Re: A question about journal creation

Klaus, that should work well. Thanks for a much simpler solution.

-pb

On Jul 25, 2013, at 2:58 PM, Klaus Wuestefeld <[email protected]> wrote:

> Since closing the prevalent system and restarting it is a cost you are
> willing to pay, if you do that before executing the transaction group,
> a new journal will be created,
> 
> On Thu, Jul 25, 2013 at 2:15 PM, Paul Bennett <[email protected]> wrote:
>> We have a situation where we would like to run a group of transactions, and
>> be able to undo them as a group. We could define inverses on the
>> transactions and re-run, but this is complicated. In the context in which
>> they run, there would only be a single user on the system (an
>> administrator). The system would leave the single-user context with either
>> the group executed as normal, or not executed at all. There would be no
>> other transactions executing on the system while this group is executed and
>> possibly undone.
>> 
>> Given these constraints, we think there is a simple implementation: simply
>> remove from the journal those transactions that need to be undone. We would
>> do this by taking a snapshot before the transactions are executed, starting
>> a new journal, and executing the transactions. Then, to undo, we simply
>> remove that new journal file, and reload the last snapshot.
>> 
>> So:
>> 
>> 1) Does this sound like a plausible solution? Have we missed something that
>> makes this unworkable?
>> 
>> 2) The only thing missing from this to be done is the ability to control
>> explicitly the transaction boundaries on which a journal is created i.e.
>> there is no equivalent of takeSnapshot() for starting a new journal.
>> Obviously this has to be synced with execution and journal sync, so we would
>> propose to implement this by adding a boolean to the execute(Transaction t)
>> method, which would ultimately add a flag to TransactionCapsule, indicating
>> it should explicitly start a new journal before serialization - similar to
>> the test in PersistentJournal#append using the isOutputJournalStillValid()
>> method.
>> 
>> Does this sound like a good/viable solution?
>> 
>> Any comments/alternative ideas would be most welcome!
>> 
>> Thanks in advance,
>> 
>> -pb
>> 
>> 
>> 
>> 
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> 
> 
> 
> -- 
> Valeu, Klaus.
> 
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