# Cache Increase the performance of **read** operations. Application server cache: local cache. Example: map, leveldb. High performance but hard to scale. Violates stateless server principle. CDN: static content cache, media/binarries ## Independent cache system * **Redis**: complex data structure. Built-in high availability * **Memcache**: simple key-value, high concurrency ## Cache writing policy write-through: write to cache and db. write-around: discard cache result, write db write-back: only write cache, don't write to DB. not consistent. If the cache content is easy to calculate, use write-through, otherwise, write-around. Cache eviction policies: LRU, LFU, FIFO. ## What to Cache TODO ## Cache Eviction Strategy TODO --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://liuzhenglaichn.gitbook.io/system-design/basics/cache.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.