Switching from MySQL to Cassandra - Pros/Cons?

问题:

For a bit of background - this question deals with a project running on a single small EC2 instance, and is about to migrate to a medium one. The main components are Django, MySQL and a large number of custom analysis tools written in python and java, which do the heavy lifting. The same machine is running Apache as well.

The data model looks like the following - a large amount of real time data comes in streamed from various networked sensors, and ideally, I'd like to establish a long-poll approach rather than the current poll every 15 minutes approach (a limitation of computing stats and writing into the database itself). Once the data comes in, I store the raw version in MySQL, let the analysis tools loose on this data, and store statistics in another few tables. All of this is rendered using Django.

Relational features I would need -

• Order by [SliceRange in Cassandra's API seems to satisy this]
• Group by
• Manytomany relations between multiple tables [Cassandra SuperColumns seem to do well for one to many]
• Sphinx on this gives me a nice full text engine, so thats a necessity too. [On Cassandra, the Lucandra project seems to satisfy this need]

My major problem is that data reads are extremely slow (and writes aren't that hot either). I don't want to throw a lot of money and hardware on it right now, and I'd prefer something that can scale easily with time. Vertically scaling MySQL is not trivial in that sense (or cheap).

So essentially, after having read a lot about NOSQL and experimented with things like MongoDB, Cassandra and Voldemort, my questions are,

• On a medium EC2 instance, would I gain any benefits in reads/writes by shifting to something like Cassandra? This article (pdf) definitely seems to suggest that. Currently, I'd say a few hundred writes per minute would be the norm. For reads - since the data changes every 5 minutes or so, cache invalidation has to happen pretty quickly. At some point, it should be able to handle a large number of concurrent users as well. The app performance currently gets killed on MySQL doing some joins on large tables even if indexes are created - something to the order of 32k rows takes more than a minute to render. (This may be an artifact of EC2 virtualized I/O as well). Size of tables is around 4-5 million rows, and there are about 5 such tables.

• Everyone talks about using Cassandra on multiple nodes, given the CAP theorem and eventual consistency. But, for a project that is just beginning to grow, does it make sense to deploy a one node cassandra server? Are there any caveats? For instance, can it replace MySQL as a backend for Django? [Is this recommended?]

• If I do shift, I'm guessing I'll have to rewrite parts of the app to do a lot more "administrivia" since I'd have to do multiple lookups to fetch rows.

• Would it make any sense to just use MySQL as a key value store rather than a relational engine, and go with that? That way I could utilize a large number of stable APIs available, as well as a stable engine (and go relational as needed). (Brett Taylor's post from Friendfeed on this - http://bret.appspot.com/entry/how-friendfeed-uses-mysql)

Any insights from people who've done a shift would be greatly appreciated!

Thanks.

回答1:

Cassandra and the other distributed databases available today do not provide the kind of ad-hoc query support you are used to from sql. This is because you can't distribute queries with joins performantly, so the emphasis is on denormalization instead.

However, Cassandra 0.6 (beta officially out tomorrow, but you can build from the 0.6 branch yourself if you're impatient) supports Hadoop map/reduce for analytics, which actually sounds like a good fit for you.

Cassandra provides excellent support for adding new nodes painlessly, even to an initial group of one.

That said, at a few hundred writes/minute you're going to be fine on mysql for a long, long time. Cassandra is much better at being a key/value store (even better, key/columnfamily) but MySQL is much better at being a relational database. :)

There is no django support for Cassandra (or other nosql database) yet. They are talking about doing something for the next version after 1.2, but based on talking to django devs at pycon, nobody is really sure what that will look like yet.

回答2:

If you're a relational database developer (as I am), I'd suggest/point out:

• Get some experience working with Cassandra before you commit to its use on a production system... especially if that production system has a hard deadline for completion. Maybe use it as the backend for something unimportant first.
• It's proving more challenging than I'd anticipated to do simple things that I take for granted about data manipulation using SQL engines. In particular, indexing data and sorting result sets is non-trivial.
• Data modelling has proven challenging as well. As a relational database developer you come to the table with a lot of baggage... you need to be willing to learn how to model data very differently.

These things said, I strongly recommend building something in Cassandra. If you're like me, then doing so will challenge your understanding of data storage and make you rethink a relational-database-fits-all-situations outlook that I didn't even realize I held.

Some good resources I've found include:

• Dominic Williams' Cassandra blog posts
• Secondary Indexes in Cassandra
• More from Ed Anuff on indexing
• Cassandra book (not fantastic, but a good start)
• "WTF is a SuperColumn" pdf

回答3:

The Django-cassandra is an early beta mode. Also Django didn't made for no-sql databases. The key in Django ORM is based on SQL (Django recommends to use PostgreSQL). If you need to use ONLY no-sql (you can mix sql and no-sql in same app) you need to risky use no-sql ORM (it significantly slower than traditional SQL orm or direct use of No-SQL storage). Or you'll need to completely full rewrite django ORM. But in this case i can't presume, why you need Django. Maybe you can use something else, like Tornado?