ClickHouse/base/poco/Foundation/include/Poco/UniqueAccessExpireCache.h

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


#ifndef Foundation_UniqueAccessExpireCache_INCLUDED
#define Foundation_UniqueAccessExpireCache_INCLUDED


#include "Poco/AbstractCache.h"
#include "Poco/UniqueAccessExpireStrategy.h"


namespace Poco
{


template <class TKey, class TValue, class TMutex = FastMutex, class TEventMutex = FastMutex>
class UniqueAccessExpireCache : public AbstractCache<TKey, TValue, UniqueAccessExpireStrategy<TKey, TValue>, TMutex, TEventMutex>
/// An UniqueAccessExpireCache caches entries for a given time span. In contrast
/// to ExpireCache which only allows to set a per cache expiration value, it allows to define
/// expiration per CacheEntry.
/// Each TValue object must thus offer the following method:
///
///    const Poco::Timespan& getTimeout() const;
///
/// which returns the relative timespan for how long the entry should be valid without being accessed!
/// The absolute expire timepoint is calculated as now() + getTimeout().
/// Accessing an object will update this absolute expire timepoint.
/// You can use the Poco::AccessExpirationDecorator to add the getExpiration
/// method to values that do not have a getExpiration function.
///
/// Be careful when using an UniqueAccessExpireCache. A cache is often used
/// like cache.has(x) followed by cache.get x). Note that it could happen
/// that the "has" call works, then the current execution thread gets descheduled, time passes,
/// the entry gets invalid, thus leading to an empty SharedPtr being returned
/// when "get" is invoked.
{
public:
    UniqueAccessExpireCache()
        : AbstractCache<TKey, TValue, UniqueAccessExpireStrategy<TKey, TValue>, TMutex, TEventMutex>(
            UniqueAccessExpireStrategy<TKey, TValue>())
    {
    }

    ~UniqueAccessExpireCache() { }

private:
    UniqueAccessExpireCache(const UniqueAccessExpireCache & aCache);
    UniqueAccessExpireCache & operator=(const UniqueAccessExpireCache & aCache);
};


} // namespace Poco


#endif // Foundation_UniqueAccessExpireCache_INCLUDED