On Jun 26, 4:21 pm, Mark Mandel <[email protected]> wrote:
> > What Transfer
> > instance is now inside that object? I thought Transfer objects kept a
> > reference to the Transfer instance that created them.
>
> They keep the Transfer instance that was created with them.... oooh.. I
> start to see the problem. While the cache is shared, the object will always
> make queries against the datasource it was originally created with. That's
> no good for this scenario.
Bingo. The locks weren't what I was talking about. :)
>
> Maybe changing datasource details on a per thread basis is actually the
> ideal way to go then.
>
> So you could have the read DB as the default database, and when you are
> ready to switch, you could do:
>
> getDatasource().setCurrentThreadDatasource("writeDatasource");
>
> getTransfer().save(fooObject);
>
> ...
>
> Anyone see any issues in that approach?
>
One major issue I see with that is it requires you to bleed the
Datasource all over an application. The datasource isn't available on
Transfer, only on the factory. It's no drop in solution, and you lose
the abstraction of the datasource name you currently get with the XML
or Configuration object.
It's especially hairy on an existing application where you'd need to
go and modify every place that talks to Transfer to manually change
the datasource name. And make sure to add a try catch each time else
you might leave the wrong datasource on a thread.
You couldn't handle this with AOP either, because if you wrapped every
save method with an advice that changed the datasource and then
changed it back, that method could potentially call methods for
validation or any number of other things on the object, causing a lazy
load of a collection or a proxy, and again issuing queries against the
writing datasource. You'd have to be extra careful and it'd be messy.
If this was handled in the main transfer configuration with two
datasource names then any existing or future application can have this
added scalability feature without any extra code, just a few more line
of XML or an extra method call on the Configuration object.
<?xml version="1.0 ?>
<datasources>
<datasource type="read">
<name>fastreplicated</name>
<username></username>
<password></password>
</datasource>
<datasource type="write">
<name>slowwriting</name>
<username></username>
<password></password>
</datasource>
</datasources>
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