Yeh - I'm struggling with this myself. My intuition is 'atom signifies
multithreaded access', but I did not see a lot of rebinding in idiomatic
Clojure.
On Tuesday, May 26, 2015 at 12:47:05 PM UTC-4, Colin Yates wrote:
>
> 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 <om...@vastseas.com <javascript:>>
> 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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