Friday, May 01, 2009

Consistent hashing vs order-preserving partitioning in distributed databases

The Cassandra distributed database supports two partitioning schemes now: the traditional consistent hashing scheme, and an order-preserving partitioner.

The reason that almost all similar systems use consistent hashing (the dynamo paper has the best description; see sections 4.1-4.3) is that it provides a kind of brain-dead load balancing from the hash algorithm spreading keys across the ring. But the dynamo authors go into some detail about how this by itself doesn't actually give good results in practice; their solution was to assign multiple tokens to each node in the cluster and they describe several approaches to that. But Cassandra's original designer considers this a hack and prefers real load balancing.

An order-preserving partitioner, where keys are distributed to nodes in their natural order, has huge advantages over consistent hashing, particularly the ability to do range queries across the keys in the system (which has also been committed to Cassandra now). This is important, because the corollary of "the partitioner uses the key to determine what node the data is on" is, "each key should only have an amount of data associated with it (see the data model explanation) that is relatively small compared to a node's capacity." Cassandra column families will often have more columns in them than you'd see in a traditional database, but "millions" is pushing it (depending on column size) and "billions" is a bad idea. So you'll want to model things such that you spread data across multiple keys and if you then pick an appropriate key naming convention, range queries will let you slice and dice that data as needed.

Cassandra is in the process of implementing load balancing still, but in the meantime order-preserving partitioning is still be useful without that if you know what your key distribution will look like in advance and can pick your node tokens accordingly. Otherwise, there's always the old-school hash-based partitioner until we get that done (for the release after the one we'll have in the next week or so).

See the introduction and getting started pages of the Cassandra wiki for more on Cassandra, and drop us a line on the mailing list or in IRC if you have questions; we're actively trying to improve our docs.


Jay Farrimond said...

Very interesting. BTW, reading the post I get the feeling you're now a part of the Cassandra development team. Is this true?

Jonathan Ellis said...

Yes, I was added as a committer at the end of March.

Jay Farrimond said...

Cool! I started to say "good for you", then I realized that it's really "good for Cassandra!"