Hi Omri, I guess I could, using an atom to indicate multiple disparate chunks
of code will update it (all within the same thread) felt idiomatic. - the
initial (binding) is at the transaction level, subject updates are within the
transaction.
> On 26 May 2015, at 17:38, Omri Hurvitz <o...@vastseas.com> wrote:
>
> If you always access *callbacks* as a thread-local, IE in the same thread,
> why do you need it to be an atom? You can just rebind it using set! instead
> of swap!.
>
> Beyond that, I think that using thread locals this way is very useful when
> you want to limit the need for implementations to know your specific
> implementation. The other way would be to pass the transaction, or a generic
> environment, around, which tends to be ugly and sometimes confusing.
>
> Just make sure not to use it a lot and to document well.
>
> Disclaimer - while I have many years of development experience, I am
> relatively new to Clojure, but I read a lot of Clojure code at this point.
>
> On Saturday, May 23, 2015 at 6:16:15 AM UTC-4, Colin Yates wrote:
> Hi,
>
> My use-case is that I need to have a bunch of state which differs for each
> (web) request but is accessible via a var. Specifically I need to allow code
> to access the current transaction and register one or callbacks that are
> executed if/after that transaction is committed.
>
> In Java I would store these as thread locals as each web request comes in as
> a new thread. In Clojure, the equivalent seems to be dynamic binding.
>
> Am I right in thinking that the following infrastructure is correct,
> specifically the use of thread-bound?. And when would I use bound? rather
> than thread-bound?
>
> To be clear, the outer 'service' will call 'do-in-tx', code evaluated as a
> result of 'do-in-tx' will access the current tx by calling 'with-tx'.
>
> (def ^:dynamic *tx*)
> (def ^:dynamic *callbacks*)
>
> (defn- execute-in-tx [db f]
> (jdbc/with-db-transaction
> [tx db]
> (binding [*tx* tx]
> (f tx))))
>
> (defn- execute-callbacks []
> (doseq [[id cb] @*callbacks*] (cb)))
>
> (defn do-in-tx [db f]
> (when (thread-bound? #'*tx*)
> (throw (IllegalArgumentException. "Nested transactions are not
> supported!.")))
> (binding [*callbacks* (atom {})]
> (execute-in-tx db f)
> (execute-callbacks)))
>
> (defn with-tx
> "Call back the specified function providing the current transaction.
>
> If there is no current transaction then an (IllegalArgument)Exception is
> thrown. "
> [f]
> (when-not (thread-bound? #'*tx*)
> (throw (IllegalArgumentException. "No transaction found!")))
> (f *tx*))
>
> (defn register-cb [id cb]
> (when-not (thread-bound? #'*callbacks*)
> (throw (IllegalArgumentException. "No transaction found!")))
> (swap! *callbacks* assoc id cb))
>
>
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