poco/Foundation/include/Poco/AbstractCache.h

//
// AbstractCache.h
//
// Library: Foundation
// Package: Cache
// Module:  AbstractCache
//
// Definition of the AbstractCache class.
//
// Copyright (c) 2006, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// SPDX-License-Identifier:	BSL-1.0
//


#ifndef Foundation_AbstractCache_INCLUDED
#define Foundation_AbstractCache_INCLUDED


#include "Poco/KeyValueArgs.h"
#include "Poco/ValidArgs.h"
#include "Poco/Mutex.h"
#include "Poco/BasicEvent.h"
#include "Poco/EventArgs.h"
#include "Poco/Delegate.h"
#include "Poco/SharedPtr.h"
#include <map>
#include <set>
#include <cstddef>


namespace Poco {


template <class TKey, class TValue, class TStrategy, class TMutex = FastMutex, class TEventMutex = FastMutex>
class AbstractCache
	/// An AbstractCache is the interface of all caches.
{
public:
	BasicEvent<const KeyValueArgs<TKey, TValue>, TEventMutex> Add;
	BasicEvent<const KeyValueArgs<TKey, TValue>, TEventMutex> Update;
	BasicEvent<const TKey, TEventMutex>                       Remove;
	BasicEvent<const TKey, TEventMutex>                       Get;
	BasicEvent<const EventArgs, TEventMutex>                  Clear;

	using DataHolder = std::map<TKey, SharedPtr<TValue>>;
	using Iterator = typename DataHolder::iterator;
	using ConstIterator = typename DataHolder::const_iterator;
	using KeySet = std::set<TKey>;

	AbstractCache()
	{
		initialize();
	}

	AbstractCache(const TStrategy& strat): _strategy(strat)
	{
		initialize();
	}

	virtual ~AbstractCache()
	{
		try
		{
			uninitialize();
		}
		catch (...)
		{
			poco_unexpected();
		}
	}

	AbstractCache(const AbstractCache& aCache) = delete;
	AbstractCache& operator = (const AbstractCache& aCache) = delete;

	void add(const TKey& key, const TValue& val)
		/// Adds the key value pair to the cache.
		/// If for the key already an entry exists, it will be overwritten.
	{
		typename TMutex::ScopedLock lock(_mutex);
		doAdd(key, val);
	}

	void update(const TKey& key, const TValue& val)
		/// Adds the key value pair to the cache. Note that adding a NULL SharedPtr will fail!
		/// If for the key already an entry exists, it will be overwritten.
		/// The difference to add is that no remove or add events are thrown in this case,
		/// just a simply silent update is performed
		/// If the key does not exist the behavior is equal to add, ie. an add event is thrown
	{
		typename TMutex::ScopedLock lock(_mutex);
		doUpdate(key, val);
	}

	void add(const TKey& key, SharedPtr<TValue > val)
		/// Adds the key value pair to the cache. Note that adding a NULL SharedPtr will fail!
		/// If for the key already an entry exists, it will be overwritten, ie. first a remove event
		/// is thrown, then a add event
	{
		typename TMutex::ScopedLock lock(_mutex);
		doAdd(key, val);
	}

	void update(const TKey& key, SharedPtr<TValue > val)
		/// Adds the key value pair to the cache. Note that adding a NULL SharedPtr will fail!
		/// If for the key already an entry exists, it will be overwritten.
		/// The difference to add is that no remove or add events are thrown in this case,
		/// just an Update is thrown
		/// If the key does not exist the behavior is equal to add, ie. an add event is thrown
	{
		typename TMutex::ScopedLock lock(_mutex);
		doUpdate(key, val);
	}

	void remove(const TKey& key)
		/// Removes an entry from the cache. If the entry is not found,
		/// the remove is ignored.
	{
		typename TMutex::ScopedLock lock(_mutex);
		Iterator it = _data.find(key);
		doRemove(it);
	}

	bool has(const TKey& key) const
		/// Returns true if the cache contains a value for the key.
	{
		typename TMutex::ScopedLock lock(_mutex);
		return doHas(key);
	}

	SharedPtr<TValue> get(const TKey& key)
		/// Returns a SharedPtr of the value. The SharedPointer will remain valid
		/// even when cache replacement removes the element.
		/// If for the key no value exists, an empty SharedPtr is returned.
	{
		typename TMutex::ScopedLock lock(_mutex);
		return doGet (key);
	}

	void clear()
		/// Removes all elements from the cache.
	{
		typename TMutex::ScopedLock lock(_mutex);
		doClear();
	}

	std::size_t size()
		/// Returns the number of cached elements
	{
		typename TMutex::ScopedLock lock(_mutex);
		doReplace();
		return _data.size();
	}

	void forceReplace()
		/// Forces cache replacement. Note that Poco's cache strategy use for efficiency reason no background thread
		/// which periodically triggers cache replacement. Cache Replacement is only started when the cache is modified
		/// from outside, i.e. add is called, or when a user tries to access an cache element via get.
		/// In some cases, i.e. expire based caching where for a long time no access to the cache happens,
		/// it might be desirable to be able to trigger cache replacement manually.
	{
		typename TMutex::ScopedLock lock(_mutex);
		doReplace();
	}

	std::set<TKey> getAllKeys()
		/// Returns a copy of all keys stored in the cache
	{
		typename TMutex::ScopedLock lock(_mutex);
		doReplace();
		ConstIterator it = _data.begin();
		ConstIterator itEnd = _data.end();
		std::set<TKey> result;
		for (; it != itEnd; ++it)
			result.insert(it->first);

		return result;
	}

	template <typename Fn>
	void forEach(Fn&& fn) const
		/// Iterates over all key-value pairs in the
		/// cache, using a functor or lambda expression.
		///
		/// The given functor must take the key and value
		/// as parameters. Note that the value is passed
		/// as the actual value (or reference),
		/// not a Poco::SharedPtr.
	{
		typename TMutex::ScopedLock lock(_mutex);
		for (const auto& p: _data)
		{
			fn(p.first, *p.second);
		}
	}

protected:
	mutable BasicEvent<ValidArgs<TKey>> IsValid;
	mutable BasicEvent<KeySet>          Replace;

	void initialize()
		/// Sets up event registration.
	{
		Add		+= Delegate<TStrategy, const KeyValueArgs<TKey, TValue>>(&_strategy, &TStrategy::onAdd);
		Update	+= Delegate<TStrategy, const KeyValueArgs<TKey, TValue>>(&_strategy, &TStrategy::onUpdate);
		Remove	+= Delegate<TStrategy, const TKey>(&_strategy, &TStrategy::onRemove);
		Get		+= Delegate<TStrategy, const TKey>(&_strategy, &TStrategy::onGet);
		Clear	+= Delegate<TStrategy, const EventArgs>(&_strategy, &TStrategy::onClear);
		IsValid	+= Delegate<TStrategy, ValidArgs<TKey>>(&_strategy, &TStrategy::onIsValid);
		Replace	+= Delegate<TStrategy, KeySet>(&_strategy, &TStrategy::onReplace);
	}

	void uninitialize()
		/// Reverts event registration.
	{
		Add		-= Delegate<TStrategy, const KeyValueArgs<TKey, TValue>>(&_strategy, &TStrategy::onAdd );
		Update	-= Delegate<TStrategy, const KeyValueArgs<TKey, TValue>>(&_strategy, &TStrategy::onUpdate);
		Remove	-= Delegate<TStrategy, const TKey>(&_strategy, &TStrategy::onRemove);
		Get		-= Delegate<TStrategy, const TKey>(&_strategy, &TStrategy::onGet);
		Clear	-= Delegate<TStrategy, const EventArgs>(&_strategy, &TStrategy::onClear);
		IsValid	-= Delegate<TStrategy, ValidArgs<TKey>>(&_strategy, &TStrategy::onIsValid);
		Replace	-= Delegate<TStrategy, KeySet>(&_strategy, &TStrategy::onReplace);
	}

	void doAdd(const TKey& key, const TValue& val)
		/// Adds the key value pair to the cache.
		/// If for the key already an entry exists, it will be overwritten.
	{
		Iterator it = _data.find(key);
		doRemove(it);

		KeyValueArgs<TKey, TValue> args(key, val);
		Add.notify(this, args);
		_data.insert(std::make_pair(key, SharedPtr<TValue>(new TValue(val))));

		doReplace();
	}

	void doAdd(const TKey& key, SharedPtr<TValue>& val)
		/// Adds the key value pair to the cache.
		/// If for the key already an entry exists, it will be overwritten.
	{
		Iterator it = _data.find(key);
		doRemove(it);

		KeyValueArgs<TKey, TValue> args(key, *val);
		Add.notify(this, args);
		_data.insert(std::make_pair(key, val));

		doReplace();
	}

	void doUpdate(const TKey& key, const TValue& val)
		/// Adds the key value pair to the cache.
		/// If for the key already an entry exists, it will be overwritten.
	{
		KeyValueArgs<TKey, TValue> args(key, val);
		Iterator it = _data.find(key);
		if (it == _data.end())
		{
			Add.notify(this, args);
			_data.insert(std::make_pair(key, SharedPtr<TValue>(new TValue(val))));
		}
		else
		{
			Update.notify(this, args);
			it->second = SharedPtr<TValue>(new TValue(val));
		}

		doReplace();
	}

	void doUpdate(const TKey& key, SharedPtr<TValue>& val)
		/// Adds the key value pair to the cache.
		/// If for the key already an entry exists, it will be overwritten.
	{
		KeyValueArgs<TKey, TValue> args(key, *val);
		Iterator it = _data.find(key);
		if (it == _data.end())
		{
			Add.notify(this, args);
			_data.insert(std::make_pair(key, val));
		}
		else
		{
			Update.notify(this, args);
			it->second = val;
		}

		doReplace();
	}

	void doRemove(Iterator it)
		/// Removes an entry from the cache. If the entry is not found
		/// the remove is ignored.
	{
		if (it != _data.end())
		{
			Remove.notify(this, it->first);
			_data.erase(it);
		}
	}

	bool doHas(const TKey& key) const
		/// Returns true if the cache contains a value for the key
	{
		// ask the strategy if the key is valid
		ConstIterator it = _data.find(key);
		bool result = false;

		if (it != _data.end())
		{
			ValidArgs<TKey> args(key);
			IsValid.notify(this, args);
			result = args.isValid();
		}

		return result;
	}

	SharedPtr<TValue> doGet(const TKey& key)
		/// Returns a SharedPtr of the cache entry, returns 0 if for
		/// the key no value was found
	{
		Iterator it = _data.find(key);
		SharedPtr<TValue> result;

		if (it != _data.end())
		{
			// inform all strategies that a read-access to an element happens
			Get.notify(this, key);
			// ask all strategies if the key is valid
			ValidArgs<TKey> args(key);
			IsValid.notify(this, args);

			if (!args.isValid())
			{
				doRemove(it);
			}
			else
			{
				result = it->second;
			}
		}

		return result;
	}

	void doClear()
	{
		static EventArgs _emptyArgs;
		Clear.notify(this, _emptyArgs);
		_data.clear();
	}

	void doReplace()
	{
		std::set<TKey> delMe;
		Replace.notify(this, delMe);
		// delMe contains the to be removed elements
		typename std::set<TKey>::const_iterator it    = delMe.begin();
		typename std::set<TKey>::const_iterator endIt = delMe.end();

		for (; it != endIt; ++it)
		{
			Iterator itH = _data.find(*it);
			doRemove(itH);
		}
	}

	TStrategy          _strategy;
	mutable DataHolder _data;
	mutable TMutex  _mutex;

};


} // namespace Poco


#endif // Foundation_AbstractCache_INCLUDED