Cache Memory, Web Cache

✅ Cache Memory

memory to decrease loading time
serve as a buffer between the main memory (RAM) and the central processing unit (CPU)

• when CPU reads from RAM, saves frequently used data on cache memory
• next time, bring the data from cache ➡️ save time

✔️ Different levels of cache

• L1: CPU will start looking for data in L1
  
• L2: if not in L1, look for data in L2
  
• L3
  

☑️ Operation of cache

1. when CPU needs to read or write a location in the main memory, check cache
2. find the memory location in cache: cache hit
3. does not find: cache miss
   • cold miss: location was called for the first time
   • conflict miss: when trying to save multiple lines in the same cache, might have to evict some of them to make room for new data
   • capacity miss: cache is too small to save data
4. performance of cache memory is measured in hit ratio

☑️ Cache mapping

1️⃣ Direct Mapping

map each memory block to one possible cache line
if new block is needed, the old block is trashed

• address space = index field ➕ tag field(cache)
• DRAM의 여러 주소가 캐시 메모리의 한 주소에 대응되는 다대일 방식

2️⃣ Associative Mapping

any block can go into any line of cache

• flexible, fastest
• 비어있는 캐시 메모리 아무대나 저장
• 저장은 간단하지만 찾는게 문제

3️⃣ Set-Associative Mapping

enhanced form of direct mapping ➕ Associative Mapping
group a few lines together to create set
block in memory can map to any line of a specific set

• 특정 행을 지정하고 그 행 안의 비어있는 열에 저장

cache 🆚 cookie

• cache: decrease loading time
  
• cookie: trace user preferences
  https://www.geeksforgeeks.org/difference-between-cache-and-cookies/?ref=header_search

✅ Web Cache

save copy of frequently used resource

• if a request requires resource in cache,
• the resource will not be provided by original server ❌ but by cache ⭕️

👍🏻 Benefits of cache

• reduce unnecessary data transmission
• burden of original server ⬇️
• network bottleneck ⬇️
• bandwidth ⬆️
• can react to sudden high traffic
• reduce latency due to distance ⬇️ (get resource from close cache server)

☑️ Cache hit, cache miss, revalidation

✔️ cache hit

• requested data is found in the cache
• fast data retrieval

✔️ cache miss

• data not in cache at time of request
• slower retrival from meain memory, origin server

☑️ cache revalidation

• data in origin server can change

• so cache has to check if the data copy it has is valid(if the copy is same w origin server)

• add to HTTP GET request If-Modified-Since header
• means get me copy only if the origin data was altered
  • if origin data not changed:
  • HTTP response HTTP 304 Not Modified
  • if origin data is different from copy that cache has:
  • HTTP response HTTP 200 OK

When to check cache validation?

• HTTP lets origin server to add expirary limit to resources
• use header such as cache-control, expires
• after this expirary limit, cache has to validate itself
• and if not valid, get the new altered resource with new expirary limit

• cache-control: max age

  • max age a resource can have
  • recorded in seconds
  • example: cache-control: max-age: 484200

• expires: data -expirary date

  • expires: Fri, 05 Jul 2025, 05:00:00 GMT

☑️ Types of Web Cache

✔️ Private cache

• 개인 전용 캐시
• cache allocated to private user
• most web browsers have Private cache
• save frequently used resources in cache in computer disk, memory

✔️ Public proxy cache

• 공용 프락시 캐시
• proxy cache provide resources in local cache
• can access server as user
• several users can access public cache, reducing unnecessary traffic ⬇️

✅ Spring cache

https://soheeparklee.github.io/posts/Spring_cache/