Database Reference
In-Depth Information
▪ QUORUM: Queries all nodes and returns the record with the most recent timestamp
after a quorum of nodes have responded, where a "quorum" is (n /2) + 1.
▪ DCQUORUM: Ensures that only nodes in the same data center are queried. Applicable
when using Rack-Aware placement strategy.
▪ ALL: Queries all nodes and returns the value with the most recent timestamp. This level
waits for all nodes to respond, and if one doesn't, it fails the read operation.
Note that there is no such thing as READ ZERO, as it doesn't make sense to specify that you
want to read some data and don't need any node to respond.
Datta C
a Centter S
d Sttrattegy
r Shard S
Decenttralliized
Cassandra is considered decentralized because it defines no master server, and instead uses
a peer-to-peer approach in order to prevent bottlenecks and single points of failure. Decent-
ralization is important in Cassandra because it is what allows it to scale up and also to scale
down; peers can enter or exit the cluster as they like, with minimal disruption.
Denormalliizattiion
In relational databases, denormalization, or the creation of redundant data, is sometimes ap-
plied in order to improve performance of read-mostly applications, such as in online analyt-
ical processing (OLAP). In Cassandra, it is typical to see denormalized data, as this improves
performance and helps account for the fact that data is structured according to the queries
you'll need, in distinction to standard relational databases where the data is typically struc-
tured around the object model independently.
Durabiilliitty
When a database is durable, it means that writes will permanently survive, even in the event
of a server crash or sudden power failure.
Cassandra accomplishes durability by appending writes to the end of the commit log, allow-
ing the server to avoid having to seek the location in the data file. Only the commit log needs
to be synced with the file system, and this happens either periodically or in a specified batch
window.
When working in a single server node, Cassandra does not immediately synchronize a file's
in-core state with the storage device. That can mean that if the server is shut down immedi-
ately after a write is performed, the write may not be present on restart. Note that a single
server node is not recommended for production.
