Well, traditional transactions map to refs in Clojure.
You could use a ref, as in any transactional stuff must be performed
in a dosync
form.
However, using a agent is a much cleaner solution for your needs IMHO.
An agent, by definition, can be executing only one operation at a given
time, and other requests/actions are queued.
So, an agent would guarantee that only 1 task is removed from the queue and
then served.
If a different thread calls dequeue, its actually sent to the agent's
internal action queue.
But dereferencing the agent would return you the value in the agent at that
given time itself, not after executing the task.
If you would actually want to wait until the de-queued task is finished,
then use await to wait until all actions sent to the agent are served.
On Wed, Mar 6, 2013 at 10:06 PM, bruce li <leilmy...@gmail.com> wrote:
> Hello,
> I'm working on a piece of code that uses congomongo to access mongodb.
> What I want to implement is to use one collection of the DB as a
> queue(let's say, it's called "task").
>
> Using congomongo, it's easy to fetch a task that is of status :queue :
> (def t (fetch-one :task :where {:status :queue})),
> and also it's simple to update it: (update! :task t (assoc t :status
> :running))
>
> But how to make a safe and consistent "dequeue" operation in concurrent
> context? A naive dequeue should look like this:
>
> (let [t (fetch-one:task :where {:status :queue})]
> (update! :task t (assoc t :status :running))
> t)
>
> But taking concurrency into consideration, this implementation is
> error-prone, at least from my point of view. Consider this:
>
> When 2 threads are fetching the task, it's very likely that the task is
> fetched twice using the previous piece of code and they would be executed
> twice.
>
> Do we have something like transaction? Something that will enforce the
> "fetch-one" and "update!" statement are both executed before another
> “fetch-one" operation is adopted?
>
> What I can think of is to use an agent. And my code looks like this:
>
> (def db-agent (agent nil))
> (defn dequeue []
> (letfn [(do-dequeue [da]
> (let [task (mongo/fetch-one :task :where {:status :queue})]
> (when task
> (mongo/update! :task feed (assoc task :status :running)))
> task))]
> (send db-agent do-dequeue)
> @db-agent)))
>
> However, I still doubt the correctness of this solution. Right before the
> @db-agent is called, won't another thread call "dequeue", which will
> involve another "send” and change the value of db-agent?
>
> I'm wondering if anyone can help. Thanks.
>
>
>
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Akhil Wali
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