System design vol.2

1. systerm design basic

Placing a cache directly on a request layer node enables the local storage of

response data. Each time a request is made to the service, the node will

quickly return local cached data if it exists. If it is not in the cache, the

requesting node will query the data from disk. The cache on one request

layer node could also be located both in memory (which is very fast) and on

the node’s local disk (faster than going to network storage).

What happens when you expand this to many nodes? If the request layer is

expanded to multiple nodes, it’s still quite possible to have each node host

its own cache. However, if your load balancer randomly distributes requests

across the nodes, the same request will go to different nodes, thus increasing

cache misses.

Two choices for overcoming this hurdle are global caches and
distributed caches.

即使用全局缓存，或分布式缓存。e.g. redis memeche

CDNs are a kind of cache that comes into play for sites serving large amounts

of static media. In a typical CDN setup, a request will first ask the CDN for a

piece of static media; the CDN will serve that content if it has it locally

available. If it isn’t available, the CDN will query the back-end servers for the

file, cache it locally, and serve it to the requesting user.

If the system we are building isn’t yet large enough to have its own CDN, we

can ease a future transition by serving the static media off a separate

subdomain (e.g. static.yourservice.com) using a lightweight HTTP server like

Nginx, and cut-over the DNS from your servers to a CDN later.

Write-through cache:

Write-around cache:

Write-back cache: