What is Cache?

Let's learn about Cache

References: jehovah's brunch

What is Cache?

• A space that stores data that has been read once in a temporary space,

  • so that the next time it is read, the result can be returned quickly

    • When the same request comes in multiple times, the Cache Server returns the result directly, reducing the load on the DB

• By utilizing the locality that appears when a program is executed, content that was used from memory or disk is stored and managed in a quickly accessible location so that it can be quickly referenced when needed next time

  • In other words, it utilizes the concept that data that has been used is likely to be used again

    • Data that is likely to be used again is stored in a faster accessible storage!

Usage Patterns

Cache Usage Pattern 1 - Data Cache

• When a Client sends a request to a web server, the web server checks whether data exists in the Cache before fetching data from the DB

  • If data exists in the Cache server,

    • The data is returned to the client from the cache without requesting it from the DB

      • This is called a Cache hit

  • If data does not exist in the Cache server,

    • This is a Cache miss situation, and the data is requested from the DB

      • The DB returns the data the user wants,

      • The Web Server saves the returned data in the cache for future use and then returns it to the client

        • Afterward, when the same request comes in, the data stored in the cache can be returned, resulting in a Cache hit

• This structure is the same as the CDN service that caches Static assets

  • CDN services like Amazon CloudFront cache data from the origin (e.g., Amazon S3) that holds the original data,

  • so that the next time the same request comes in, the result can be returned immediately without requesting the origin data

Cache Usage Pattern 2 - When Simultaneous Writes Occur

• When simultaneous writes occur, write requests can suddenly flood the DB and potentially crash it; in this case, Cache can be utilized

  • The Client sends a write request to the web server,

    • and the web server writes the data to the cache and immediately returns the result

  • Cache typically uses memory, so the speed is quite fast

    • Separately running worker servers

      • fetch data from the cache server,

      • perform the work,

      • and write the results to the DB

    • By doing this, the DB can process transactions sequentially

Terms

• Cache hit

  • When the data you want to reference exists in the cache

• Cache miss

  • When the data you want to reference does not exist in the cache