Add a .Net Core distributed cache

[fredericDelaporte]

This implementation is not standalone: an IDistributedCache
implementation has to be provided through an IDistributedCacheFactory.
Four such factories are provided as additional packages, covering the three caches from https://github.com/aspnet/Caching and Memcached through EnyimMemcachedCore. (Other implementations exist.)

Fix #28

[fredericDelaporte]

Yuck!

[fredericDelaporte]

See this comment on this PR matching issue.

[fredericDelaporte]

I may add two "companion" packages, one for a Redis factory and the other for SQL Server distributed cache.

[fredericDelaporte]

Last commit was causing tests to fail, but this was not detected by the CI script. Now the test failure is fixed (and the factories names slightly simplified by the way).

[fredericDelaporte]

WIP again: going to add a Memcached factory, and adjust the factory interface for being a bit more adaptable to new needs without breaking changes.

[fredericDelaporte]

I have checked two other implementations: Regex and StringBuilder. They were no match in execution time, so keeping this basic Replace one.

For reference, here is how I have checked:

[Test]
public void PerfTest()
{
	var keys = new[]
	{
		"abcdefghijklmnopkrstuvwxyz",
		"abcde\fghijklm\nopk\r \tu\vwxyz",
		"abcdefghijklmnopkrstuvwxyzabcdefghijklmnopkrstuvwxyzabcdefghijklmnopkrstuvwxyzabcdefghijklmnopkrstuvwxyz" +
		"abcdefghijklmnopkrstuvwxyzabcdefghijklmnopkrstuvwxyzabcdefghijklmnopkrstuvwxyzabcdefghijklmnopkrstuvwxyz",
		"abcde\fghijklm\nopk\r \tu\vwxyzabcde\fghijklm\nopk\r \tu\vwxyzabcde\fghijklm\nopk\r \tu\vwxyzabcde\fghijklm\nopk\r \tu\vwxyz" +
		"abcde\fghijklm\nopk\r \tu\vwxyzabcde\fghijklm\nopk\r \tu\vwxyzabcde\fghijklm\nopk\r \tu\vwxyzabcde\fghijklm\nopk\r \tu\vwxyz"
	};

	var sw = new Stopwatch();
	Regex purgeForbiddenChar = null;
	HashSet<char> forbiddenCharSet = null;
	char[] forbiddenChar = null;

	var algos = new[]
	{
		new Tuple<string, System.Action, Func<string, string>>("Regex",
			() => purgeForbiddenChar = new Regex("[ \n\r\t\v\f]", RegexOptions.Compiled),
			s => purgeForbiddenChar.Replace(s, "-")),
		new Tuple<string, System.Action, Func<string, string>>("Builder",
			() => forbiddenCharSet = new HashSet<char> { ' ', '\n', '\r', '\t', '\v', '\f' },
			s =>
			{
				var sanitizedKey = new StringBuilder();
				foreach (var c in s)
				{
					sanitizedKey.Append(forbiddenCharSet.Contains(c) ? '-' : c);
				}
				return sanitizedKey.ToString();
			}),
		new Tuple<string, System.Action, Func<string, string>>("Replace",
			() => forbiddenChar = new[] { '\n', ' ', '\r', '\t', '\v', '\f' },
			s =>
			{
				foreach (var forbidden in forbiddenChar)
				{
					s = s.Replace(forbidden, '-');
				}
				return s;
			})
	};

	foreach (var algo in algos)
	{
		sw.Restart();
		algo.Item2();
		foreach (var k in keys)
		{
			Console.WriteLine(algo.Item3(k));
		}
		Console.WriteLine($"{sw.Elapsed} for {algo.Item1} warmup");
		foreach (var k in keys)
		{
			sw.Restart();
			for (var i = 0; i < 1000; i++)
			{
				algo.Item3(k + i);
			}
			Console.WriteLine($"{sw.Elapsed} for {algo.Item1} key {k}");
		}
	}
}

[fredericDelaporte]

It is a bit messy to integrate a library meant to be used with the .Net Core default dependency injection system with NHibernate factories/configuration logic.

EnyimMemcachedCore was the worst case, due to its dependency on a logger factory (I have provided it through a wrapping of the NHibernate one) and its structured configuration (thus the JSON trick I have choosen for it).

Maybe should each factory also expose directly a constructor with the cache dependencies (at least IMemcachedClientConfiguration and maybe ILoggerFactory for this case)for users who would set the factory programmatically.

[fredericDelaporte]

EnyimMemcachedCore does not have strong name. This creates no issues for netcoreapp2.0 tests and compilation whatever the target, but if we sign NHibernate.Caches.CoreDistributedCache.Memcached, then net461 tests fail loading the dependency, complaining System.IO.FileLoadException : Could not load file or assembly 'EnyimMemcachedCore, Version=2.1.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. A strongly-named assembly is required.

It seems it is recommended to no more strongly name assemblies in most cases, so not signing this package should be acceptable.

[fredericDelaporte]

I wonder if we can and should actually trust this. NHibernate creates many caches, at least one by region which could mean one per mapped collection (and indeed much more, see nhibernate/nhibernate-core#1480).

Each RedisCache hold its own Redis "database" instance. If that mean some kind of permanently opened connection on Redis, that would be bad to create many instances of RedisCache.

[fredericDelaporte]

Same question than for RedisCache, but with a different concern: each SqlServerCache may spawn its own background task for deleting expired items, at each operation on itself, if enough time has elapsed since the last time the instance have done it. So the more instances we create, the more such background task may be run concurrently. Moreover this causes the expired items deletion to occur overall much more frequently than asked through ExpiredItemsDeletionInterval.

[fredericDelaporte]

After that, for information there are two more things I intend to do (within their own PR): generate the xml documentation (at least for NuGet packages) and generate release artifacts on AppVeyor. Then I will proceed with releasing caches.

Update: now done and this PR is rebased.

[fredericDelaporte]

I intend to merge and publish 5.3 next monday, unless someone wants to take the time reviewing this pending PR.