how a cache works?

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A cache is a high-speed storage layer that temporarily stores frequently accessed data, reducing the time and resources required to retrieve it. Here’s how it works:

1. Basic Concept

  • Purpose: A cache stores a copy of data from a slower, primary storage system (like a database or a hard drive) to provide faster access.
  • Speed Advantage: Caches are usually built using faster storage technologies (e.g., RAM) compared to the main storage.

2. How It Works

  • Request Flow:
    1. Cache Check: When a system requests data, the cache is checked first. This is known as a cache hit.
      • If the requested data is in the cache, it’s retrieved directly (avoiding the slower primary storage).
      • If the data isn’t in the cache (cache miss), it’s retrieved from the primary storage and then stored in the cache for future requests.
    2. Cache Population: On a miss, the cache decides what to store based on caching strategies (see below).

3. Caching Strategies

To determine what data to store and how long to keep it, caches use strategies like:

  • Least Recently Used (LRU): Removes the least recently accessed items when space is needed.
  • First In, First Out (FIFO): Removes the oldest items first.
  • Time-to-Live (TTL): Items are stored for a fixed amount of time, after which they expire.

4. Cache Types

Caches can be used in various layers of a system:

  1. CPU Cache:
    • Built into processors for storing frequently used instructions and data.
    • Multiple levels (L1, L2, L3) provide hierarchical caching close to the CPU.
  2. Disk Cache:
    • Stored on faster storage (like SSDs or RAM) to cache data from slower hard drives.
  3. Web Cache:
    • Stores web resources (like images, HTML pages) on a user’s device or servers to reduce loading times.
  4. Application-Level Cache:
    • Tools like Redis or Memcached store frequently accessed database queries or computations.
  5. Content Delivery Network (CDN):
    • Globally distributed servers cache web content closer to users.

5. Benefits

  • Speed: Dramatically faster data access.
  • Reduced Load: Less strain on primary storage systems.
  • Cost Savings: Reduces bandwidth and computational costs.

6. Drawbacks

  • Stale Data: If the original data changes, the cache may become outdated.
  • Memory Usage: Caches require additional memory or storage.
  • Complexity: Cache management adds complexity to system design.

Example in Action

Suppose you visit a website:

  1. The browser checks its cache for the webpage.
  2. If found (cache hit), it loads instantly.
  3. If not (cache miss), it fetches the data from the web server and saves it in the cache for the next visit.

This reduces future load times and network traffic!

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