About
An entity is one thing which can be distinctly identified.
Example:
- A specific person
- A specific company
- A specific group of persons,
- A specific place,
- A specific event
It's a concept to abstractly group similar things.
An entity is a single occurrence of an entity set, it's also known as an instance
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Example of entity implementation by domain
An entity would be represented:
- In a database model, by a row where the Primary Key identifies a single instance
- in Mathematics, by a tuple.
- in Statistics, by a unit, an observation
- in Olap, by a member
- in a graph/network by a Node
- In code, by a a class (or struct).
- in a file system, by a file
Properties
Attribute
An attribute is a descriptive property or characteristic of an entity.
Value
The value about an entity is expressed by a set of attribute-value pairs
<math> \begin{array}{rrr} att1 & \mapsto & Foo \\ att2 & \mapsto & Blue \\ att3 & \mapsto & 20 \\ att4 & \mapsto & Sunny \\ \end{array} </math>
id
An id is attribute that describes the identity of an entity.
Relationship
Entities are associated via relationship
Type
Permanent
A permanent entity is always part of the system, never entering or leaving.
Temporary
A temporary entity is not always present in a system. It enters the system at some point in time, stays for a while, and then leaves again at some other point in time.
A transaction, a customer in a restaurant is an example of a temporary entity.
There is a constant flow of temporary entity.
Active
An active entity (also known as dynamic model components) are entities that follow a process lifecycle (hence active and not passive).
Dimension are generally not an active component whereas a transaction is.
Communication
Asynchronous communication between entities is based on signal and reception