# Core Building Blocks — 12 Fundamental Concepts (Part 6)

## The 12 Concept are :

Requirements Clarification (Part 1)

Back-of-Envelope Estimation (Part 1)

Databases — SQL vs NoSQL (Part 2)

Caching (Part 2)

Load Balancing (Part 3)

API Design (Part 3)

Message Queues + Event-Driven Systems (Part 4)

Consistency vs Availability (CAP Theorem) (Part 4)

Scaling Strategies (Part 5)

Fault Tolerance + Reliability (Part 5)

.CDN (Content Delivery Network) (Part 6)

Data Partitioning (Sharding) (Part 6)

But in this article we cover only two concept

# 11\. CDN (Content Delivery Network)

**What it is:** A globally distributed network of servers that cache content close to users.

**WHY it exists:** Network latency increases with physical distance. A CDN in Singapore serves Singapore users faster than a server in Virginia.

**What to put on CDN:**

*   Static assets (JS, CSS, images)
    
*   Video content
    
*   Large downloadable files
    

**What NOT to put on CDN:**

*   Personalized dynamic content
    
*   Real-time data
    
*   Private user data (security risk)
    

**Push vs Pull CDN:**

*   Push: You upload content to CDN proactively. Good for known, large files.
    
*   Pull: CDN fetches from origin on first miss, caches for subsequent requests. Good for unpredictable access.
    

* * *

# 12\. Data Partitioning (Sharding)

**What it is:** Splitting a large dataset across multiple database nodes.

**WHY it exists:** A single DB node has limited storage and throughput. Sharding breaks that limit.

**Sharding strategies:**

| Strategy | How | Good | Bad |
| --- | --- | --- | --- |
| Range-based | Shard by value range (A-M, N-Z) | Simple | Hot spots if distribution is skewed |
| Hash-based | Hash(key) % num\_shards | Even distribution | Range queries become scatter-gather |
| Directory-based | Lookup table maps key to shard | Flexible | Lookup table is a bottleneck |

**Shard key selection rule:** The key should distribute data evenly AND be present in most queries.

**Problems with sharding:**

*   Cross-shard joins are very hard
    
*   Cross-shard transactions even harder
    
*   Resharding (adding shards) is painful
    

**Beginner mistake:** Choosing user\_id as shard key when most queries are by timestamp. Your shard key must match your query patterns.