private static TItem GetInternal <TItem, TSerialized>(string cacheKey, TimeSpan localExpiration, TimeSpan remoteExpiration,
string groupKey, Func <TItem> loader, Func <TItem, TSerialized> serialize, Func <TSerialized, TItem> deserialize)
where TItem : class
where TSerialized : class
{
ulong?groupGeneration = null;
ulong?groupGenerationCache = null;
string itemGenerationKey = cacheKey + GenerationSuffix;
var localCache = LocalCache.Provider;
var distributedCache = DistributedCache.Provider;
// retrieves distributed cache group generation number lazily
Func <ulong> getGroupGenerationValue = delegate()
{
if (groupGeneration != null)
{
return(groupGeneration.Value);
}
groupGeneration = distributedCache.Get <ulong?>(groupKey);
if (groupGeneration == null || groupGeneration == 0)
{
groupGeneration = RandomGeneration();
distributedCache.Set(groupKey, groupGeneration.Value);
}
groupGenerationCache = groupGeneration.Value;
// add to local cache, use 5 seconds from there
LocalCache.Add(groupKey, groupGenerationCache, GenerationCacheExpiration);
return(groupGeneration.Value);
};
// retrieves local cache group generation number lazily
Func <ulong> getGroupGenerationCacheValue = delegate()
{
if (groupGenerationCache != null)
{
return(groupGenerationCache.Value);
}
// check cached local value of group key
// it expires in 5 seconds and read from server again
groupGenerationCache = localCache.Get <object>(groupKey) as ulong?;
// if its in local cache, return it
if (groupGenerationCache != null)
{
return(groupGenerationCache.Value);
}
return(getGroupGenerationValue());
};
// first check local cache, if item exists and not expired (group version = item version) return it
var cachedObj = localCache.Get <object>(cacheKey);
if (cachedObj != null)
{
// check local cache, if exists, compare version with group one
var itemGenerationCache = localCache.Get <object>(itemGenerationKey) as ulong?;
if (itemGenerationCache != null &&
itemGenerationCache == getGroupGenerationCacheValue())
{
// local cached item is not expired yet
if (cachedObj == DBNull.Value)
{
return(null);
}
return((TItem)cachedObj);
}
// local cached item is expired, remove all information
if (itemGenerationCache != null)
{
localCache.Remove(itemGenerationKey);
}
localCache.Remove(cacheKey);
cachedObj = null;
}
// if serializer is null, than this is a local store only item
if (serialize != null)
{
// no item in local cache or expired, now check distributed cache
var itemGeneration = distributedCache.Get <ulong?>(itemGenerationKey);
// if item has version number in distributed cache and this is equal to group version
if (itemGeneration != null &&
itemGeneration.Value == getGroupGenerationValue())
{
// get item from distributed cache
var serialized = distributedCache.Get <TSerialized>(cacheKey);
// if item exists in distributed cache
if (serialized != null)
{
cachedObj = deserialize(serialized);
LocalCache.Add(cacheKey, (object)cachedObj ?? DBNull.Value, localExpiration);
LocalCache.Add(itemGenerationKey, getGroupGenerationValue(), localExpiration);
return((TItem)cachedObj);
}
}
}
// couldn't find valid item in local or distributed cache, produce value by calling loader
var item = loader();
// add item and its version to cache
LocalCache.Add(cacheKey, (object)item ?? DBNull.Value, localExpiration);
LocalCache.Add(itemGenerationKey, getGroupGenerationValue(), localExpiration);
if (serialize != null)
{
var serializedItem = serialize(item);
// add item and generation to distributed cache
if (remoteExpiration == TimeSpan.Zero)
{
distributedCache.Set(cacheKey, serializedItem);
distributedCache.Set(itemGenerationKey, getGroupGenerationValue());
}
else
{
distributedCache.Set(cacheKey, serializedItem, remoteExpiration);
distributedCache.Set(itemGenerationKey, getGroupGenerationValue(), remoteExpiration);
}
}
return(item);
}