Navigational database
Encyclopedia
A navigational database is a type of database
characterized by the fact that objects (or records) in it are found primarily by following references from other objects. Traditionally navigational interfaces are procedural, though one could characterize some modern systems like XPath
as being simultaneously navigational and declarative.
Navigational access is traditionally associated with the network model
and hierarchical model
of database
interfaces, and some have even acquired set-oriented features. Navigational techniques use "pointers" and "paths" to navigate among data records (also known as "nodes"). This is in contrast to the relational model
(implemented in relational database
s), which strives to use "declarative" or logic programming
techniques in which you ask the system for what you want instead of how to navigate to it.
For example, to give directions to a house, the navigational approach would resemble something like, "Get on highway 25 for 8 miles, turn onto Horse Road, left at the red barn, then stop at the 3rd house down the road"; whereas, the declarative approach would resemble, "Visit the green house(s) within the following coordinates...."
Hierarchical models are also considered navigational because one "goes" up (to parent), down (to leaves), and there are "paths", such as the familiar file/folder paths in hierarchical file systems. In general, navigational systems will use combinations of paths and prepositions such as "next", "previous", "first", "last", "up", "down", "owner", etc.
"Paths" are often formed by concatenation of node
names or node addresses. Example:
Node6.Node4.Node5.Node1
Or
Node6/Node4/Node5/Node1
If there is no link between given nodes, then an error condition is usually triggered with a message such as "Invalid Path". The path "Node6.Node2.Node1" would be invalid in most systems because there is no direct link between Node 6 and Node 2.
The usage of the term "navigational" allegedly is derived from a statement by Charles Bachman
in which he describes the "programmer as navigator" while accessing his favored type of database. http://portal.acm.org/citation.cfm?id=362534&coll=portal&dl=ACM
Except for hierarchical file systems (which some consider a form of database), navigational techniques fell out of favor by the 1980s. However, object oriented programming and XML
have kindled a renewed, but controversial interest in navigational techniques.
Critics of navigational techniques view them as "unstructured spaghetti messes", and liken them to the "goto" of pre-structured programming
. In other words, they are allegedly to data organization what goto's were to behavior flow. In this view, relational techniques provide improved discipline and consistency to data organization and usage because of its roots in set theory
and predicate calculus.
Some also suggest that navigational database engines are easier to build and take up less memory (RAM) than relational equivalents. However, the existence of relational or relational-based products of the late 1980s that possessed small engines (by today's standards) because they didn't use SQL suggest this is not necessarily the case. Whatever the reason, navigational techniques are still the preferred way to handle smaller-scale structures.
A current example of navigational structuring can be found in the Document Object Model
(DOM) often used in web browsers and closely associated with JavaScript
. The DOM "engine" is essentially a light-weight navigational database. The World Wide Web
itself and Wikipedia could potentially be considered forms of navigational databases, though they focus on human-readable text rather than data (on a large scale, the Web is a network model and on smaller or local scales, such as domain and URL partitioning, it uses hierarchies). In contrast, the Linked Data
facet of the Semantic Web
is specifically concerned with network-scale machine-readable data, and follows precisely the 'follow your nose' paradigm implied by the navigational idea.
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
characterized by the fact that objects (or records) in it are found primarily by following references from other objects. Traditionally navigational interfaces are procedural, though one could characterize some modern systems like XPath
XPath
XPath is a language for selecting nodes from an XML document. In addition, XPath may be used to compute values from the content of an XML document...
as being simultaneously navigational and declarative.
Navigational access is traditionally associated with the network model
Network model
The network model is a database model conceived as a flexible way of representing objects and their relationships. Its distinguishing feature is that the schema, viewed as a graph in which object types are nodes and relationship types are arcs, is not restricted to being a hierarchy or lattice.The...
and hierarchical model
Hierarchical model
A hierarchical database model is a data model in which the data is organized into a tree-like structure. The structure allows representing information using parent/child relationships: each parent can have many children, but each child has only one parent...
of database
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
interfaces, and some have even acquired set-oriented features. Navigational techniques use "pointers" and "paths" to navigate among data records (also known as "nodes"). This is in contrast to the relational model
Relational model
The relational model for database management is a database model based on first-order predicate logic, first formulated and proposed in 1969 by Edgar F...
(implemented in relational database
Relational database
A relational database is a database that conforms to relational model theory. The software used in a relational database is called a relational database management system . Colloquial use of the term "relational database" may refer to the RDBMS software, or the relational database itself...
s), which strives to use "declarative" or logic programming
Logic programming
Logic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...
techniques in which you ask the system for what you want instead of how to navigate to it.
For example, to give directions to a house, the navigational approach would resemble something like, "Get on highway 25 for 8 miles, turn onto Horse Road, left at the red barn, then stop at the 3rd house down the road"; whereas, the declarative approach would resemble, "Visit the green house(s) within the following coordinates...."
Hierarchical models are also considered navigational because one "goes" up (to parent), down (to leaves), and there are "paths", such as the familiar file/folder paths in hierarchical file systems. In general, navigational systems will use combinations of paths and prepositions such as "next", "previous", "first", "last", "up", "down", "owner", etc.
"Paths" are often formed by concatenation of node
Node (computer science)
A node is a record consisting of one or more fields that are links to other nodes, and a data field. The link and data fields are often implemented by pointers or references although it is also quite common for the data to be embedded directly in the node. Nodes are used to build linked, often...
names or node addresses. Example:
Or
Node6/Node4/Node5/Node1
If there is no link between given nodes, then an error condition is usually triggered with a message such as "Invalid Path". The path "Node6.Node2.Node1" would be invalid in most systems because there is no direct link between Node 6 and Node 2.
The usage of the term "navigational" allegedly is derived from a statement by Charles Bachman
Charles Bachman
Charles William "Charlie" Bachman is an American computer scientist, who spent his entire career as an industrial researcher rather than in academia...
in which he describes the "programmer as navigator" while accessing his favored type of database. http://portal.acm.org/citation.cfm?id=362534&coll=portal&dl=ACM
Except for hierarchical file systems (which some consider a form of database), navigational techniques fell out of favor by the 1980s. However, object oriented programming and XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
have kindled a renewed, but controversial interest in navigational techniques.
Critics of navigational techniques view them as "unstructured spaghetti messes", and liken them to the "goto" of pre-structured programming
Structured programming
Structured programming is a programming paradigm aimed on improving the clarity, quality, and development time of a computer program by making extensive use of subroutines, block structures and for and while loops - in contrast to using simple tests and jumps such as the goto statement which could...
. In other words, they are allegedly to data organization what goto's were to behavior flow. In this view, relational techniques provide improved discipline and consistency to data organization and usage because of its roots in set theory
Set theory
Set theory is the branch of mathematics that studies sets, which are collections of objects. Although any type of object can be collected into a set, set theory is applied most often to objects that are relevant to mathematics...
and predicate calculus.
Some also suggest that navigational database engines are easier to build and take up less memory (RAM) than relational equivalents. However, the existence of relational or relational-based products of the late 1980s that possessed small engines (by today's standards) because they didn't use SQL suggest this is not necessarily the case. Whatever the reason, navigational techniques are still the preferred way to handle smaller-scale structures.
A current example of navigational structuring can be found in the Document Object Model
Document Object Model
The Document Object Model is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. Aspects of the DOM may be addressed and manipulated within the syntax of the programming language in use...
(DOM) often used in web browsers and closely associated with JavaScript
JavaScript
JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....
. The DOM "engine" is essentially a light-weight navigational database. The World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
itself and Wikipedia could potentially be considered forms of navigational databases, though they focus on human-readable text rather than data (on a large scale, the Web is a network model and on smaller or local scales, such as domain and URL partitioning, it uses hierarchies). In contrast, the Linked Data
Linked Data
In computing, linked data describes a method of publishing structured data so that it can be interlinked and become more useful. It builds upon standard Web technologies such as HTTP and URIs, but rather than using them to serve web pages for human readers, it extends them to share information in a...
facet of the Semantic Web
Semantic Web
The Semantic Web is a collaborative movement led by the World Wide Web Consortium that promotes common formats for data on the World Wide Web. By encouraging the inclusion of semantic content in web pages, the Semantic Web aims at converting the current web of unstructured documents into a "web of...
is specifically concerned with network-scale machine-readable data, and follows precisely the 'follow your nose' paradigm implied by the navigational idea.