I was trying to construct a simple example of what I actually have in
my apps that use pooling on top of java.jdbc. My actual code *does*
work to create a singleton but you're right, I've contracted my code
too far in trying to create a simple example of it... I'll have
another attempt!
Sean
On Aug 22, 3:04 am, "Meikel Brandmeyer (kotarak)" <m...@kotka.de>
wrote:
> Am Montag, 22. August 2011 11:53:32 UTC+2 schrieb faenvie:
> > yesterday i came across the following use of
> > the delay-macro:
>
> > (defn pooled-data-source
> > [db-connection-settings]
> > ; this Fn creates and returns object of type PooledDataSource
> > )
>
> > (defn pooled-data-source-as-singleton
> > [db-connection-settings]
> > (let [datasource (delay (pooled-data-source db-connection-
> > settings))]
> > @datasource))
>
> > i am not sure, what the exact semantic of this is and if
> > it's safe in a multithreaded context.
>
> > What is the difference to using def/defonce ?
>
> > Someone who can explain ?
>
> This snippet does not work as the original author intended. You can remove
> the whole delay incantations and everything will work as before. That's
> because each call of pooled-data-source-as-singleton creates a new delay,
> forces it and throws it away immediately. What probably was intended, is the
> use of memoize:
>
> (def ^{:arglists ([db-connection-settings])} pooled-data-source-as-singleton
> (memoize pooled-data-source))
>
> The intention is to delay the creation of the datapool instance until
> runtime and not do it on load time. This is also nice if you have your db
> connection configurable in some non-code form. If you have only one db
> connection, which is hardwired in some Var at load time, you can go with the
> following:
>
> (let [db-connection-pool (delay (pooled-data-source
> db-connection-settings))]
> (defn pooled-data-source-as-singleton
> []
> @db-connection-pool))
>
> This is maybe, what the original author had in mind.
>
> Hope, I'm not too far off.
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