Database Reference
In-Depth Information
turned among the data response received by a local quorum of replica
nodes.
•
EACH_QUORUM:
With the read consistency level
EACH_QUORUM, the last updated data (based on timestamp) is re-
turned among the data response received by each quorum of replica
nodes.
•
ALL:
With the read consistency level ALL, the last updated data (based
on timestamp) returned among the data response received from all rep-
lica nodes. Since responses with the latest timestamp are returned
among all replica nodes, ALL is the highest consistency level.
•
SERIAL:
With the read consistency level SERIAL, it would return the
latest set of columns committed or in progress. Uncommitted transac-
tions discovered during read would result in implicit commit of running
transactions and return to the latest column values.
•
TWO:
With the read consistency level TWO, the latest column values
will be returned from the two closest replica nodes.
•
THREE:
With the read consistency level THREE, the latest column
values will be returned from three of the closest replica nodes.
Based on the above-mentioned consistency level configurations, the user can al-
ways configure each read/write request with a desired consistency level. For example,
to ensure the lowest write consistency but the highest read consistency, we can opt for
ANY as write consistency and ALL for read consistency level.
Cassandra Query Language (CQL)
One of the key features of Cassandra from an end user perspective is ease-of-use rather
than familiarity. Cassandra query language (CQL) was introduced with Cassandra 0.8
release with the intention of having a RDBMS style structured query language (SQL).
Since its inception CQL has gone through many changes. Many new features have
been introduced in later releases along with lots of performance-related enhancement
work. CQL adds a flavor of known data definition language (ddl) and data manipula-
tion language (dml) statements.
During the course of this topic, we will be covering most of the CQL features.
