Consumer/producer problem: pause production on slow consumption

Denis Kulagin :

I have a producer that reads blocks of text from the disk. Multiple consumers are doing computations on that blocks.

I would like producer to pause reading data from the disk if there are more that n blocks currently being computed over.

Have put it in pseudocode to illustrate what I would like to achieve.

// "produceBlocks" reads blocks from disk one by one
// and feeds them to lambda
produceBlocks(block -> {
  // (!) if activeCounter exceeds a THRESHOLD, then pause

  executorService.submit(() -> { 
     activeCounter.incrementAndGet();

     // do some work

     activeCounter.decrementAndGet();
  });
});

Alexei Kaigorodov :

"I would like producer to pause reading data from the disk if there are more that n blocks currently being computed over." The real task descrition is slightly different: the producer, before reading data from the disk, should aquire a permit to do so. If your producer is a thread, then natural facility to manage permits is Semaphore. Initialy it contains n permits. The producer, to read a block, aguires 1 permit with Semaphore::aquire. When the block is processed by the consumer, consumer releases 1 permit with Semaphore::release.

Another approach is to combine blocks and permits. Similary to the output queue from producer to consumer, create an input blocking queue for blocks. Initualy put there n blocks. The producer, to read a block, first takes the next block from that queue. Consumer, after handling a block, returns it to the input queue.