Convention over configuration
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Convention over Configuration (aka Coding by convention) is a software design paradigm which seeks to decrease the number of decisions that developers need to make, gaining simplicity, but not necessarily losing flexibility.
The phrase essentially means a developer only needs to specify unconventional aspects of the application. For example, if there's a class Sale in the model, the corresponding table in the database is called sales by default. It is only if one deviates from this convention, such as calling the table "products_sold", that one needs to write code regarding these names.
When the convention implemented by the tool you are using matches your desired behavior, you enjoy the benefits without having to write configuration files. When your desired behavior deviates from the implemented convention, then you configure your desired behavior.
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[edit] Motivation
Traditionally, frameworks have needed multiple configuration files, each with many settings. These provide information specific to each project, ranging from URLs to mappings between classes and database tables. With the complexity of an application the size and number of those files grows as well.
For example, early versions of the well-known Java persistence mapper Hibernate mapped entities and their fields to the database by describing these relationships in XML files. Most of this information could have been revealed by conventionally mapping class names to the identically named database table and the fields to its columns, respectively. Later versions did away with the XML configuration file and instead employed these very conventions, deviations of which may be overridden through the use of Java annotations.
[edit] Usage
Many modern frameworks use a convention over configuration approach. A few such frameworks include: Spring; Ruby on Rails; Kohana PHP; Grails; Grok; Zend Framework; CakePHP; symfony; Maven.[citation needed]
The concept is older, however, and can be spotted even in the roots of Java libraries; for example, the JavaBean specification relies on it heavily. To quote the JavaBeans specification 1.01 [1]: As a general rule we don’t want to invent an enormous java.beans.Everything class that people have to inherit from. Instead we’d like the JavaBeans runtimes to provide default behaviour for “normal” objects, but to allow objects to override a given piece of default behaviour by inheriting from some specific java.beans.something interface.
