# Caching Layers from Browser to Database — Caching Strategies & CDN Design

Source: https://www.skillbyai.com/en/caching-strategies/f-layers

> Name the cache layers in a typical web system and what each is good for.

## Caches at every hop

A request from a user can be answered by caches at many layers. The **browser cache** stores responses on the user's device according to HTTP headers; nothing is faster than not making a request. A **CDN** (content delivery network) caches responses at **edge locations** near users worldwide. A **reverse proxy** in front of your servers (NGINX, Varnish) can cache whole pages or API responses. Inside the application, an **in-process cache** (a hash map with eviction, such as Caffeine in Java) answers in nanoseconds but is per instance, while a **distributed cache** such as Redis or Memcached is shared by all instances. Finally, **databases** cache data pages and query plans in memory, and some workloads use **materialised views**. Each outer layer serves more users with less effort but is harder to invalidate precisely; inner layers are easier to control but every request still travels further.

## Who answers the request?

The earlier a layer can answer, the less work everything behind it does.

```text
browser cache     -> 0 network round trips; per user; controlled by Cache-Control / ETag
CDN edge          -> nearest PoP; shared by everyone in a region; purge by URL or tag
reverse proxy     -> in your datacentre; full pages / API responses
app in-process    -> nanoseconds; per instance; may diverge between instances
distributed cache -> ~sub-millisecond; shared by all instances; Redis / Memcached
database buffers  -> automatic; hot pages and indexes in RAM
```

## Groceries at home, the corner shop and the wholesale market

Your fridge (browser) is instant, the corner shop (CDN edge) is a short walk, the supermarket (distributed cache) is a drive, and the wholesale market (database) is far away. You only drive to the market for what the closer places do not stock.

**Quiz:** Which cache layer is shared by all application instances but sits inside your own infrastructure?

- [ ] The browser cache
- [ ] An in-process hash map
- [ ] The CPU L1 cache
- [x] A distributed cache such as Redis

*Answer:* A distributed cache such as Redis. A distributed cache is shared across instances, unlike per-process caches.