Im looking for a way to synchronize multiple asynchronous operations. I'd like to use a BlockingQueue with a size equal to my operations but who can i wait till the Queue is full?
Im looking for something like a reversed Blocking Queue.
I need to gather the Results of each Thread at the End.
The AsyncHandler is fixed, its already a ThreadExecutor underlying, i cannot start new Threads.
//3 Times
makeAsync(new AsyncHandler() {
onSuccess() {
..
queue.put(result)
}
onFailure() {
..
}
});
//Blocking till the Queue is full
List<Results> = queue.takeAll()
Bonus Question: I need a way to end the wait when one of my Requests fails
I've never had need to do this sort of thing, but you might have some luck using a CountDownLatch or CyclicBarrier from your various threads.
What you describe with
//Blocking till the Queue is full
List<Results> results = queue.takeAll();
does not differ semantically from “take as much items as the queue’s capacity”. If you know the capacity you can achieve this by:
// preferably a constant which you also use to construct the bounded queue
int capacity;
…
List<Results> results = new ArrayList<>(capacity);
queue.drainTo(results, capacity);
while(result.size()<capacity)
queue.drainTo(results, capacity-result.size());
This will block until it has received as much items as the capacity which is, as said, the same as waiting for the queue to become full (has a size equal to its capacity) and than take all items. The only difference is that the event of the queue becoming full is not guaranteed to happen, e.g. if you intend your async operations to offer items until the queue is full, it does not work this way.
If you don’t know the capacity, you are out of luck. There is not even a guaranty that an arbitrary BlockingQueue is bounded, read, it might have an unlimited capacity.
On the other hand, if the asynchronous operations are able to detect when they have finished, they could simply collect the items in a list locally and put the entire list into a BlockingQueue<List<Results>> as a single item once they are done. Then your code waiting for it needs only a single take to get the entire list.
If you're using Java 8, do the following:
With each call to makeAsync, create a CompletableFuture<Result> instance and make it available to the AsyncHandler, and have the caller keep a reference too, say in a list.
When an async task completes normally, have it call complete(result) on its CompletableFuture instance.
When an async task completes with an error, have it call completeExceptionally(exception) on its CompletableFuture instance.
After initiating all the asynchronous tasks, have the caller call CompletableFuture.allOf(cfArray).join(). Unfortunately this takes an array, not a list, so you have to convert. The join() call will throw an exception if any one of the tasks completed with an error. Otherwise, you can collect the results from the individual CompletableFuture instances by calling their get() methods.
If you don't have Java 8, you'll have to sort of roll your own mechanism. Initialize a CountDownLatch to the number of async tasks you're going to fire off. Have each async task store its result (or an exception, or some other means of indicating failure) into a thread-safe data structure and then decrement ('countDown`) the latch. Have the caller wait for the latch to reach zero and then collect the results and errors. This isn't terribly difficult, but you have to determine a means for storing valid results as well as recording whether an error occurred, and also maintain a count manually.
If you can modify methodAsync(), then it's as simple as to use a CountDownLatch after each time you put some elements in the queue and have the main thread wait for such a CountDownLatch.
If unfortunately you cannot modify methodAsync(), then simply wrap the queue and give it a count down latch, and then override the add() method to count down this latch. The main method just wait it to be done.
Having said the above, your program structure smells not well organized.
Related
Im implimenting a java game with multiplayer and i have a sender thread to send a messages from a Queue to another player.
I have read here:
https://developer.android.com/reference/java/util/concurrent/BlockingQueue.html
"A Queue that additionally supports operations that wait for the queue to become non-empty"
which operetions and how to use them , and i know that an infinte loop that always checks if something Queue is what i want to avoid.
Since there appears to be no out-of-the-box solution to this problem, I had to implement my own.
So, what I do is that my void block() method invokes poll() and saves the item returned. Then, every method that obtains an item from the queue first checks if we have a saved item, and if so, returns it and clears it, otherwise it delegates to the underlying java blocking queue.
Luckily I only consume items from a single thread, so I do not have to worry about race conditions.
take() will allow you to wait until an element becomes available in the queue or use poll(long timeout, TimeUnit unit) to wait until a specified time.
I'm writing an application in Java which uses ExecutorService for running multiple threads.
I wish to submit multiple tasks (thousands at a time) to the Executor as Callables and when done, retrieve their result. The way I'm approaching this is each time I call submit() function, I get a Future which I store in an ArrayList. Later I pass the List to a thread which keeps iterating over it, calling future.get() function with a timeout to see if the task completed. is this the right approach or is to too inefficient?
EDIT --- More info ---
Another problem is that each Callable takes different amount of processing time. So if I simply take the first element out of the List and call get() on it, it will block while results of others may become available and the program will not know. That is why I need to keep iterating with timeouts.
thanks in advance
is this the right approach or is to too inefficient?
This is not the correct approach per se. You are needlessly iterating over ArrayList checking for task completion.
This is very simple: Just use: CompletionService. You can easily wrap your existing executor into it. From JavaDocs:
Producers submit tasks for execution. Consumers take completed tasks
and process their results in the order they complete.
In essence, CompletionService provides a way to get the result back simply by calling take(). It is a blocking function and the caller will block until the results are available.
Note that the call to Future.get will block until the answer is available. So you don't need another thread to iterate over the array list.
See here https://blogs.oracle.com/CoreJavaTechTips/entry/get_netbeans_6
new SynchronousQueue()
new LinkedBlockingQueue(1)
What is the difference? When I should use SynchronousQueue against LinkedBlockingQueue with capacity 1?
the SynchronousQueue is more of a handoff, whereas the LinkedBlockingQueue just allows a single element. The difference being that the put() call to a SynchronousQueue will not return until there is a corresponding take() call, but with a LinkedBlockingQueue of size 1, the put() call (to an empty queue) will return immediately.
I can't say that i have ever used the SynchronousQueue directly myself, but it is the default BlockingQueue used for the Executors.newCachedThreadPool() methods. It's essentially the BlockingQueue implementation for when you don't really want a queue (you don't want to maintain any pending data).
As far as I understand code above do the same things.
No, the code is not the same at all.
Sync.Q. requires to have waiter(s) for offer to succeed. LBQ will keep the item and offer will finish immediately even if there is no waiter.
SyncQ is useful for tasks handoff. Imagine you have a list w/ pending task and 3 threads available waiting on the queue, try offer() with 1/4 of the list if not accepted the thread can run the task on its own. [the last 1/4 should be handled by the current thread, if you wonder why 1/4 and not 1/3]
Think of trying to hand the task to a worker, if none is available you have an option to execute the task on your own (or throw an exception). On the contrary w/ LBQ, leaving the task in the queue doesn't guarantee any execution.
Note: the case w/ consumers and publishers is the same, i.e. the publisher may block and wait for consumers but after offer or poll returns, it ensures the task/element is to be handled.
One reason to use SynchronousQueue is to improve application performance. If you must have a hand-off between threads, you will need some synchronization object. If you can satisfy the conditions required for its use, SynchronousQueue is the fastest synchronization object I have found. Others agree. See: Implementation of BlockingQueue: What are the differences between SynchronousQueue and LinkedBlockingQueue
[Just trying to put it in (possibly) more clearer words.]
I believe the SynchronousQueue API docs states things very clearly:
A blocking queue in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.
A synchronous queue does not have any internal capacity, not even a capacity of one. You cannot peek at a synchronous queue because an element is only present when you try to remove it; you cannot insert an element (using any method) unless another thread is trying to remove it; you cannot iterate as there is nothing to iterate.
The head of the queue is the element that the first queued inserting thread is trying to add to the queue; if there is no such queued thread then no element is available for removal and poll() will return null.
And BlockingQueue API docs:
A Queue that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.
So the difference is obvious and somewhat critically subtle, especially the 3rd point below:
If the queue is empty when you are retrieving from BlockingQueue, the operation block till the new element is inserted. Also, if the queue is full when you are inserting in the BlockingQueue, the operation will block till the element is removed from the queue and a space is made for the new queue. However note that in SynchronousQueue, as operation is blocked for opposite operation (insert and remove are opposite of each other) to occur on another thread. So, unlike BlockingQueue, the blocking depends on the existence of the operation, instead of existence or non existence of an element.
As the blocking is dependent on existence of opposite operation, the element never really gets inserted in the queue. Thats why the second point: "A synchronous queue does not have any internal capacity, not even a capacity of one."
As a consequence, peek() always returns null (again, check the API doc) and iterator() returns an empty iterator in which hasNext() always returns false. (API doc). However, note that the poll() method neatly retrieves and removes the head of this queue, if another thread is currently making an element available and if no such thread exists, it returns null. (API doc)
Finally, a small note, both SynchronousQueue and LinkedBlockingQueue classes implement BlockingQueue interface.
SynchronousQueue works in a similar fashion with following major differences:
1) The size of SynchronousQueue is 0
2) put() method will only insert an element if take() method will be able to fetch that element from the queue at the same moment i.e. an element cannot be inserted if the consumer take() call is going to take some time to consume it.
SynchronousQueue - Insert only when someone will receive it at that moment itself.
Synchronous queues are basically used for handoff purposes. They do not have any capacity and a put operation is blocked until some other thread performs get operation.
If we want to safely share a variable between two threads, we can put that variable in synchrounous queue and let other thread take it from the queue.
Code Sample from https://www.baeldung.com/java-synchronous-queue
ExecutorService executor = Executors.newFixedThreadPool(2);
SynchronousQueue<Integer> queue = new SynchronousQueue<>();
Runnable producer = () -> {
Integer producedElement = ThreadLocalRandom
.current()
.nextInt();
try {
queue.put(producedElement);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
};
Runnable consumer = () -> {
try {
Integer consumedElement = queue.take();
} catch (InterruptedException ex) {
ex.printStackTrace();
}
};
executor.execute(producer);
executor.execute(consumer);
executor.awaitTermination(500, TimeUnit.MILLISECONDS);
executor.shutdown();
assertEquals(queue.size(), 0);
They are also used in CachedThreadPool to achieve an effect of unlimited(Integer.MAX) thread creation as tasks arrive.
CachedPool has coreSize as 0 and maxPoolSize as Integer.MAX with synchronous queue
As tasks arrive onto queue, other tasks are blocked until the first one is fetched out. Since it does not have any queue capacity, thread pool will create one thread and this thread will take out task allowing more tasks to be put onto the queue. This will continue until thread creation reaches maxPoolSize. Based on timeOut, idle threads maybe terminated and new ones are created without crossing the maxPoolSize.
The getQueue() method provides access to the underlying blocking queue in the ThreadPoolExecutor, but this does not seem to be safe.
A traversal over the queue returned by this function might miss updates made to the queue by the ThreadPoolExecutor.
"Method getQueue() allows access to the work queue for purposes of monitoring and debugging. Use of this method for any other purpose is strongly discouraged."
What would you do if you wanted to traverse the workQueue used by the ThreadPoolExecutor? Or is there an alternate approach?
This is a continuation of..
Choosing a data structure for a variant of producer consumer problem
Now, I am trying the multiple producer multiple consumer, but I want to use some existing threadpool, since I don't want to manage the threadpool myself, and also I want a callback when ThreadPoolExecutor has finished executing some task alongwith the ability to examine in a thread safe way the "inprogress transactions" data structure.
You can override the beforeExecute and afterExecute methods to let you know that a task has started and finished. You can override execute() to know when a task is added.
The problem you have is that the Queue is not designed to be queried and a task can be consumed before you see it. One way around this is to create you own implementation of a Queue (perhaps overriding/wrapping a ConcurrentLinkedQueue)
BTW: The queue is thread-safe, however it is not guaranteed you will see every entry.
A ConcurrentLinkedQueue.iterator() is documented as
Returns an iterator over the elements in this queue in proper sequence. The returned iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.
If you wish to copy the items in the queue and ensure that what you have in the queue has not been executed, you might try this:
a) Introduce the ability to pause and resume execution. See: http://download.oracle.com/javase/1,5.0/docs/api/java/util/concurrent/ThreadPoolExecutor.html
b) first pause the queue, then copy the queue, then resume the queue.
And then i have my own question. The problem i see is that while you execute your "Runnable", that "Runnable" is not placed in the queue, but a FutureTask "wrapper", and i cannot find any way to determine just which one of my runnables i'm looking at. So, grabbing and examining the queue is pretty useless. Does anybody know aht i missed there?
If you are following Jon Skeet's advice in your accepted answer from your previous question, then you'll be controlling access to your queues via locks. If you acquire a lock on the in-progress queue then you can guarantee that a traversal will not miss any items in it.
The problem with this of course is that while you are doing the traverse all other operations on the queue (other producers and consumers trying to access it) will block, which could have a pretty dire effect on performance.
I have a series of concurrent tasks to run. If any one of them fails, I want to interrupt them all and await termination. But assuming none of them fail, I want to wait for all of them to finish.
ExecutorCompletionService seems like almost what I want here, but there doesn't appear to be a way to tell if all of my tasks are done, except by keeping a separate count of the number of tasks. (Note that both of the examples of in the Javadoc for ExecutorCompletionService keep track of the count "n" of the tasks, and use that to determine if the service is finished.)
Am I overlooking something, or do I really have to write this code myself?
Yes, you do need to keep track if you're using an ExecutorCompletionService. Typically, you would call get() on the futures to see if an error occurred. Without iterating over the tasks, how else could you tell that one failed?
If your series of tasks is of a known size, then you should use the second example in the javadoc.
However, if you don't know the number of tasks which you will submit to the CompletionService, then you have a sort of Producer-Consumer problem. One thread is producing tasks and placing them in the ECS, another would be consuming the task futures via take(). A shared Semaphore could be used, allowing the Producer to call release() and the Consumer to call acquire(). Completion semantics would depend on your application, but a volatile or atomic boolean on the producer to indicate that it is done would suffice.
I suggest a Semaphore over wait/notify with poll() because there is a non-deterministic delay between the time a task is produced and the time that task's future is available for consumption. Therefore the consumer and producer needs to be just slightly smarter.