| Title: 4.1 - Schema |
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| # 4.1 - Schema |
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| A **Schema** is a set of elements that are used by the **LDAP Server** to describe the data that are stored. The top two elements are **ObjectClass** and **AttributeType**. An **ObjectClass** is a container, describing what an **LDAP** entry can contain, while an **AttributeType** describe each singe element stored in this container. |
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| The key here is that an **LDAP** entry uses **ObjectClasses** to list all the possible **AttributeTypes** that an entry can contain, some of them being mandatory, some other optional. |
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| Now, there is more. We have many other **Schema Objects** that are colateral: |
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| * **LdapComparator** (_ApacheDS_ specific) |
| * **DitContentRule** |
| * **DitStructureRule** |
| * **MatchingRule** |
| * **MatchingRuleUse** |
| * **NameForm** |
| * **Normalizer** (_ApacheDS_ specific) |
| * **Syntax** |
| * **SyntaxChecker** (_ApacheDS_ specific) |
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| Most of those elements are defined in [RFC 4512](https://tools.ietf.org/html/rfc4512), we have added three others (**LDdapComparator**, **Normalizer** and **SyntaxChecker**) in order to make the server easy to extend by adding some new ones. |
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| Those **SchemaObjects** are used to refine the relation between elements, or define their syntax, or some other constraints. The most important ones are **LdapComparator**, **MatchingRule**, **Normalizer**, **Syntax** and **SyntaxChecker**. All of them have dependencies, exposed in this picture: |
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