Clojure agents consuming from a queue

Question

I'm trying to figure out the best way to use agents to consume items from a Message Queue (Amazon SQS). Right now I have a function (process-queue-item) that grabs an items from the queue, and processes it.

I want to process these items concurrently, but I can't wrap my head around how to control the agents. Basically I want to keep all of the agents busy as much as possible without pulling to many items from the Queue and developing a backlog (I'll have this running on a couple of machines, so items need to be left in the queue until they are really needed).

Can anyone give me some pointers on improving my implementation?

(def active-agents (ref 0))

(defn process-queue-item [_]
  (dosync (alter active-agents inc))
  ;retrieve item from Message Queue (Amazon SQS) and process
  (dosync (alter active-agents dec)))

(defn -main []
  (def agents (for [x (range 20)] (agent x)))

  (loop [loop-count 0]

    (if (< @active-agents 20)
      (doseq [agent agents]
        (if (agent-errors agent)
          (clear-agent-errors agent))
        ;should skip this agent until later if it is still busy processing (not sure how)
        (send-off agent process-queue-item)))

    ;(apply await-for (* 10 1000) agents)
    (Thread/sleep  10000)
    (logging/info (str "ACTIVE AGENTS " @active-agents))
    (if (> 10 loop-count)
      (do (logging/info (str "done, let's cleanup " count))
       (doseq [agent agents]
         (if (agent-errors agent)
           (clear-agent-errors agent)))
       (apply await agents)
       (shutdown-agents))
      (recur (inc count)))))

Answer 1

What you are asking for is a way to keep handing out tasks but with some upper limit. One simple approach to this is to use a semaphore to coordinate the limit. Here is how I would approach it:

(let [limit (.availableProcessors (Runtime/getRuntime))
      ; note: you might choose limit 20 based upon your problem description
      sem (java.util.concurrent.Semaphore. limit)]
  (defn submit-future-call
    "Takes a function of no args and yields a future object that will
    invoke the function in another thread, and will cache the result and
    return it on all subsequent calls to deref/@. If the computation has
    not yet finished, calls to deref/@ will block. 
    If n futures have already been submitted, then submit-future blocks
    until the completion of another future, where n is the number of
    available processors."  
    [#^Callable task]
    ; take a slot (or block until a slot is free)
    (.acquire sem)
    (try
      ; create a future that will free a slot on completion
      (future (try (task) (finally (.release sem))))
      (catch java.util.concurrent.RejectedExecutionException e
        ; no task was actually submitted
        (.release sem)
        (throw e)))))

(defmacro submit-future
  "Takes a body of expressions and yields a future object that will
  invoke the body in another thread, and will cache the result and
  return it on all subsequent calls to deref/@. If the computation has
  not yet finished, calls to deref/@ will block.
  If n futures have already been submitted, then submit-future blocks
  until the completion of another future, where n is the number of
  available processors."  
  [& body] `(submit-future-call (fn [] ~@body)))

#_(example
    user=> (submit-future (reduce + (range 100000000)))
    #<core$future_call$reify__5782@6c69d02b: :pending>
    user=> (submit-future (reduce + (range 100000000)))
    #<core$future_call$reify__5782@38827968: :pending>
    user=> (submit-future (reduce + (range 100000000)))
    ;; blocks at this point for a 2 processor PC until the previous
    ;; two futures complete
    #<core$future_call$reify__5782@214c4ac9: :pending>
    ;; then submits the job

With that in place now you just need to coordinate how the tasks themselves are taken. It sounds like you already have the mechanisms in place to do that. Loop (submit-future (process-queue-item))

Answer 2

Perhaps you could use the seque function? Quoting (doc seque):

clojure.core/seque
([s] [n-or-q s])
  Creates a queued seq on another (presumably lazy) seq s. The queued
  seq will produce a concrete seq in the background, and can get up to
  n items ahead of the consumer. n-or-q can be an integer n buffer
  size, or an instance of java.util.concurrent BlockingQueue. Note
  that reading from a seque can block if the reader gets ahead of the
  producer.

What I have in mind is a lazy sequence getting queue items over the network; you'd wrap this in seque, put that in a Ref and have worker Agents consume items off of this seque. seque returns something which looks just like a regular seq from the point of view of your code, with the queue magic happening in a transparent way. Note that if the sequence you put inside is chunked, then it'll still be forced a chunk at a time. Also note that the initial call to seque itself seems to block until an initial item or two is obtained (or a chunk, as the case may be; I think that's more to do with the way lazy sequences work than seque itself, though).

A sketch of the code (a really sketchy one, not tested at all):

(defn get-queue-items-seq []
  (lazy-seq
   (cons (get-queue-item)
         (get-queue-items-seq))))

(def task-source (ref (seque (get-queue-items-seq))))

(defn do-stuff []
  (let [worker (agent nil)]
    (if-let [result
             (dosync
               (when-let [task (first @task-source)]
                (send worker (fn [_] (do-stuff-with task)))))]
      (do (await worker)
          ;; maybe do something with worker's state
          (do-stuff))))) ;; continue working

(defn do-lots-of-stuff []
  (let [fs (doall (repeatedly 20 #(future (do-stuff))))]
    fs)))

Actually you'd probably want a more complex producer of the queue item seq so that you can ask it to stop producing new items (a necessity if the whole thing is to be able to be shut down gracefully; the futures will die when the task source runs dry, use future-done? to see if they've done so already). And that's just something I can see at first glance... I'm sure there's more things to polish here. I think that the general approach would work, though.

Answer 3

(let [switch (atom true) ; a switch to stop workers
      workers (doall 
                (repeatedly 20 ; 20 workers pulling and processing items from SQS
                  (future (while @switch 
                            (retrieve item from Amazon SQS and process)))))]
  (Thread/sleep 100000) ; arbitrary rule to decide when to stop ;-)
  (reset! switch false) ; stop !
  (doseq [worker workers] @worker)) ; waiting for all workers to be done