Backend authorization cache handlers

[mikz]

Extract how cache is treated based on the backend response into a module.
It allows extension by adding custom handlers like:

local cache_handler = require('backend.cache_handler')

function cache_handler.handlers.custom(cache, cached_key, response, ttl)
  cache:add(...)
  return true or false
end

• strict is the original handler

  • it removes cached key on any non successful response

• resilient is the new handler

  • stores response in cache only when it was not a server error

It also allows overriding proxy.shared_cache method to create custom cache instance for all proxies.
That can be useful when replacing local shared memory cache with something like redis.

TODO

• integration test

[octobot]

Spell Checker found issues

doc/parameters.md

Line	Typo
3	APIcast v2 has a number of parameters co
3	ariables) that can modify the behavior of the gateway. The following
5	e that when deploying APIcast v2 with OpenShift, some of thee
15	Default: **stderr**
17	or more information. The file pathcan be either absolute, or relati
21	nfo
21	warn
46	Default: "apicast"
77	Values: a number > **60**
78	Default: 0
80	r. The value should be set to 0 or more than 60. For example,
80	ould be set to 0 or more than 60. For example, if `APICAST_CON
80	ONFIGURATION_CACHE` is set to 120, the gateway will reload the
80	eload the configuration every 2 minutes (120 seconds).
80	onfiguration every 2 minutes (120 seconds).
84	Default: "127.0.0.1"
86	ning Redis instance for OAuth 2.0 flow. REDIS_HOST parameter
90	Default: 6379
92	ning Redis instance for OAuth 2.0 flow. REDIS_PORT parameter
98	ning Redis instance for OAuth 2.0 flow. REDIS_URL parameter c
98	e used to set the full URI as DSN format like: `redis://PASSWOR
102	pty, the DNS resolver will be autodiscovered.
154	Setting it to particual version will make it not auto
166	tificate bundle generated by [export-builtin-trusted-certs](https://github.com/openresty

Generated by 🚫 Danger

[mayorova]

Haven't tried yet, but looks good.
How can proxy.shared_cache be overriden?..

[mikz]

@mayorova

proxy.shared_cache = function()
end