StrictTransactionCensor.java

//Prevayler(TM) - The Free-Software Prevalence Layer.
//Copyright (C) 2001-2003 Klaus Wuestefeld
//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.
//Contributions: Jon Tirs�n.

package org.prevayler.implementation.publishing.censorship;

import java.io.Serializable;

import org.prevayler.foundation.serialization.Serializer;
import org.prevayler.implementation.PrevalentSystemGuard;
import org.prevayler.implementation.TransactionTimestamp;
import org.prevayler.implementation.snapshot.GenericSnapshotManager;

public class StrictTransactionCensor<P> implements TransactionCensor {

	private final PrevalentSystemGuard<P> _king;
	private PrevalentSystemGuard<P> _royalFoodTaster;
	private final Serializer _snapshotSerializer;

	public StrictTransactionCensor(GenericSnapshotManager<P> snapshotManager) {
		_king = snapshotManager.recoveredPrevalentSystem();
		// The _royalFoodTaster cannot be initialized here, or else the pending transactions will not be applied to it.
		_snapshotSerializer = snapshotManager.primarySerializer();
	}

	public void approve(TransactionTimestamp transactionTimestamp) throws RuntimeException, Error {
		try {
			TransactionTimestamp timestampCopy = transactionTimestamp.cleanCopy();
			PrevalentSystemGuard<P> royalFoodTaster = royalFoodTaster(transactionTimestamp.systemVersion() - 1);
			royalFoodTaster.receive(timestampCopy);
		} catch (RuntimeException rx) {
			letTheFoodTasterDie();
			throw rx;
		} catch (Error error) {
			letTheFoodTasterDie();
			throw error;
		}
	}

	private void letTheFoodTasterDie() {
		_royalFoodTaster = null;  // At this moment there might be transactions that have already been approved by this censor but have not yet been applied to the _king. It is a requirement, therefore, that the _royalFoodTaster must not be initialized now, but only when the next transaction arrives to be approved.
	}

	private PrevalentSystemGuard<P> royalFoodTaster(long systemVersion) {
		if (_royalFoodTaster == null) produceNewFoodTaster(systemVersion);
		return _royalFoodTaster;
	}

	private void produceNewFoodTaster(long systemVersion) {
		try {
			_royalFoodTaster = _king.deepCopy(systemVersion, _snapshotSerializer);
			_royalFoodTaster.guaranteeTransactionDeepCopy();
		} catch (Exception ex) {
			ex.printStackTrace();
			throw new RuntimeException("Unable to produce a copy of the prevalent system for trying out transactions before applying them to the real system.");
		}
	}

}