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| <article class="doc"> |
| <h1 class="page">Overview</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Apache Cassandra is an open source, distributed, NoSQL database. It |
| presents a partitioned wide column storage model with eventually |
| consistent semantics.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Apache Cassandra was initially designed at |
| <a href="https://www.cs.cornell.edu/projects/ladis2009/papers/lakshman-ladis2009.pdf">Facebook</a> |
| using a staged event-driven architecture |
| (<a href="http://www.sosp.org/2001/papers/welsh.pdf">SEDA</a>) to implement a |
| combination of Amazon’s |
| <a href="http://courses.cse.tamu.edu/caverlee/csce438/readings/dynamo-paper.pdf">Dynamo</a> |
| distributed storage and replication techniques and Google’s |
| <a href="https://static.googleusercontent.com/media/research.google.com/en//archive/bigtable-osdi06.pdf">Bigtable</a> |
| data and storage engine model. Dynamo and Bigtable were both developed |
| to meet emerging requirements for scalable, reliable and highly |
| available storage systems, but each had areas that could be improved.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Cassandra was designed as a best-in-class combination of both systems to |
| meet emerging largescale, both in data footprint and query volume, |
| storage requirements. As applications began to require full global |
| replication and always available low-latency reads and writes, it became |
| imperative to design a new kind of database model as the relational |
| database systems of the time struggled to meet the new requirements of |
| global scale applications.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Systems like Cassandra are designed for these challenges and seek the |
| following design objectives:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Full multi-master database replication</p> |
| </li> |
| <li> |
| <p>Global availability at low latency</p> |
| </li> |
| <li> |
| <p>Scaling out on commodity hardware</p> |
| </li> |
| <li> |
| <p>Linear throughput increase with each additional processor</p> |
| </li> |
| <li> |
| <p>Online load balancing and cluster growth</p> |
| </li> |
| <li> |
| <p>Partitioned key-oriented queries</p> |
| </li> |
| <li> |
| <p>Flexible schema</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="features"><a class="anchor" href="#features"></a><a class="link" href="#features">Features</a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Cassandra provides the Cassandra Query Language (<a href="../cql/ddl.html" class="page">CQL</a>), an SQL-like |
| language, to create and update database schema and access data. CQL |
| allows users to organize data within a cluster of Cassandra nodes using:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><strong>Keyspace</strong>: Defines how a dataset is replicated, per datacenter. |
| Replication is the number of copies saved per cluster. |
| Keyspaces contain tables.</p> |
| </li> |
| <li> |
| <p><strong>Table</strong>: Defines the typed schema for a collection of partitions. |
| Tables contain partitions, which contain rows, which contain columns. |
| Cassandra tables can flexibly add new columns to tables with zero downtime.</p> |
| </li> |
| <li> |
| <p><strong>Partition</strong>: Defines the mandatory part of the primary key all rows in |
| Cassandra must have to identify the node in a cluster where the row is stored. |
| All performant queries supply the partition key in the query.</p> |
| </li> |
| <li> |
| <p><strong>Row</strong>: Contains a collection of columns identified by a unique primary |
| key made up of the partition key and optionally additional clustering |
| keys.</p> |
| </li> |
| <li> |
| <p><strong>Column</strong>: A single datum with a type which belongs to a row.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>CQL supports numerous advanced features over a partitioned dataset such |
| as:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Single partition lightweight transactions with atomic compare and set |
| semantics.</p> |
| </li> |
| <li> |
| <p>User-defined types, functions and aggregates</p> |
| </li> |
| <li> |
| <p>Collection types including sets, maps, and lists.</p> |
| </li> |
| <li> |
| <p>Local secondary indices</p> |
| </li> |
| <li> |
| <p>(Experimental) materialized views</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Cassandra explicitly chooses not to implement operations that require |
| cross partition coordination as they are typically slow and hard to |
| provide highly available global semantics. For example Cassandra does |
| not support:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Cross partition transactions</p> |
| </li> |
| <li> |
| <p>Distributed joins</p> |
| </li> |
| <li> |
| <p>Foreign keys or referential integrity.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="operating"><a class="anchor" href="#operating"></a><a class="link" href="#operating">Operating</a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Apache Cassandra configuration settings are configured in the |
| <code>cassandra.yaml</code> file that can be edited by hand or with the aid of |
| configuration management tools. Some settings can be manipulated live |
| using an online interface, but others require a restart of the database |
| to take effect.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Cassandra provides tools for managing a cluster. The <code>nodetool</code> command |
| interacts with Cassandra’s live control interface, allowing runtime |
| manipulation of many settings from <code>cassandra.yaml</code>. The |
| <code>auditlogviewer</code> is used to view the audit logs. The <code>fqltool</code> is used |
| to view, replay and compare full query logs. The <code>auditlogviewer</code> and |
| <code>fqltool</code> are new tools in Apache Cassandra 4.0.</p> |
| </div> |
| <div class="paragraph"> |
| <p>In addition, Cassandra supports out of the box atomic snapshot |
| functionality, which presents a point in time snapshot of Cassandra’s |
| data for easy integration with many backup tools. Cassandra also |
| supports incremental backups where data can be backed up as it is |
| written.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Apache Cassandra 4.0 has added several new features including virtual |
| tables, transient replication (experimental), audit logging, full query logging, and |
| support for Java 11 (experimental).</p> |
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