What is a High-Level System Diagram?

A High-Level System Diagram (HLD) is a visual representation of the overall architecture of a system or application. It shows the main components, modules, or services of the system and how they interact with each other, external systems, or users — without going into technical details or implementation specifics.

Purpose:

• To provide a big-picture view of the system.
• To illustrate key components and their relationships.
• To explain how data and control flow between components.
• To communicate system architecture to both technical and non-technical stakeholders.

Typical Audience:

• Product Owners
• Management
• Stakeholders
• Development Teams (as a starting point)

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Common Elements / Controls in a High-Level System Diagram

Here are typical visual elements you’ll find in HLDs, and what they mean:

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1. Boxes (Rectangles) — Components or Modules

• What: Rectangular shapes represent major components, systems, services, or modules.
• Purpose: To visually group and identify parts of the system.

• Example:

  • Web Server
  • Database
  • Authentication Service
  • Third-party API

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2. Arrows — Flow or Interaction

• What: Lines with arrowheads connecting boxes.
• Purpose: Represent direction of data flow, control flow, or communication between components.

• Arrow Types:

  • Solid Arrow: Indicates synchronous or direct communication (e.g., API calls).
  • Dashed Arrow: Indicates asynchronous communication, events, or notifications.
  • Double-headed Arrow: Two-way communication or bidirectional data exchange.

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3. Cloud Shapes — External Systems or Networks

• What: Cloud-like shape to represent external systems or network boundaries.
• Purpose: To indicate systems outside the control or boundary of the system being designed.

• Example:

  • External Payment Gateway
  • User’s Browser or Mobile App
  • Third-party Authentication Provider

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4. Database Symbol (Cylinder)

• What: Cylinder shape represents data storage like databases.
• Purpose: To show where data is persisted or accessed.

• Example:

  • Relational DB
  • NoSQL DB
  • File Storage

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5. Containers / Grouping Boxes

• What: Large boxes or containers that group related components.
• Purpose: Show subsystems or layers within the overall architecture.

• Example:

  • Frontend Layer
  • Backend Services
  • Infrastructure Layer

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6. Annotations / Text

• What: Notes, labels, or descriptions attached to components or arrows.
• Purpose: Provide extra context, such as protocols used (HTTP, gRPC), data formats (JSON, XML), or important behaviors.

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Example & Explanation of Controls in a Simple High-Level System Diagram

[User's Browser] ---> (HTTP Request) ---> [Web Server] ---> (Queries) ---> [Database]

          \                                         |
           \---> (API Call) ---> [Authentication Service] --|

• Boxes: Represent User's Browser, Web Server, Database, Authentication Service.
• Solid Arrows: Represent synchronous communication or request/response flows.
• Labels on Arrows: Indicate the type of interaction, like "HTTP Request", "API Call", or "Queries".
• Grouping (not shown here): Could group Web Server + Authentication Service as "Backend Layer".

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Summary Table of Controls in High-Level System Diagrams

Control Type	Visual Representation	Purpose/Meaning
Box (Rectangle)	Simple rectangle	Component, module, service
Arrow (Solid)	Solid line with arrowhead	Synchronous flow / direct call
Arrow (Dashed)	Dashed line with arrowhead	Asynchronous flow / event
Arrow (Double-headed)	Line with arrowheads both ends	Bidirectional communication
Cloud	Cloud shape	External system or boundary
Cylinder (Database)	Cylinder shape	Data storage or database
Grouping Box	Large rectangle grouping others	Subsystem or architectural layer
Text / Annotation	Labels near arrows or boxes	Clarification / protocols / notes