metadata

Metadata means data about other data.

It’s extra information that describes something, rather than being the main content itself.

Simple example

Suppose you have a photo:

Photo = the actual data

Its metadata could be:

filename: vacation.jpg
size: 3.2 MB
type: image/jpeg
created_at: 2026-09-11
width: 1920
height: 1080

The photo is the main data. The information about the photo is metadata.

---

In your backend

Suppose you have an audit log:

{
  "action": "UPDATE_USER_ROLE",
  "resource_id": 87,
  "metadata": {
    "oldRole": "USER",
    "newRole": "ADMIN"
  }
}

Here:

action       → what happened
resource_id  → what was affected
metadata     → additional details about what happened

The metadata gives you information that isn’t worth creating a separate column for every possible detail.

For example, another audit event could have:

{
  "action": "CHANGE_TASK_STATUS",
  "resource_id": 15,
  "metadata": {
    "oldStatus": "TODO",
    "newStatus": "DONE"
  }
}

Different actions can have different metadata.

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Metadata appears everywhere

HTTP

A response:

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 1500

The actual response body is the data:

{
  "name": "Reyhan"
}

While headers provide metadata about the response:

Content-Type
Content-Length
Cache-Control

---

Database

A database record:

User:
    id: 42
    username: reyhan

Metadata might include:

created_at
updated_at
created_by

These describe the record rather than being the core user information.

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Logs

{
  "level": "ERROR",
  "message": "Database connection failed",
  "metadata": {
    "service": "user-service",
    "requestId": "abc123",
    "database": "postgres",
    "retryCount": 3
  }
}

The main data is:

"Database connection failed"

The metadata tells you where, when, and in what context it happened.

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The easiest definition

Data = the thing.
Metadata = information about the thing.

For example:

Book
├── Data: the actual text of the book
└── Metadata:
    ├── title
    ├── author
    ├── publication date
    ├── page count
    └── ISBN

This concept is useful throughout backend development because you’ll see metadata in HTTP, databases, APIs, logs, files, authentication tokens, and audit logs.

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Back-End My-Journey-In-Codeless