Re: ACIDity of Prevayler 2.6

Thanks Justin. It's more clear for me now.

But only a small I/O is required to mark the tx as valid in the journal 
after a successful execution (not a full tx write on disk). It's 
applicable even for a continous stream.

So the the additional (potential) latency for queries is just the "mark" 
I/O at the end of the tx execution.
For some categories of server applications, this is a small price to pay 
to be sure that the journal never contains tx that could not be executed 
correctly.


On 17/11/2012 08:44, Justin T. Sampson wrote:
> Arnaud -- You've got things a bit backwards. Yes, larger latencies are
> acceptable for transactions than for queries, but that's exactly how
> Prevayler behaves: Queries execute and return immediately whereas
> transactions have to wait for writing to disk before executing and
> returning. If transactions were written to disk after executing, then
> both queries and transactions would execute immediately but both would
> wait for writing to disk before returning. Throughput would be similar,
> but query latency would be worse than it is now.
>
> Transactions are already serialized concurrently (in memory) without
> holding any locks, so don't worry about that. And the journal is written
> as a continuous stream, not individual files, so moving/renaming is not
> an option (and would be significantly slower).
>
>
> On Fri, Nov 16, 2012 at 2:06 AM, Arnaud Masson <[email protected]
> <mailto:[email protected]>> wrote:
>
>     Many systems have much more queries than transactions,
>     so transaction latency wouldn't be so critical.
>
>     Moreover a transaction could be physically pre-serialized (or maybe in
>     parallel thread) on the disk, in some temporary location, before the
>     lock scope.
>     After tx successful execution, just a quick "file move/rename" operation
>     would be required to commit it to the journal, not a full serialization.
>
>
>
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