Glossary - Cassandra: The Definitive Guide

Database Reference

In-Depth Information

Hecttor

An open source project created by Ran Tavory of Outbrain and hosted at GitHub, Hector is

a Cassandra client written in Java. It wraps Thrift and offers JMX, connection pooling, and

failover.

Hiintted H

d Handof

This is a mechanism to ensure availability, fault tolerance, and graceful degradation. If a write

operation occurs and a node that is intended to receive that write goes down, a note (the

“hint”) is given (“handed off”) to a different live node to indicate that it should replay the

write operation to the unavailable node when it comes back online. This does two things: it

reduces the amount of time that it takes for a node to get all the data it missed once it comes

back online, and it improves write performance in lower consistency levels. That is, a hinted

handoff does not count as a sufficient acknowledgment for a write operation if the consist-

ency level is set to ONE, QUORUM, or ALL. A hint does count as a write for consistency

level ANY, however. Another way of putting this is that hinted writes are not readable in and

of themselves.

The node that received the hint will know very quickly when the unavailable node comes

back online again, because of Gossip . If, for some reason, the hinted handoff doesn't work,

the system can still perform a read repair.

Key

See Row Key .

Keyspace

A keyspace is a container for column families. It is roughly analogous to the database in the

relational model, used in Cassandra to separate applications. Where a relational database is a

collection of tables, a keyspace is an ordered collection of column families. You define your

application's keyspace in the Cassandra configuration file or by using the definition methods

in the API. When you define a keyspace, you can also define its replication factor and its

replica placement strategy. Within a given Cassandra cluster, you can have one or more key-

spaces, typically one for each application.