Re: [Prevayler-coders] The PrevaylerJr Game

[Justin T. S. <ju...@kr...>]

Well, queries aren't supposed to modify the prevalent system at all,
so it's actually okay for a query to throw an Error. The system can
keep on running without experiencing inconsistency.

The only exception might be if you're doing some kind of lazy
computations inside queries -- conceptually read-only, but actually
read-write. In that case, queries must not run concurrently with each
other anyway.

-Justin


On Wed, Jul 2, 2008 at 11:57 PM, Klaus Wuestefeld
<kla...@gm...> wrote:

> Justin,
>
> Do you see any way of extending this "no inconsistency" guarantee for
> queries running in parallel when one of them throws an Error?
>
> Will some sort of gate (AtomicReference<Boolean>?), just before the
> queries return, work? Or might the scheduler play tricks on us?
>
> Klaus.
>
>
> On Thu, Jul 3, 2008 at 3:50 AM, Klaus Wuestefeld
> <kla...@gm...> wrote:
>>> 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;}
>>
>> }
>>
>
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