internal void Acquire(Transaction transaction, LockType requestedType)
{
string tname = "T" + transaction.Id;
string message = "";
if (requestedType == Kind && OwnerIds.Contains(transaction))
{
return;
}
switch (Kind)
{
//When a lock is UNLOCKED the transaction always gets the requested lock
case LockType.UNLOCKED:
switch (requestedType)
{
case LockType.SHARED:
GetShared(transaction);
break;
case LockType.EXCLUSIVE:
GetExclusive(transaction);
break;
default:
break;
}
break;
case LockType.SHARED:
switch (requestedType)
{
//A SHARED lock can always be acquired, if the current state of the lock is SHARED
case LockType.SHARED:
GetShared(transaction);
break;
case LockType.EXCLUSIVE:
//There is more than one transaction using the SHARED lock. In this case the EXCLUSIVE lock is denied and the
//requesting transaction is placed in queue.
if (OwnerIds.Exists(t => t.Id != transaction.Id))
{
message = tname + " <-- " + "Wait(" + ObjectName + ")\n";
Logger.Log += message;
Console.Write(message);
AddToQueue(transaction, requestedType);
}
//If there exists no other transaction owning the SHARED lock, then the transaction can upgrade its SHARED lock
//to an EXCLUSIVE lock.
else
{
GetExclusive(transaction);
}
break;
}
break;
case LockType.EXCLUSIVE:
//if the lock is EXCLUSIVE, then any request to acquire it is denied an the requesting transaction is placed in queue
switch (requestedType)
{
case LockType.SHARED:
message = tname + " <-- " + "Wait(" + ObjectName + ")\n";
Logger.Log += message;
Console.Write(message);
AddToQueue(transaction, requestedType);
break;
case LockType.EXCLUSIVE:
message = tname + " <-- " + "Wait(" + ObjectName + ")\n";
Logger.Log += message;
Console.Write(message);
AddToQueue(transaction, requestedType);
break;
}
break;
}
}
public TransactionExecution(Scheduler scheduler, Transaction transaction)
{
this.scheduler = scheduler;
this.transaction = transaction;
}
//add a transaction to queue and pause it
private void AddToQueue(Transaction transaction, LockType lockType)
{
TransactionQueue.Enqueue(new Tuple <Transaction, LockType>(transaction, lockType));
transaction.Paused = true;
}
//When releasing the lock, the current transaction is removed from the owners. This function reports
//whether the queue is empty or not.
internal bool Release(Transaction transaction)
{
OwnerIds.Remove(transaction);
return(OwnerIds.Count == 0);
}
