Added to TODO.detail/replication.
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> > I had thought that the pre-commit information could be stored in an
> > auxiliary table by the middleware program ; we would then have
> > to re-implement some sort of higher-level WAL (I thought of the list
> > of the commands performed in the current transaction, with a sequence
> > number for each of them that would guarantee correct ordering between
> > concurrent transactions in case of a REDO). But I fear I am missing
>
> This wouldn't work for READ COMMITTED isolation level.
> But why do you want to log commands into WAL where each modification
> is already logged in, hm, correct order?
> Well, it has sense if you're looking for async replication but
> you need not in two-phase commit for this and should aware about
> problems with READ COMMITTED isolevel.
>
> Back to two-phase commit - it's easiest part of work required for
> distributed transaction processing.
> Currently we place single commit record to log and transaction is
> committed when this record (and so all other transaction records)
> is on disk.
> Two-phase commit:
>
> 1. For 1st phase we'll place into log "prepared-to-commit" record
> and this phase will be accomplished after record is flushed on disk.
> At this point transaction may be committed at any time because of
> all its modifications are logged. But it still may be rolled back
> if this phase failed on other sites of distributed system.
>
> 2. When all sites are prepared to commit we'll place "committed"
> record into log. No need to flush it because of in the event of
> crash for all "prepared" transactions recoverer will have to
> communicate other sites to know their statuses anyway.
>
> That's all! It is really hard to implement distributed lock- and
> communication- managers but there is no problem with logging two
> records instead of one. Period.
>
> Vadim
>
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