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{{Short description|Abstract interface to a computer database}}
{{About|the software design pattern|the Microsoft library|Jet Data Access Objects}}

In [[software]], a '''data access object''' ('''DAO''') is a pattern that provides an abstract [[Interface (computer science)|interface]] to some type of [[database]] or other [[Persistence (computer science)|persistence mechanism]]. By mapping application calls to the persistence layer, the DAO provides data operations without exposing database details. This isolation supports the [[single responsibility principle]]. It separates the data access the application needs, in terms of domain-specific objects and data types (the DAO's public interface), from how these needs can be satisfied with a specific [[Database Management System|DBMS]] (the implementation of the DAO).

Although this [[design pattern]] is applicable to most programming languages, most software with persistence needs, and most databases, it is traditionally associated with [[Java Platform, Enterprise Edition|Java EE]] applications and with [[relational databases]] (accessed via the [[JDBC]] API because of its origin in [[Sun Microsystems]]' best practice guidelines<ref>
{{cite web
| title = Core J2EE Patterns - Data Access Objects
| publisher = Sun Microsystems Inc.
| url = http://www.oracle.com/technetwork/java/dataaccessobject-138824.html
| date = 2007-08-02
}}</ref> "Core J2EE Patterns".

This object can be found in the [[Data_access_layer|Data Access layer]] of the [[Multitier_architecture|3-Tier Architecture]].

There are various ways in which this object can be implemented:
* One DAO for each table.
* One DAO for all the tables for a particular DBMS.
* Where the SELECT query is limited only to its target table and cannot incorporate JOINS, UNIONS, subqueries and Common Table Expressions (CTEs)
* Where the SELECT query can contain anything that the DBMS allows.

== Advantages ==

Using data access objects (DAOs) offers a clear advantage: it separates two parts of an application that don't need to know about each other. This separation allows them to evolve independently. If business logic changes, it can rely on a consistent DAO interface. Meanwhile, modifications to persistence logic won't affect DAO clients.<ref>{{Cite web |date=2022-08-03 |title=Data Access Object(DAO) Design Pattern |url=https://www.digitalocean.com/community/tutorials/dao-design-pattern |access-date=2024-07-08 |website=DigitalOcean |language=en-US}}</ref><ref>{{Cite web |date=2017-08-26 |title=Data Access Object(DAO) Design Pattern |url=https://www.geeksforgeeks.org/data-access-object-pattern/#8-advantages-of-the-data-access-object-designdao-pattern |access-date=2024-07-08 |website=GeeksforGeeks |language=en-US}}</ref>

All details of storage are hidden from the rest of the application (see [[information hiding]]). [[Unit testing]] code is facilitated by substituting a [[test double]] for the DAO in the test, thereby making the tests independent of the persistence layer.

In the context of the [[Java (programming language)|Java]] programming language, DAO can be implemented in various ways. This can range from a fairly simple interface that separates data access from the application logic, to frameworks and commercial products.

Technologies like [[Java Persistence API]] and [[Enterprise JavaBeans]] come built into application servers and can be used in applications that use a Java EE application server. Commercial products such as [[TopLink]] are available based on [[object–relational mapping]] (ORM). Popular open source ORM software includes [[Doctrine (PHP)|Doctrine]], [[Hibernate (Java)|Hibernate]], [[iBATIS]] and JPA implementations such as [[Apache OpenJPA]].<ref>{{Cite web |date=2017-08-26 |title=Data Access Object(DAO) Design Pattern |url=https://www.geeksforgeeks.org/data-access-object-pattern/ |access-date=2024-01-29 |website=GeeksforGeeks |language=en-US}}</ref>

== Disadvantages ==
Potential disadvantages of using DAO include [[leaky abstraction]],{{Citation needed|reason=What kind of leak?|date=August 2014}} [[Duplicate code|code duplication]], and [[abstraction inversion]]. In particular, the abstraction of the DAO as a regular Java object can obscure the high cost of each database access. Developers may inadvertently make multiple database queries to retrieve information that could be returned in a single operation. If an application requires multiple DAOs, the same create, read, update, and delete code may have to be written for each DAO.<ref>See http://www.ibm.com/developerworks/java/library/j-genericdao/index.html for workarounds</ref>

Note that these disadvantages only appear when you have a separate DAO for each table and the SELECT query is prevented from accessing anything other than the target table.

== Tools and frameworks ==
* [[ODB (C++)|ODB]] compiler-based [[object–relational mapping]] (ORM) system for C++
* ORMLite: Lightweight [[object–relational mapping]] (ORM) framework in Java for JDBC and Android<ref>{{cite book|last1=Hodgson|first1=Kyle|last2=Reid|first2=Darren|title=ServiceStack 4 Cookbook|publisher=Packt Publishing Ltd|isbn=9781783986576|page=Chapter 4|url=https://books.google.com/books?id=x9hpBgAAQBAJ&q=%22ORMLite%22&pg=PA129|access-date=22 June 2016|language=en|date=2015-01-23}}</ref>
* [[Entity Framework|Microsoft Entity Framework]]
* DBIx::Class [[object–relational mapping]] (ORM) module for [[Perl]]
* TuxORM: Simple [[object–relational mapping]] (ORM) library in Java for JDBC
* [[Persist (Java tool)]] Java-based [[object–relational mapping]] and data access object tool

== See also ==
* [[Create, read, update and delete]] (CRUD)
* [[Data access layer]]
* [[Service Data Objects]]
* [[Object–relational mapping]]

== References ==
{{Reflist}}

{{Design Patterns Patterns}}

[[Category:Architectural pattern (computer science)]]
[[Category:Software design patterns]]

Data access object - Revision history

imported>Toast1454: Reverted edits by 2804:29B8:518C:67A:E2:F2C0:8FBB:8BF7 (talk) (AV)

94.135.171.70: rm whitespace