Re: [Prevayler-coders] The PrevaylerJr Game

[Klaus W. <kla...@gm...>]

> There's a remaining bug -- while transactions will now be prevented
> from executing since the journal has been closed, queries are still
> allowed to execute against the corrupted prevalent system.

Yup. Fixed below.

Anything else? Is that it?

Is this what 90% of all apps (that fit in RAM) out there need in terms
of persistence?

Klaus

--------------------

import java.io.EOFException;
import java.io.IOException;

public class PrevaylerJr {
	private final Object _system;
	private final AcidOutputStream _journal;

	public PrevaylerJr(Object initialState, String storageFile) throws Exception {
		AcidInputStream input = new AcidInputStream(storageFile);
		try {
			initialState = input.readObject();

			while (true)
				((Command)input.readObject()).executeOn(initialState);
		
		} catch (EOFException expected) {}
		
		_system = initialState;
		_journal = new AcidOutputStream(storageFile, _system);
	}

	synchronized public Object executeTransaction(Command transaction)
throws Exception {
		checkSystemConsistency();
		_journal.append(transaction);
		return tryToExecute(transaction);
	}

	synchronized public Object executeQuery(Command query) {
		checkSystemConsistency();
		return tryToExecute(query);
	}

	private Object tryToExecute(Command command) {
		try {
			return command.executeOn(_system);
		} catch (Error err) {
			_journal.close();
			throw err;
		}
	}
	
	private void checkSystemConsistency() {
		if (_journal.isClosed())
			throw new IllegalStateException("An Error such as OutOfMemoryError
or StackOverflowError was thrown while executing some previous
transaction or query. The system might be in an inconsistent state.");
	}

	public static interface Command {
		Object executeOn(Object system);
	}
}

class AcidInputStream {

	public AcidInputStream(String fileName) throws IOException {}
	public Object readObject() throws IOException {return null;}

}

class AcidOutputStream {

	/** Atomically and durably replaces the previous journal file with
the new entry using file rename manoeuvres.*/
	public AcidOutputStream(String fileName, Object firstEntry) throws
IOException {}
	public void append(Object entry) throws IOException {}
	public void close() {}
	public boolean isClosed() {return false;}

}