/*-------------------------------------------------------------------------
 *
 *   FILE
 *	pqxx/isolation.hxx
 *
 *   DESCRIPTION
 *      definitions of transaction isolation levels
 *   Policies and traits describing SQL transaction isolation levels
 *   DO NOT INCLUDE THIS FILE DIRECTLY; include pqxx/isolation instead.
 *
 * Copyright (c) 2003-2013, Jeroen T. Vermeulen <jtv@xs4all.nl>
 *
 * See COPYING for copyright license.  If you did not receive a file called
 * COPYING with this source code, please notify the distributor of this mistake,
 * or contact the author.
 *
 *-------------------------------------------------------------------------
 */
#ifndef PQXX_H_ISOLATION
#define PQXX_H_ISOLATION

#include "pqxx/compiler-public.hxx"
#include "pqxx/compiler-internal-pre.hxx"

#include "pqxx/util"

namespace pqxx
{

/// Transaction isolation levels.
/** These are as defined in the SQL standard.  But there are a few notes
 * specific to PostgreSQL.
 *
 * First, postgres does not support "read uncommitted."  The lowest level you
 * can get is "read committed," which is better.  PostgreSQL is built on the
 * MVCC paradigm, which guarantees "read committed" isolation without any
 * additional performance overhead, so there was no point in providing the
 * lower level.
 *
 * Second, "repeatable read" also makes more isolation guarantees than the
 * standard requires.  According to the standard, this level prevents "dirty
 * reads" and "nonrepeatable reads," but not "phantom reads."  In postgres,
 * it actually prevents all three.
 *
 * Third, "serializable" is only properly supported starting at postgres 9.1.
 * If you request "serializable" isolation on an older backend, you will get
 * the same isolation as in "repeatable read."  It's better than the "repeatable
 * read" defined in the SQL standard, but not a complete implementation of the
 * standard's "serializable" isolation level.
 *
 * In general, a lower isolation level will allow more surprising interactions
 * between ongoing transactions, but improve performance.  A higher level
 * gives you more protection from subtle concurrency bugs, but sometimes it
 * may not be possible to complete your transaction without avoiding paradoxes
 * in the data.  In that case a transaction may fail, and the application will
 * have to re-do the whole thing based on the latest state of the database.
 *
 * Study the levels and design your application with the right level in mind.
 */
enum isolation_level
{
  // read_uncommitted,
  read_committed,
  repeatable_read,
  serializable
};

/// Traits class to describe an isolation level; primarly for libpqxx's own use
template<isolation_level LEVEL> struct isolation_traits
{
  static isolation_level level() throw () { return LEVEL; }
  static const char *name() throw ();
};


template<> inline const char *isolation_traits<read_committed>::name() throw ()
	{ return "READ COMMITTED"; }
template<> inline const char *isolation_traits<repeatable_read>::name() throw ()
	{ return "REPEATABLE READ"; }
template<> inline const char *isolation_traits<serializable>::name() throw ()
	{ return "SERIALIZABLE"; }

}


#include "pqxx/compiler-internal-post.hxx"

#endif

