path: root/cpp/include/IceUtil/Cache.h

// **********************************************************************
//
// Copyright (c) 2003-2006 ZeroC, Inc. All rights reserved.
//
// This copy of Ice is licensed to you under the terms described in the
// ICE_LICENSE file included in this distribution.
//
// **********************************************************************

#ifndef ICE_UTIL_CACHE_H
#define ICE_UTIL_CACHE_H

#include <IceUtil/Handle.h>
#include <IceUtil/Mutex.h>
#include <IceUtil/CountDownLatch.h>
#include <map>

namespace IceUtil
{

//
// An abstraction to efficiently populate a Cache, without holding
// a lock while loading from a database.
//

template<typename Key, typename Value>
class Cache
{
public:
    
    //
    // Latch and CacheValue are implementation details;
    // application code should not use them.
    //

    struct Latch : public CountDownLatch
    {
	Latch() :
	    CountDownLatch(1),
	    useCount(0)
	{ 
	}
	int useCount;
    };

    struct CacheValue
    {
	CacheValue(const Handle<Value>& o) :
	    obj(o),
	    latch(0)
	{
	}
	
	Handle<Value> obj;
	Latch* latch;
    };

    typedef typename std::map<Key, CacheValue>::iterator Position;


    Handle<Value> getIfPinned(const Key&) const;

    void unpin(Position);
    bool unpin(const Key&);

    void clear();
    size_t size() const;

    bool pin(const Key&, const Handle<Value>&);
    
    Handle<Value> pin(const Key&);
    Handle<Value> putIfAbsent(const Key&, const Handle<Value>&);

protected:

    virtual Handle<Value> load(const Key&) = 0;

    virtual void pinned(const Handle<Value>&, Position)
    {
    }
    
    virtual ~Cache()
    {
    }

private:

    Handle<Value> pinImpl(const Key&, const Handle<Value>&);

    typedef std::map<Key, CacheValue> CacheMap;
    
    Mutex _mutex;
    CacheMap  _map;
};


template<typename Key, typename Value> Handle<Value> 
Cache<Key, Value>::getIfPinned(const Key& key) const
{
    Mutex::Lock sync(_mutex);
    typename CacheMap::const_iterator p = _map.find(key);
    if(p != _map.end())
    {
	return (*p).second.obj;
    }
    else
    {
	return 0;
    }
}

template<typename Key, typename Value> void
Cache<Key, Value>::unpin(typename Cache::Position p)
{
    Mutex::Lock sync(_mutex);
    _map.erase(p);
}

template<typename Key, typename Value> bool
Cache<Key, Value>::unpin(const Key& key)
{
    Mutex::Lock sync(_mutex);
    return _map.erase(key) > 0;
}

template<typename Key, typename Value> void 
Cache<Key, Value>::clear()
{
    Mutex::Lock sync(_mutex);
    _map.clear();
}

template<typename Key, typename Value> size_t 
Cache<Key, Value>::size() const
{
    Mutex::Lock sync(_mutex);
    return _map.size();
}
    
template<typename Key, typename Value> bool 
Cache<Key, Value>::pin(const Key& key, const Handle<Value>& obj)
{
    Mutex::Lock sync(_mutex);
#if defined(_MSC_VER) && (_MSC_VER < 1300)
    std::pair<CacheMap::iterator, bool> ir = 
#else
    std::pair<typename CacheMap::iterator, bool> ir =
#endif 

#if defined(_MSC_VER)
       _map.insert(CacheMap::value_type(key, CacheValue(obj)));
#else
       _map.insert(typename CacheMap::value_type(key, CacheValue(obj)));
#endif       


    if(ir.second)
    {
	pinned(obj, ir.first);
    }
    return ir.second;
}

template<typename Key, typename Value> Handle<Value> 
Cache<Key, Value>::pin(const Key& key)
{
    return pinImpl(key, 0);
}

template<typename Key, typename Value> Handle<Value> 
Cache<Key, Value>::putIfAbsent(const Key& key, const Handle<Value>& obj)
{
    return pinImpl(key, obj);
}

template<typename Key, typename Value> Handle<Value> 
Cache<Key, Value>::pinImpl(const Key& key, const Handle<Value>& newObj)
{
    Latch* latch = 0;
    Position p;

    for(;;)
    {
	{
	    Mutex::Lock sync(_mutex);
	
	    //
	    // Clean up latch from previous loop
	    //
	    if(latch != 0)
	    {
		if(--latch->useCount == 0)
		{
		    delete latch;
		}
		latch = 0;
	    }
    
#if defined(_MSC_VER) && (_MSC_VER < 1300)
	    std::pair<CacheMap::iterator, bool> ir = 
#else
	    std::pair<typename CacheMap::iterator, bool> ir =
#endif 		
	    
#if defined(_MSC_VER)
      	        _map.insert(CacheMap::value_type(key, CacheValue(0)));
#else
      	        _map.insert(typename CacheMap::value_type(key, CacheValue(0)));
#endif    

	    if(ir.second == false)
	    {
		CacheValue& val = ir.first->second;
		if(val.obj != 0)
		{
		    return val.obj;
		}

		//
		// Otherwise wait
		//
		if(val.latch == 0)
		{
		    // 
		    // The first queued thread creates the latch 
		    // 
		    val.latch = new Latch; 
		}
		latch = val.latch;
		latch->useCount++;
	    }
	   
	    p = ir.first;	   
	}
	        
	if(latch != 0) 
	{ 
	    latch->await();
	    
	    // 
	    // p could be stale now, e.g. some other thread pinned and unpinned the 
	    // object while we were waiting. 
	    // So start over. 
	    // 
	    continue;
	} 
	else 
	{          
	    Handle<Value> obj;
	    try
	    {
		obj = load(key);
	    }
	    catch(...)
	    {
		{
		    Mutex::Lock sync(_mutex);
		    latch = p->second.latch;
		    p->second.latch = 0;
		    _map.erase(p);
		}
		if(latch != 0)  
		{ 
		    assert(latch->getCount() == 1);
		    latch->countDown();
		}
		throw;
	    }

	    {
		Mutex::Lock sync(_mutex);

		latch = p->second.latch;
		p->second.latch = 0;

		try
		{
		    if(obj != 0) 
		    {  
			p->second.obj = obj;
			pinned(obj, p);
		    } 
		    else 
		    { 
			if(newObj == 0)
			{
			    //
			    // pin() did not find the object
			    //
			    
			    // 
			    // The waiting threads will have to call load() to see by themselves. 
			    // 
			    _map.erase(p);
			}
			else
			{
			    //
			    // putIfAbsent() inserts key/newObj
			    //
			    p->second.obj = newObj;
			    pinned(newObj, p);
			}
		    }
		}
		catch(...)
		{
		    if(latch != 0)  
		    { 
			assert(latch->getCount() == 1);
			latch->countDown();
		    }
		    throw;
		}
	    }
	    if(latch != 0)  
	    { 
		assert(latch->getCount() == 1);
		latch->countDown();
	    }
	    return obj;
	}  
    }
}


}

#endif