What is Caching?
💡 Concept Name
Caching – The process of storing copies of data in a temporary storage location for faster access.
📘 Quick Intro
Caching improves performance by temporarily storing frequently accessed data in faster storage (e.g., memory), reducing the need to fetch or compute the same data repeatedly.
🧠 Analogy / Short Story
Think of caching like putting your frequently used tools in a desk drawer instead of the garage — much quicker to reach and use them when needed.
🔧 Technical Explanation
- ⚡ Reduces latency and load on databases, APIs, and file systems.
- 🧠 Uses RAM or in-memory data structures for fast retrieval.
- ♻️ Can be local (in-memory), distributed, or persistent.
- 🛠️ Often used with key-value pairs (e.g., Dictionary, Redis).
- 🕒 Requires cache invalidation policies like LRU, TTL.
🎯 Purpose & Use Case
- ✅ Speeding up web APIs and database queries.
- ✅ Storing configuration or frequently queried data.
- ✅ Reducing server load in high-traffic applications.
💻 Real Code Example
// Example using IMemoryCache in .NET
public class DataService
{
private readonly IMemoryCache _cache;
public DataService(IMemoryCache cache)
{
_cache = cache;
}
public string GetUserData(int userId)
{
return _cache.GetOrCreate("User_" + userId, entry =>
{
entry.AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(10);
return GetFromDatabase(userId);
});
}
private string GetFromDatabase(int id) => $"User Data for {id}";
}
❓ Interview Q&A
Q1: What is caching?
A: A way to store and reuse data quickly without fetching it repeatedly from the original source.
Q2: Why use caching?
A: To reduce latency and improve application speed.
Q3: What are some common caching tools?
A: Redis, MemoryCache, NCache, Memcached.
Q4: What is cache invalidation?
A: The process of clearing or updating outdated cache data.
Q5: What are TTL and LRU?
A: TTL: Time-To-Live, LRU: Least Recently Used – both are cache eviction policies.
🧾 Cache Writing Strategies
- Write-Through: Writes to cache and backing store simultaneously. Ensures consistency.
- Write-Around: Writes directly to store, cache is updated only on next read.
- Write-Back (Write-Behind): Writes to cache first, then persists later. Improves write speed, risk of loss.
- Read-Through: Automatically loads data to cache on miss and returns it.
- Refresh-Ahead: Proactively updates cache before it expires.
- Write-Once: Data is cached once and never updated (for immutable data).
🚫 Cache Invalidation
Cache Invalidation is the process of removing or updating outdated data from the cache to ensure the application returns fresh and accurate information.
- 🧹 Manual Invalidation: Developer explicitly clears or updates the cache.
- 🕒 Time-based (TTL): Cached data expires after a set duration (Time To Live).
- ♻️ Event-based: Cache is updated when data source changes (e.g., database triggers).
- 👀 LRU (Least Recently Used): Evicts old or least-accessed entries when space is needed.
- 📉 Stale-while-revalidate: Serves stale content while refreshing cache in background.
- ⚠️ Incorrect or missing invalidation leads to stale data bugs.
📝 MCQs
Q1. What is caching used for?
- Storing logs
- Security
- Faster access to data
- Authorization
Q2. Where is cache usually stored?
- On disk
- On network
- In-memory
- In database
Q3. Which is a distributed cache system?
- LINQ
- ASP.NET
- Redis
- Entity Framework
Q4. What happens when cache expires?
- App crashes
- Data is lost
- Fresh data is retrieved
- Nothing
Q5. What’s a con of caching?
- Slower apps
- More memory
- Can serve stale data
- Security risk
Q6. What is cache invalidation?
- Writing logs
- Restarting server
- Clearing or updating stale data
- Refreshing UI
Q7. Which pattern fits caching best?
- Relational
- Key-value store
- Graph
- Queue
Q8. Which .NET interface supports caching?
- IDatabase
- IMemoryCache
- ICacheable
- ICacheService
Q9. What policy removes oldest data?
- FIFO
- TTL
- LRU
- GC
Q10. Which caching policy is time based?
- LRU
- TTL
- MRU
- CLR
💡 Bonus Insight
While caching boosts performance, always balance freshness vs. speed. Use eviction and invalidation wisely.
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