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Entity Header

Entity headers are HTTP headers that provide information about the body of a message. They can appear in both requests and responses, describing properties of the content such as type, length, encoding, or last modification date.

Important Entity Headers:

1. Content-Type

  • Specifies the media type (MIME type) of the content.
  • Example:
Content-Type: application/json; charset=UTF-8

2. Content-Length

  • Indicates the size of the content in bytes.
  • Example:
Content-Length: 1024

3. Content-Encoding

  • Shows if the content has been compressed (e.g., gzip).
  • Example:
Content-Encoding: gzip

4. Content-Language

  • Specifies the language of the content.
  • Example:
Content-Language: de-DE

5. Cache-Location

  • Indicates the URL or storage location of the actual resource.
  • Example:
Content-Location: /files/document.pdf

6. Last-Modified

  • Specifies when the content was last changed.
  • Example:
Last-Modified: Tue, 30 Jan 2025 14:20:00 GMT

7. ETag

  • A unique identifier for a version of the resource, useful for caching.
  • Example:
ETag: "abc123xyz"

8. Expires

  • Indicates when the content should be considered outdated.
  • Example:
Expires: Fri, 02 Feb 2025 12:00:00 GMT

9. Allow

  • Lists the allowed HTTP methods for a resource.
  • Example:
Allow: GET, POST, HEAD

10. Refresh  (Not standardized but often used)

  • Instructs the browser to refresh the page after a specified time.
  • Example:
Refresh: 10; url=https://example.com

These headers help describe the content of an HTTP message, optimize caching strategies, and ensure correct rendering.




Entity

An Entity is a central concept in software development, particularly in Domain-Driven Design (DDD). It refers to an object or data record that has a unique identity and whose state can change over time. The identity of an entity remains constant, regardless of how its attributes change.

Key Characteristics of an Entity:

  1. Unique Identity: Every entity has a unique identifier (e.g., an ID) that distinguishes it from other entities. This identity is the primary distinguishing feature and remains the same throughout the entity’s lifecycle.

  2. Mutable State: Unlike a value object, an entity’s state can change. For example, a customer’s properties (like name or address) may change, but the customer remains the same through its unique identity.

  3. Business Logic: Entities often encapsulate business logic that relates to their behavior and state within the domain.

Example of an Entity:

Consider a Customer entity in an e-commerce system. This entity could have the following attributes:

  • ID: 12345 (the unique identity of the customer)
  • Name: John Doe
  • Address: 123 Main Street, Some City

If the customer’s name or address changes, the entity is still the same customer because of its unique ID. This is the key difference from a Value Object, which does not have a persistent identity.

Entities in Practice:

Entities are often represented as database tables, where the unique identity is stored as a primary key. In an object-oriented programming model, entities are typically represented by a class or object that manages the entity's logic and state.