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| <div class="section" id="overview"> |
| <span id="id1"></span><h1>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">¶</a></h1> |
| <p>Apache Cassandra is an open source, distributed, NoSQL database. It presents |
| a partitioned wide column storage model with eventually consistent semantics.</p> |
| <p>Apache Cassandra was initially designed at <a class="reference external" href="https://www.cs.cornell.edu/projects/ladis2009/papers/lakshman-ladis2009.pdf">Facebook</a> |
| using a staged event-driven architecture (<a class="reference external" href="http://www.sosp.org/2001/papers/welsh.pdf">SEDA</a>) to implement a combination of |
| Amazon’s <a class="reference external" href="http://courses.cse.tamu.edu/caverlee/csce438/readings/dynamo-paper.pdf">Dynamo</a> |
| distributed storage and replication techniques combined with Google’s <a class="reference external" 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> |
| <p>Cassandra was designed as a best in class combination of both systems to meet |
| emerging large scale, 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> |
| <p>Systems like Cassandra are designed for these challenges and seek the |
| following design objectives:</p> |
| <ul class="simple"> |
| <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 class="section" id="features"> |
| <h2>Features<a class="headerlink" href="#features" title="Permalink to this headline">¶</a></h2> |
| <p>Cassandra provides the Cassandra Query Language (CQL), 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> |
| <ul class="simple"> |
| <li><p><strong>Keyspace</strong>: defines how a dataset is replicated, for example in which |
| datacenters and how many copies. Keyspaces contain tables.</p></li> |
| <li><p><strong>Table</strong>: defines the typed schema for a collection of partitions. Cassandra |
| tables have flexible addition of new columns to tables with zero downtime. |
| Tables contain partitions, which contain partitions, which contain columns.</p></li> |
| <li><p><strong>Partition</strong>: defines the mandatory part of the primary key all rows in |
| Cassandra must have. 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 belong to a row.</p></li> |
| </ul> |
| <p>CQL supports numerous advanced features over a partitioned dataset such as:</p> |
| <ul class="simple"> |
| <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> |
| <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> |
| <ul class="simple"> |
| <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 class="section" id="operating"> |
| <h2>Operating<a class="headerlink" href="#operating" title="Permalink to this headline">¶</a></h2> |
| <p>Apache Cassandra configuration settings are configured in the <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></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> |
| <p>Cassandra provides tools for managing a cluster. The <code class="docutils literal notranslate"><span class="pre">nodetool</span></code> command |
| interacts with Cassandra’s live control interface, allowing runtime manipulation |
| of many settings from <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code>. The <code class="docutils literal notranslate"><span class="pre">auditlogviewer</span></code> is used |
| to view the audit logs. The <code class="docutils literal notranslate"><span class="pre">fqltool</span></code> is used to view, replay and compare |
| full query logs. The <code class="docutils literal notranslate"><span class="pre">auditlogviewer</span></code> and <code class="docutils literal notranslate"><span class="pre">fqltool</span></code> are new tools in |
| Apache Cassandra 4.0.</p> |
| <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> |
| <p>Apache Cassandra 4.0 has added several new features including virtual tables. |
| transient replication, audit logging, full query logging, and support for Java |
| 11. Two of these features are experimental: transient replication and Java 11 |
| support.</p> |
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