On Sun, Nov 28, 2010 at 11:35 AM, Tom Melendez <t...@supertom.com> wrote: > On Sun, Nov 28, 2010 at 12:28 AM, David Boxenhorn <da...@lookin2.com> wrote: >> As our launch date approaches, I am getting increasingly nervous about >> Cassandra tuning. It is a mysterious black art that I haven't mastered even >> at the low usages that we have now. I know of a few more things I can do to >> improve things, but how will I know if it is enough? All this is >> particularly ironic since - as we are just starting out - we don't have >> scalability problems yet, though we hope to! >> > How are your load tests looking? Of course, there's nothing like > going live, but I expect you'll be able to simulate 2x-3x your initial > launch traffic. > >> Luckily, I have completely wrapped Cassandra in an entity mapper, so that I >> can easily trade in something else, perhaps temporarily, until we really >> need Cassandra's scalability. >> >> So, I'm thinking of emulating Cassandra with mySQL. I would use mySQL either >> as a simple key-value store, without joins, or map Cassandra supercolumns to >> mySQL columns, probably of type CLOB. >> >> Does anyone want to talk me out of this? >> > > As you said, I think you just have some cold feet. > > My feeling is that you did some original research and decided on > Cassandra for various reasons. I think if you put the MySQL solution > in now, you won't go back to the Cassandra solution, because once its > live, it will be much riskier to switch. And if you feel you made a > mistake in your original assessment, then great, at least you found > out before launch. > > Whatever you choose, I would flesh out my my fears with as much detail > as possible. Invest in load tests and develop contingency plans. I > talked about this in 2009 a little bit here - see slide 22, we call > these "Defcon Levels". > > http://www.slideshare.net/supertom/building-configurable-applications-for-the-web > > The idea is prioritizing what REALLY is important if the shit hits the > fan (watch out, biz folks think everything is always important) and > having processes to implemen and knobs to turn and levers to pull > should you get slashdotted (or "facebooked", "tweeted", "oprahed", > "techcrunched" or whatever we call it these days). > > Good luck with your launch. > > Thanks, > > Tom >
You should always worry about everything, but you should also have confidence in your decisions. If your worry is how your cluster will perform under load, then you should find a way to test under load. Tweeks and tunes do not make scalability (they help), hardware does. If you want to be ready to be 'slashdotted' you better have a rack of servers idling. If you just need a key-value store you may not need Cassandra. Cassandra is scalable in a different way then MySQL would be. You want convincing... (Ill try) Cassandra shards through node joins and handles replication. If you start off with a Mysql master/slave architecture, or using id hash(key) mod 3. It is not clear how you grow that cluster with demand. If you make a choice that is not scalable, when you get 'slashdotted' you will not be ready. What is worse you will have no easy way out of the problem.
