a two player game you can half your latency by talking directly to each other. Assuming you both aren't behind a non configurable firewall, or a proxy server. http is good as a backup protocol though. Because some fascists don't allow anything but access to the internet except through a proxy. Or maybe you are playing a game in a web browser :) Note that some dbs have async interfaces(eg postgres). So you wouldn't need a thread. btw, anyone know if sendfile is in (or going to be in python soon)? http://mail.python.org/pipermail/python-dev/2002-March/021498.html
(Python is quite slow enough already, thanks.) You can't just use a cache because the data changes _all the time_, and you have to care about it from everywhere that you care about your data. Working with your objects directly in memory is close to the only option.
What about berkley dbs? I think bsddb3 was 3-4 times slower than a python dict in general. They are very quick, and you don't need to lock them. You can use transactions. Locking can kill performance. Quickness depends on memory, and data sets really. These are approximate speedinesses of python dictionary like things for different data sizes and memory: say 700MB of memory: 200 MB key value data - python dicts, kjDicts, berklydb3 on disk db. 400 MB key value data - kjDicts(as python dicts use more memory and begin swapping), berkley db3-4 on disk, python dicts.
2 gigs of key value data - berkley db3-4 on disk, kjDicts(the kjdict starts swapping before here), python dicts.
Of course deleting large python dicts is *really* slow. I used to kill my python processes with kill -9 so that I didn't have to wait for the reference counting garbage collecting beast to do lots of free()s on the dicts memory. kjDicts and bsddb dbs were faster for deleting. Some people do use RDBMS for large online games. Check out gamasutra.com for some articles. Seems they have lots of fun performance problems. Compressing the hash would be nice. Maybe memory mapped files on a compressed file system would be quick for this ;) Distributed hashes would also be nice! Any good distruted hashes for python? I think memory is one of the first things that kill apache performance(when using preforking). Especially when it has massive php compiled in! You can get big speed boosts by using different apache configs for different request types(even on the same machine). Eg set one up for static files(eg images), and one for your bloated php.
If you're writing a real-time financial data system, you do want to use a database, but you want to very carefully control your access to it. Certainly, you don't want to equate 'web hit' with 'database query', as the LAMP model is wont to do.
Or maybe you're writing an application that has to operate as a client-side proxy, and you don't have the leisure of a DBA at every desk, so you can't require that an RDBMS gets set up with each installation. This might require some hackish workarounds with the filesystem that you'd rather not do, but nevertheless, it's better than having the user editing pg_hba.conf themselves.
SQLlite and berkley db are good for easy to bundle dbs. One good thing about LAMP though is that lots of servers have it installed, and it can be quite cheap to use as a platform. As you say, not everything is a blog. There are too many different factors for a one size fits all 'this way is best' solution. Besides everyone knows twisted rules ;) Have fun!