Discuss the main characteristics of NOSQL systems in the area related to data models and query languages.? Discuss the main characteristics of NOSQL systems in the area related to distributed s

NoSQL Systems for Big Data Management

Venkat N Gudivada

Weisburg Division of Computer Science

Marshall University

Huntington, WV, USA

[email protected]

Dhana Rao

Biological Sciences Department

Marshall University

Huntington, WV, USA

[email protected]

Vijay V. Raghavan

Center for Advanced Computer Studies

University of Louisiana at Lafayette

Lafayette, LA, USA

[email protected]

Abstract—The advent of Big Data created a need for out-of-the-box horizontal scalability for data management systems. This ushered in an array of choices for Big Data management under the umbrella term NoSQL. In this paper, we provide a taxonomy and unified perspective on NoSQL systems. Using this perspective, we compare and contrast various NoSQL systems using multiple facets including system architecture, data model, query language, client API, scalability, and availability. We group current NoSQL systems into seven broad categories: Key-Value, Table-type/Column, Document, Graph, Native XML, Native Object, and Hybrid databases. We also describe application scenarios for each category to help the reader in choosing an appropriate NoSQL system for a given application. We conclude the paper by indicating future research direc- tions.

Keywords-Data Models, NoSQL, NewSQL, Big Data, Graph Databases, Document Databases, Native XML Databases

I. Introduction

Until recently, relational database management sys- tems (RDBMS) were the mainstay for managing all types of data irrespective of their naturally fit to the relational data model. The emergence of Big Data and mobile computing necessitated new database functionality to support applications such as real-time logfile analysis, cross-selling in eCommerce, location based services, and micro-blogging. Many of these applications exhibit a preponderance of insert and retrieve operations on a very large scale. Relational databases were found to be inadequate in handling the scale and varied structure of data. The above requirements ushered in an array of

choices for Big Data management under the umbrella term NoSQL [1], [2], [3]. NoSQL (meaning ‘not only SQL’) has come to describe a large class of databases which do not have properties of traditional relational databases and are generally not queried with SQL (structured query language). NoSQL systems provide data partitioning and replication as built-in features. They typically run on cluster computers made from commodity hardware and provide horizontal scalability. Developing applications using NoSQL systems is

quite different from the process used with RDBMS [4].

NoSQL databases require developer-centric approach from the application inception to completion. For exam- ple, data modeling is done by the application architect or developer, whereas in RDBMS based applications, data architects and data modelers complete the data modeling tasks. They begin by constructing conceptual and logical data models. Database transactions and queries come into play only in the design of a phys- ical database. In contrast, NoSQL database approaches begin by identifying application queries and structure the data model to efficiently support these queries. In other words, there is a strong coupling between the data model and application queries. Any changes to the queries will necessitate changes to the data model. This approach is in stark contrast to the time-tested RDBMS principles of logical and physical data independence. Traditionally, data is viewed as a strategic and shared

corporate resource with well established policies and procedures for data governance and quality control. In contrast, NoSQL systems promote data silos, each silo geared towards meeting the performance and scalability requirements of just one or more applications. This runs against the ethos of enterprise data integration, redundancy control, and integrity constraints [5]. Given the above backdrop, following are the contribu-

tions of this paper. We provide a taxonomy and unified perspective to help the reader understand the func- tionality, strengths and limitations of NoSQL databases. Using this perspective, we compare and contrast vari- ous NoSQL systems using multiple facets which include system architecture, data model, query language, client API, scalability, and availability. We group the current NoSQL systems into seven broad categories: Key-Value, Table-type/Columnar, Document, Graph, Native XML, Native Object, and Hybrid databases. Members of these categories are not always disjoint, with some systems falling into more than one category. We also describe application scenarios for each category to help the reader choose an appropriate NoSQL system for a given application. We conclude the paper by indicating future research direction. We begin with a discussion of the principal features

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of NoSQL systems (section II). In section III, we describe concepts that are essential to understanding NoSQL systems from both a technical standpoint and business perspective. Sections IV through X describe the classes of NoSQL systems along with their application scenar- ios. Section XI concludes the paper.

II. Principal Features of NoSQL

The need for NoSQL database solutions primarily emanated from the requirements of eCommerce, mobile computing and location based services, Web services, and social media applications. eCommerce applications require predictive analytics, personalization, dynamic pricing, superior customer service, fraud and anomaly detection, real-time order status through supply chain visibility, and Web server access log analysis. All of the above functions are characterized by massive data which need to be fused from multiple sources and processed in real-time.

A. Big Data Characteristics

Data that is too big and complex to capture, store, process, analyze, and interpret using the state-of-the- art tools and methods is referred to as Big Data. It is characterized by five Vs: Volume, Velocity, Variety, Value, and Veracity. Data size (Volume) is in the order of terabytes or even petabytes, and is rapidly heading towards exabytes. Velocity refers to the speed at which the data is generated. Enterprise Big Data is typically heterogeneous (Variety) and is comprised of structured, semi-structured, and unstructured data. Value refers to extracting useful and actionable information from massive data sets. Due to diversity in sources and evo- lution of data through intermediate processing, issues of security, privacy, trust, and accountability need to be addressed using secure data provenance (Veracity).

B. Flexible Data Models

All RDBMS databases are based on the relational data model. In contrast, each class of NoSQL system employs a different data model. NoSQL data models inherently support horizontal data partitioning across processors in a cluster or even processors across data centers. RDBMS applications typically work with fixed

database schema, though the schema may evolve slowly. Schema change is a well managed activity and the application goes through rigorous software testing. In contrast, many NoSQL systems treat schema as a con- struct that inherently evolves over time. These systems also require schema that allows great variation in row data without incurring the NULL value problems. A NoSQL data model closely reflects an individual

application need rather than needs of several appli- cations. The data model naturally mirrors the way a

subject matter expert (SME) thinks about the data in the domain in the context of a specific application. In other words, a NoSQL application works with a data model that is specifically designed and optimized for that application.

C. Eventual Consistency

Create/Insert, Read, Update, and Delete (CRUD) are four operations that databases are expected to provide and execute efficiently. NoSQL data models are struc- tured to support massively concurrent insert and read operations relative to updates. The notion of eventual consistency is embraced to support extremely fast insert and read operations. Eventual consistency implies that applications are aware of the non-repeatable read issue due to latency in consistency. Some NoSQL systems employ data models to efficiently support only insert and read to the exclusion of update and delete.

D. Partial Record Updates

RDBMS data models are optimized for row-wise pro- cessing based on the assumption that applications process most of an entity’s attributes. In contrast, some NoSQL data models are designed to efficiently perform column-wise processing to support computing aggregates on one or two attributes across all entities. For example, data models of column-oriented NoSQL databases resemble a data warehouse star schema where the fact table at the center stores de-normalized row data and each dimension table stores all columns of a column-family.

E. Application Synthesized Relationships

RDBMS data models are designed to capture one-to- one, one-to-many, and many-to-many relationships be- tween entities. In contrast, many NoSQL data models do not explicitly and inherently model relationships (graph data model based NoSQL systems are an exception). Most NoSQL systems do not support relational join operation. NoSQL applications are expected to synthe- size relationships by processing the data. If the data is inherently rich in relationships, many NoSQL data models are a poor fit.

F. Optimized MapReduce Processing

In RDBMS applications, employing MapReduce pro- cessing requires moving the data from the RDBMS to another memory address space within the same ma- chine or to a different one. In contrast, some NoSQL systems feature MapReduce as a native functionality to obviate the data movement.

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G. Native Support for Versioning

Mainstream, off-the-shelf RDBMS do not provide sup- port for data versioning. However, an RDBMS data model can be designed to support versioning of an attribute by including two additional attributes (begin date and end date). This is a convoluted approach with strong performance penalties. In contrast, some NoSQL data models are designed to provide native support for versioning.

H. New Query Languages

RDBMS SQL queries in practice tend to be complex, involve joins across multiple tables, consume substan- tial database resources to execute, and are expensive to write, debug, and maintain. In contrast, many NoSQL systems avoid SQL altogether. Some provide a SQL-like query language such as XQuery, while others require writing programs for querying the database. NoSQL databases take a multi-faceted approach

to querying including allowing only simple queries through interfaces such as REST API, caching data in main memory, or using database stored procedures as queries. For example, in document-oriented NoSQL sys- tems, a database query is a JSON document (technically, a JavaScript expression) specified across all the docu- ments in the collection. For graph model based NoSQL systems, queries involve traversing graph structures. Interfaces for graph traversal include JavaScript-based interface (JIG), and special query languages such as SPARQL, RDFS++, and Cypher.

I. Horizontal Scalability

RDBMS for Big Data applications are forced to use distributed storage. Reasons for this include data that exceeds the disk size limits, need for partitioned table storage, recovery from hard disk failures through data redundancy, and data replication to enable high avail- ability. A typical RDBMS query requires join operations across multiple tables. How fast a query is processed is limited by how quickly data can be moved from hard disks to primary storage, and the amount of data moved. Database update operations are usually run as database transactions. If the table data is partitioned, update operations run slowly due to contention for exclusive locks. Securing and releasing write locks, coordinating this

across various disks, and ACID (atomicity, consistency, isolation, and durability) compliance slows down trans- action throughput. Long running transactions exacer- bate the transaction throughput further. Vertical scaling is often proposed as a solution to the transaction throughput problem, which involves adding more CPUs, more cores per CPU, and additional main memory.

However, vertical scaling quickly reaches its satura- tion point. Other techniques for improving transaction throughput include relaxing ACID compliance and using methods such as disabling database logging.

Horizontal scalability, in contrast, refers to scaling by adding new processors complete with their own disks (aka nodes). Closely associated with horizontal scaling is the partitioning of data. Each node/processor contains only a subset of the data. The process of assigning data to each node is referred to as sharding, which is done automatically in some NoSQL systems. It is easier to achieve horizontal scalability with

NoSQL databases. The complexity involved in enforc- ing ACID compliance simply does not exist for most NoSQL systems as they are ACID non-compliant by design. Furthermore, only insert and read operations dominate in NoSQL databases as update and delete operations fade into insignificance in terms of volume. NoSQL systems also achieve horizontal scalability by delegating two-phase commit required for transaction implementation to applications. For example, MongoDB does not guarantee ACID compliance for concurrent operations. This may infrequently result in undesirable outcomes such as phantom reads.

III. NoSQL Concepts

NoSQL databases draw upon several concepts and techniques to realize flexible data modeling and associ- ated query languages, and horizontal scalability. These concepts include but are not limited to shared-nothing architecture, Hash trees, consistent hashing, REST API, Protocol buffers (Protobuf), Apache Thrift, JSON and BSON, BASE, MapReduce, vector clocks, column family, keyspace, memory-mapped files, and the CAP theorem.

A. Shared-Nothing Architecture

In this architecture, each node is self-sufficient and acts independently to remove single point of resource contention. Nodes share neither memory nor disk stor- age. Database systems based on shared-nothing ar- chitecture can scale almost linearly by adding new nodes assembled using inexpensive commodity hard- ware components. Data is distributed across the nodes in a non-overlapping manner, which is called sharding. Though this concept existed for long with its roots in distributed databases, it gained prominence with the advent of NoSQL systems.

B. Hash Trees and Consistent Hashing

A hash tree (aka Merkle tree) is a tree in which every non-leaf node is labeled with the hash of the labels of its child nodes. Hash trees enable efficient and secure verification of data transmitted between computers for

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veracity. They are also used to efficiently determine the differences between a document and its copy. In traditional hashing, a change in the number of

slots in the hash table results in nearly all keys remapped to new slots. If K is the number of keys and n is the number of slots, consistent hashing guarantees that on average no more than K/n keys are remapped to new slots.

C. REST API, Protobuf and Apache Thrift

Representational State Transfer (REST) is an HTTP API. It uses four HTTP methods – GET, POST, PUT and DELETE – to execute different operations. Many NoSQL systems enable client interaction through this API. Protocol buffers (Protobuf) is a method for efficiently

serializing and transmitting structured data. It is used to perform remote procedure calls (RPC) as well as for storing data. Thrift is a software framework and an interface definition language (IDL) for cross-language services and API development.

D. JSON and BSON

JavaScript Object Notation (JSON) is a lightweight, text-based, open standard format for exchanging data between a server and a Web application. Though it is originally derived from the JavaScript language, it is a language-neutral data format. BSON is a format for binary-coded serialization of JSON-like documents. Compared to Protobuf, BSON is more flexible with schema but not as space efficient.

E. BASE Properties

In RDBMS, the consistency property ensures that all transactions transform a database from one valid state to another. Once a transaction updates a database item, all database clients (i.e., programs and users) will see the same value for the updated item. ACID properties are to RDBMS as BASE is to NoSQL

systems. BASE refers to basic availability, soft state, and eventual consistency. Basic availability implies discon- nected client operation and delayed synchronization, and tolerance to temporary inconsistency and its impli- cations. Soft state refers to state change without input, which is required for eventual consistency. Eventual consistency means that if no further updates are made to an updated database item for long enough period of time, all clients will see the same value for the updated item.

F. Minimizing Data Movement with MapReduce

MapReduce is a computational paradigm for pro- cessing massive datasets in parallel if the computation fits a pattern characterized by three steps: map, shard and reduce. The map process involves several parallel

map processes concurrently processing different parts of data and each process produces (key, value) pairs. The shard process (second step) acts as a barrier to ensure that all mapper processes have completed their work, collects the generated key-value pairs from each mapper process, sorts them, and partitions the sorted key-value pairs. Next, the shard process assigns each partition to a different reduce process (third step). Each reduce process essentially receives all related key-value pairs and produces one result.

G. Vector Clocks

A vector clock is an algorithm to reason about events based on event timestamps. It is an extension of multi- version concurrency control (MVCC) used in RDBMS to multiple servers. Each server keeps its copy of vector clock. When servers send and receive messages among themselves, vector clocks are incremented and attached with messages. A partial order relationship is defined based on server vector clocks, and is used to derive causal relationships between database item updates.

H. Column Families and Keyspaces

A column family is a collection of rows, and each row can contain different number of columns. Row keys are unique within a column family, but can be reused in other column families. This enables storing unrelated data about the same key in different column families. Some NoSQL systems use the term column-family to refer to a group of related columns within a row. A keyspace is a data container. It is similar to the

schema concept in RDBMS. Keyspaces are used to group column families together. Data replication is specified at the keyspace level. Therefore, data with different replication requirements reside in separate keyspaces.

I. Memory-Mapped Files

A memory-mapped file is a segment of virtual mem- ory which is associated with an operating system file or file-like resource (e.g., a device, shared memory) on a byte-for-byte correspondence. Memory-mapped files increase I/O performance especially for large files.

J. CAP Theorem

Consistency, availability, and partition tolerance (CAP) are the three primary concerns that determine which data management system is suitable for a given application. Consistency feature guarantees that all clients of a data management system have the same view of data. Availability assures that all clients can al- ways read and write. Finally, partition tolerance ensures that the system works well with data that is distributed across physical network partitions. The CAP theorem states that it is impossible for any data management

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system to achieve all these three features at the same time. For example, to achieve partition tolerance, a system may need to give up consistency or availability.

IV. Key-Value Databases

The defining characteristics of key-value databases include real-time processing of Big Data, horizontal scalability across nodes in a cluster or data centers, reliability and high availability. Their use cases include applications where response is needed in milliseconds. They are used for session management for Web appli- cations; configuration management; distributed locks; messaging; personalization of user experience and pro- viding engaging user interactions in social media, mo- bile platforms, and Internet gaming; real-time bidding and online trading; Ad servers; recommendation en- gines; and multi-channel retailing and eCommerce. As the name suggests, these systems store data as

key-value pairs or maps (aka dictionaries). Though all of them store data as maps, they differ widely in func- tionality and performance. Some systems store data ordered on the key, while others do not. Some keep entire data in memory and while others persist data to disk. Data can also be distributed across a cluster of nodes. Representative systems in this category include

Memcached, Aerospike, Redis, Riak, Kyoto Cabinet, Membase, Amazon DynamoDB, CouchDB, BerkeleyDB, EHCache, Apache Cassandra [6], and Voldermot. Ta- ble I summarizes characteristics of some key-value databases. A hallmark of key-value database is rapid applica-

tions development and extremely fast response times even with commodity type processors. For example, 100K – 200K simple write/read operations have been achieved with an Intel Core 2 Duo, 2.6 GHz processor. In another case, replacement of a RDBMS with Redis for user profile management (100K+ profiles with over 10 filter attributes) was reduced from 3 – 6 sec to 50 ms.

V. Table-type/Column Databases

RDBMS are row-based systems as their processing is row-centric. They are designed to efficiently return entire rows of data. Rows are uniquely identified by system generated row ids. As the name implies, column databases are column-centric. Conceptually, a columnar database is like an RDBMS with an index on every column without incurring the overhead associated with the latter. It is also useful to think of column databases as nested key-value systems. Column database applications are characterized by

tolerance to temporary inconsistency, need for ver- sioning, flexible database schema, sparse data, partial

record access, and high speed insert and read opera- tions. When a value changes, it is stored as a different version of the same value using a timestamp. In other words, the notion of update is effectively nonexistent. Partial record access contributes to dramatic perfor- mance improvements for certain applications. Colum- nar databases perform aggregate operations such as computing maxima, minima, average, and sum on large datasets with extreme efficiency. Recall that a column family is a set of related

columns. Column databases require predefining col- umn families, and not columns. A column family may contain any number of columns of any type of data, as long as the latter can be persisted as byte arrays. Columns in a family are logically related to each other, and are physically stored together. Performance gain is achieved by grouping columns with similar access characteristics into the same family. Database schema evolution is achieved by adding columns to column families. A column family is similar to the column concept in RDBMS. Systems in this category include Google BigTable

(available through Google App Engine), Apache Cassan- dra, Apache HBase, Hypertable, Cloudata, Oracle RDBMS Columnar Expression, Microsoft SQL Server 2012 En- terprise Edition. Table II summarizes characteristics of some columnar databases.

VI. Graph Databases

A graph data model is at the heart of graph databases [7]. In some applications, relationships between objects is even more important than the objects themselves. Relationships can be static or dynamic. Such data is called connected data. Twitter, Facebook, Google, and LinkedIn data are naturally modeled using graphs. In addition, graph data models are used in other indus- tries including airlines, freight companies, healthcare, retail, gaming, oil and gas. Graph databases are also popular for implementing access control and authoriza- tion subsystems for applications that serve millions of end users. Graph databases include FlockDB, InfiniteGraph, Ti-

tan, Microsoft Trinity, HyperGraphDB, AllegroGraph, Affinity, OrientDB, DEX, Facebook Open Graph, Google Knowledge Graph, and Neo4J. Table III summarizes characteristics of two graph databases.

VII. Document Databases

These databases are not the document/full-text databases in the traditional sense. They are also not content management systems. Document databases are used to manage semi-structured data mostly in the form of key-value pairs packaged as JSON documents.

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Table I Key-Value Databases

Name Salient Characteristics

Memcached Shared-nothing architecture, in-memory object caching systems with no disk persistence. Automatic sharding but no replication. Client libraries for popular programming languages including Java, .Net, PHP, Python, and Ruby.

Aerospike Shared-nothing architecture, in-memory database with disk persistence. Automatic data partitioning and synchronous replication. Data structures support for string, integer, BLOB, map, and list. ACID with relax option, backup and recovery, high availability. Cross data center replication. Client access through Java, Lua, and REST.

Cassandra Shared-nothing, master-master architecture, in-memory database with disk persistence. Key range based automatics data partitioning. Synchronous and asynchronous replication across multiple data centers. High availability. Client interfaces include Cassandra Query Language (CQL), Thrift, and MapReduce. Largest known Cassandra cluster has over 300 TB of data in over 400-node cluster.

Redis Shared-nothing architecture, in-memory database with disk persistence, ACID transactions. Supports several data structures including sets, sorted sets, hashes, strings, and blocking queues. Backup and recovery. High availability. Client interface through C and Lua.

Riak Shared-nothing architecture, in-memory database with disk persistence, data teated as BLOBs, automatic data partitioning, eventually consistency, backup and recovery, and high availability through multi data center replication. Client API includes Erlang, JavaScript, MapReduce queries, full text search, and REST.

Voldemort Shared-nothing architecture, in-memory database with disk persistence, automatic data partitioning and replication, versioning, map and list data structures, ACID with relax option, backup and recovery, high availability. Protocol Buffers, Thrift, Avro and Java serialization options. Client access through Java API.

Table II Column Databases

Name Salient Characteristics

BigTable A sparse, persistent, distributed, multi-dimensional sorted map. Features strict consistency and runs on distributed commodity clusters. Ideal for data that is in the order of billions of rows with millions of columns.

HBase Open Source Java implementation of BigTable. No data types, and everything is a byte array. Client access tools: shell (i.e., command line), Java API, Thrift, REST, and Avo (binary protocol). Row keys are typically 64-bytes long. Rows are byte-ordered by their row keys. Uses distributed deployment model. Works with Hadoop Distributed File System (HDFS), but uses filesystem API to avoid strong coupling. HBase can also be used with CloudStore.

Cassandra Provides eventual consistency. Client interfaces: phpcasa (a PHP wrapper), Pycassa (Python binding), command line/shell, Thrift, and Cassandra Query Language (CQL). Popular for developing financial services applications.

Table III Graph Databases

Name Salient Characteristics

Neo4J In-memory or in-memory with persistence. Full support for transactions. Nodes/vertices in the graph are described using properties and the relationships between nodes are typed and relationsh

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