TableCache.h

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//# TableCache.h: Cache of open tables
//# Copyright (C) 1994,1995,1997,1999
//# Associated Universities, Inc. Washington DC, USA.
//#
//# This library is free software; you can redistribute it and/or modify it
//# under the terms of the GNU Library General Public License as published by
//# the Free Software Foundation; either version 2 of the License, or (at your
//# option) any later version.
//#
//# This library is distributed in the hope that it will be useful, but WITHOUT
//# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
//# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
//# License for more details.
//#
//# You should have received a copy of the GNU Library General Public License
//# along with this library; if not, write to the Free Software Foundation,
//# Inc., 675 Massachusetts Ave, Cambridge, MA 02139, USA.
//#
//# Correspondence concerning AIPS++ should be addressed as follows:
//#        Internet email: aips2-request@nrao.edu.
//#        Postal address: AIPS++ Project Office
//#                        National Radio Astronomy Observatory
//#                        520 Edgemont Road
//#                        Charlottesville, VA 22903-2475 USA
//#
//# $Id$

#ifndef TABLES_TABLECACHE_H
#define TABLES_TABLECACHE_H

//# Includes
#include <casacore/casa/aips.h>
#include <casacore/casa/IO/FileLocker.h>
#include <casacore/casa/OS/Mutex.h>
#include <map>

namespace casacore { //# NAMESPACE CASACORE - BEGIN

//# Forward Declarations
class PlainTable;
class TableLock;
template<class T> class Vector;


// <summary>
// Cache of open tables
// </summary>

// <use visibility=local>

// <reviewed reviewer="UNKNOWN" date="before2004/08/25" tests="">
// </reviewed>

// <prerequisite>
//# Classes you should understand before using this one.
// </prerequisite>

// <etymology>
// TableCache represents a cache of open tables.
// </etymology>

// <synopsis> 
// A TableCache object keeps track of the tables which have already
// been opened in a program. It maps the name of a table to its
// PlainTable object.
// In principle only one TableCache object (statically defined in
// class PlainTable) exists in a process.
// The cache is used to prevent a table from being opened more than
// once, which is not only a waste of space, but more importantly,
// may give rise to synchronization problems.
// Synchronization between the same table in multiple processes must
// be done by a locking mechanism.
//
// TableCache is used by class Table and PlainTable.
// Before opening a table, Table will first look in the cache.
// Newly opened or created tables will be added to the cache.
// When a table is actually closed, it will be removed from the cache.
// </synopsis> 

// <motivation>
// When a RefTable is read back, it will also read back the table it
// references. However, that table may have been opened before and
// it is bad to have a table open more than once in the same program.
// The TableCache class catches this and will not reopen the table.
// </motivation>

// <todo asof="$DATE:$">
//# A List of bugs, limitations, extensions or planned refinements.
//   <li> Currently only PlainTables are taken into account.
//          Maybe RefTables should be too.
// </todo>


class TableCache
{
public:

    // Construct an empty cache of open tables.
    TableCache();

    ~TableCache();

    // Try to find a table with the given name in the cache.
    // Return a pointer to a table if found (thus if already open).
    // Return a zero pointer if not found.
    PlainTable* operator() (const String& tableName) const;

    // Add an open table to the cache.
    void define (const String& tableName, PlainTable*);

    // Remove an open table.
    void remove (const String& tableName);

    // Rename an open table.
    // If oldName is not in the cache, nothing will be done.
    void rename (const String& newName, const String& oldName);

    // Determine the number of locked tables opened with the AutoLock option
    // (Locked table means locked for read and/or write).
    uInt nAutoLocks();

    // Unlock locked tables opened with the AutoLock option.
    // If <src>all=True</src> all such tables will be unlocked.
    // If <src>all=False</src> only tables requested by another process
    // will be unlocked.
    void relinquishAutoLocks (Bool all);

    // Get the names of the tables in the cache.
    Vector<String> getTableNames() const;

    // Get the names of tables locked in this process.
    // By default all locked tables are given (note that a write lock
    // implies a read lock), but it is possible to select on lock type
    // FileLocker::Write and on option (TableLock::AutoLocking,
    // TableLock::ReadLocking, or TableLock::PermanentLocking).
    Vector<String> getLockedTables (FileLocker::LockType,
                                    int lockOption);

    // Flush a possibly cached Table.
    void flushTable (const String& tableName,
                     Bool fsync, Bool recursive);

    // Look in the cache if the table is already open.
    // If so, check if table option matches.
    // If needed reopen the table for read/write and merge the lock options.
    PlainTable* lookCache (const String& name, int tableOption,
                           const TableLock& tableInfo);

private:
    // The copy constructor is forbidden.
    TableCache (const TableCache&);
    // The assignment operator is forbidden.
    TableCache& operator= (const TableCache&);

    // Get the table without doing a mutex lock (for operator()).
    PlainTable* getTable (const String& tableName) const;

    //# void* iso. PlainTable* is used in the map declaration
    //# to reduce the number of template instantiations.
    //# The .cc file will use (fully safe) casts.
    std::map<String,void*> tableMap_p;
    //# A mutex to synchronize access to the cache.
    mutable Mutex itsMutex;
};



} //# NAMESPACE CASACORE - END

#endif