Symbol Guide & Visual Language
Why Visual Language Matters
Diagrams and animations show how CDNs work. We use one set of symbols and colors in every lesson. So the visuals are easy to read.
Use the gallery above as your main reference. Click any icon to see its name, description, and where it appears in the course. This lesson helps you read our diagrams.
Network Entities
These are the key parts of any CDN setup. See them in action in the gallery above.
| Name | Description | |
|---|---|---|
| End User. | The browser or client that makes HTTP requests. | They can be anywhere in the world. |
| Edge Server. | CDN node at a Point of Presence (PoP) near the user. | It gives fast, low-latency responses. |
| Origin Shield. | A cache layer in front of origin. | It groups requests from all edge servers. |
| Mid-Tier Cache. | A cache layer between edge and shield. | It handles caching for a region. |
| Origin Server. | Your web server or cloud storage. | That is where your content lives. |
| DNS Server. | Turns domain names into CDN edge IP addresses. | It works via GeoDNS or anycast. |
Cache States
When you view cache behavior, these states show freshness:
| State | Meaning | |
|---|---|---|
| HIT. | Served straight from the edge cache. | The fastest path. No origin contact. |
| MISS. | The content is not in cache. | So it must be fetched from origin. Slower path. |
| STALE. | Cached content has expired by now. | It may still be served while it checks again. |
| REVALIDATE. | Checks with origin if cached content is still valid. | This works as a conditional request. |
| BYPASS. | The cache is skipped on purpose. | Use it for authenticated requests or cookies. |
Color Semantics
Colors show status fast in all course visuals:
| Color | Meaning | Example Usage |
|---|---|---|
| Green | Success / Cache HIT | Content served from edge cache |
| Red. | Blocked / Attack. | Security, geoblocking, or rate limits block it. |
| Yellow / Amber. | Warning / Stale. | Stale content. Needs revalidation. |
| Blue. | Neutral / In Progress. | Active request, DNS lookup. |
| Gray | Cache Miss | Not in cache, request goes to origin |
Data Flow Notation
Arrows and lines show how requests move through the CDN:
- Solid arrow (right). A request from client to origin.
- Solid arrow (left). The response flows back to client.
- Dashed arrow. Conditional or optional request path.
- Curved arrow. It redirects to a different URL.
- X mark. Blocked request. Security, geoblocking, rate limiting.
Diagram Types
The course uses a few diagram types:
- Request Flow Diagrams. Path of a single request from user to content. Read left to right.
- Cache Hierarchy Diagrams. Layers of caching (edge, shield, origin). Read top to bottom.
- Geographic Maps. Global spread of PoPs and user routing. Color intensity shows traffic volume.
- Sequence Diagrams. They show the order of operations over time.
- Header Tables. HTTP headers shown with syntax highlighting. They show directives and values too.
Animation Controls
You can play, pause, and restart course animations:
- Play/Pause. This starts or stops the animation.
- Restart. This sends you back to the start.
- Speed control. Slow down (0.5x) or speed up (1.5x, 2x) playback.
- Loop. Some animations play on a loop (set by the author).
Use slower speeds when learning new concepts, and faster speeds for review.
In the course diagrams, what does the color green mean?
Green shows success in the diagrams.
- A cache HIT means the edge serves the content.
- The request never reaches the origin.
- Gray signals a miss, and amber signals stale content.
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