#2246 Review Core cache and redesign Regex caching

[raman-m]

Fixes #2246

• Memory leak in Regex caching logic #2246

Proposed Changes

• Added new useful method to the IOcelotCache<T> interface: bool TryGetValue(string, string, out T).
  As a result, the Ocelot.Cache.CacheManager package must be released❗

• Refactored the DefaultMemoryCache<T> class to use concurrent collections for regions, as the previous ones were not thread-safe.

• Proposed implementing a real cache in the UpstreamTemplatePatternCreator class to initialize the Pattern property of the class with a cached Regex object when the UpstreamPathTemplate is being created.
  Note that the design with a static concurrent dictionary was acceptable due to the constant number of routes (and Regex objects) in the collection. However, for dynamic routing scenarios, it is better to switch to a real caching design for future use.

• ❗Resolved memory leak in the DownstreamUrlCreatorMiddleware class by refactoring the RemoveQueryStringParametersThatHaveBeenUsedInTemplate method, where all Regex objects were saved into _regex, an internal static collection❗ This faulty design was unfortunately released in version 23.4.0 as PR Best practices for regular expressions versus Regex performance review #1348❗ We must learn from this incident because sometimes optimizations for CPU may adversely affect memory consumption 🙈
  Ultimately, the algorithm was refactored to remove Regex in favor of StringBuilder.

• Refactored the IUrlPathToUrlTemplateMatcher interface, the UrlMatch class, and the associated logic.

• Added load test to reproduce the problem with a memory leak. It helped to develop the fix. It should not run in the CI/CD environment because it consumes high CPU resources; thus, the test is skipped.

[raman-m]

@donnytian Welcome to review!

[ggnaegi]

Just asking, why should we have region as parameter since we don't use it?

[raman-m]

Regions are used in the Administration API to delete cached objects within specific region:

Ocelot/docs/features/administration.rst

Lines 139 to 145 in d1ecfaf

	DELETE {adminPath}/outputcache/{region}
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	This clears a region of the cache. If you are using a backplane, it will clear all instances of the cache!
	Giving your the ability to run a cluster of Ocelots and cache over all of them in memory and clear them all at the same time, so just use a distributed cache.
	The region is whatever you set against the **Region** field in the `FileCacheOptions <https://github.com/search?q=repo%3AThreeMammals%2FOcelot%20FileCacheOptions&type=code>`_ section of the Ocelot configuration.

[raman-m]

Regions have been defined in the Caching Configuration since the early releases:

Ocelot/docs/features/caching.rst

Lines 41 to 47 in d1ecfaf

	"Region": "europe-central",
	"Header": "OC-Caching-Control",
	"EnableContentHashing": false // my route has GET verb only, assigning 'true' for requests with body: POST, PUT etc.
	}
	In this example **TtlSeconds** is set to 15 which means the cache will expire after 15 seconds.
	The **Region** represents a region of caching.

Target: Emptying cache region by Administration API or even by custom C# code

[raman-m]

Ideally, the region name should be the route name, but we propose to assign a custom name. Consequently, the DefaultMemoryCache service could be redesigned and enhanced.

[ggnaegi]

@raman-m ok, fine