| .. Licensed to the Apache Software Foundation (ASF) under one |
| .. or more contributor license agreements. See the NOTICE file |
| .. distributed with this work for additional information |
| .. regarding copyright ownership. The ASF licenses this file |
| .. to you under the Apache License, Version 2.0 (the |
| .. "License"); you may not use this file except in compliance |
| .. with the License. You may obtain a copy of the License at |
| .. |
| .. http://www.apache.org/licenses/LICENSE-2.0 |
| .. |
| .. Unless required by applicable law or agreed to in writing, software |
| .. distributed under the License is distributed on an "AS IS" BASIS, |
| .. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| .. See the License for the specific language governing permissions and |
| .. limitations under the License. |
| |
| .. _overview: |
| |
| Overview |
| ======== |
| |
| Apache Cassandra is an open source, distributed, NoSQL database. It presents |
| a partitioned wide column storage model with eventually consistent semantics. |
| |
| Apache Cassandra was initially designed at `Facebook |
| <https://www.cs.cornell.edu/projects/ladis2009/papers/lakshman-ladis2009.pdf>`_ |
| using a staged event-driven architecture (`SEDA |
| <http://www.sosp.org/2001/papers/welsh.pdf>`_) to implement a combination of |
| Amazon’s `Dynamo |
| <http://courses.cse.tamu.edu/caverlee/csce438/readings/dynamo-paper.pdf>`_ |
| distributed storage and replication techniques combined with Google's `Bigtable |
| <https://static.googleusercontent.com/media/research.google.com/en//archive/bigtable-osdi06.pdf>`_ |
| 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. |
| |
| 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. |
| |
| Systems like Cassandra are designed for these challenges and seek the |
| following design objectives: |
| |
| - Full multi-master database replication |
| - Global availability at low latency |
| - Scaling out on commodity hardware |
| - Linear throughput increase with each additional processor |
| - Online load balancing and cluster growth |
| - Partitioned key-oriented queries |
| - Flexible schema |
| |
| Features |
| -------- |
| |
| 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: |
| |
| - **Keyspace**: defines how a dataset is replicated, for example in which |
| datacenters and how many copies. Keyspaces contain tables. |
| - **Table**: 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. |
| - **Partition**: defines the mandatory part of the primary key all rows in |
| Cassandra must have. All performant queries supply the partition key in |
| the query. |
| - **Row**: contains a collection of columns identified by a unique primary key |
| made up of the partition key and optionally additional clustering keys. |
| - **Column**: A single datum with a type which belong to a row. |
| |
| CQL supports numerous advanced features over a partitioned dataset such as: |
| |
| - Single partition lightweight transactions with atomic compare and set |
| semantics. |
| - User-defined types, functions and aggregates |
| - Collection types including sets, maps, and lists. |
| - Local secondary indices |
| - (Experimental) materialized views |
| |
| 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: |
| |
| - Cross partition transactions |
| - Distributed joins |
| - Foreign keys or referential integrity. |
| |
| Operating |
| --------- |
| |
| Apache Cassandra configuration settings are configured in the ``cassandra.yaml`` |
| 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. |
| |
| Cassandra provides tools for managing a cluster. The ``nodetool`` command |
| interacts with Cassandra's live control interface, allowing runtime manipulation |
| of many settings from ``cassandra.yaml``. The ``auditlogviewer`` is used |
| to view the audit logs. The ``fqltool`` is used to view, replay and compare |
| full query logs. The ``auditlogviewer`` and ``fqltool`` are new tools in |
| Apache Cassandra 4.0. |
| |
| 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. |
| |
| 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. |
