[throwaway4good]

I am sure you have good reasons for your concrete design but in the general case: Why not simply build the caching into your backend services rather than having a proxy do it based on the specifics of http protocol? It would be simpler and far more powerful.

[sneak]

Because the backend app servers were extremely fat (memory and disk intensive) and every request they served that could have been served from an upstream cache was a request that they weren't serving that actually required all of their resources to serve.

Caching upstream on (vastly cheaper) instances permitted a huge cost savings for the same requests/sec.

[mkranjec]

Proxy caching / leveraging caching to external service makes sense when you have a LOT of users scattered around the globe - let Akamai/Cloudflare take care of edge node caching and maintenance. In the end it saves you a lot of engineering time and infrastructure costs not mentioning user experience. YMMV but it pays off in our current setup.

[tatersolid]

Because if you use HTTP caching, you can use a CDN with 100s of global locations. Which is quite a bit more powerful than any custom solution.