Answering my own question:
- It isn't possible to have a solution that is completely non-blocking. Even
ScheduledThreadPoolExecutor keeps at least one thread around waiting for the queue to return a new task.
ThreadPoolExecutor sits on top of a BlockingQueue. When there are no tasks left, it blocks on BlockingQueue.take()- The solution has 3 moving pieces:
- A rate limiter.
- A
BlockingQueue that hides elements until the rate limiter allows their consumption.
- A
ThreadPoolExecutor that sits on top of the BlockingQueue.
The Rate Limiter
I provide my own rate limiter based on the Token Bucket algorithm algorithm to overcome RateLimiter's limitations. The source-code can be found here.
The BlockingQueue
I implementing a BlockingDeque (which extends BlockingQueue) because in the future I want to try pushing failed tasks back to the front of the queue.
RateLimitedBlockingDeque.java
import java.time.Duration;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.BlockingDeque;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.TimeUnit;
import static org.bitbucket.cowwoc.requirements.core.Requirements.requireThat;
/**
* A blocking deque of elements, in which an element can only be taken when the deque-wide delay has expired.
* <p>
* The optional capacity bound constructor argument serves as a way to prevent excessive expansion. The capacity, if
* unspecified, is equal to {@link Integer#MAX_VALUE}.
* <p>
* Even though methods that take elements, such as {@code take} or {@code poll}, respect the deque-wide delay the
* remaining methods treat them as normal elements. For example, the {@code size} method returns the count of both
* expired and unexpired elements.
* <p>
* This class and its iterator implement all of the <em>optional</em> methods of the {@link Collection} and {@link
* Iterator} interfaces.
*
* @param <E> the type of elements in the deque
* @author Gili Tzabari
*/
public final class RateLimitedBlockingDeque<E> implements BlockingDeque<E>
{
private final int capacity;
private final LinkedBlockingDeque<E> delegate;
private final Bucket rateLimit = new Bucket();
/**
* Creates a {@code RateLimitedBlockingDeque} with a capacity of {@link Integer#MAX_VALUE}.
*/
public RateLimitedBlockingDeque()
{
this.capacity = Integer.MAX_VALUE;
this.delegate = new LinkedBlockingDeque<>();
}
/**
* Creates a {@code RateLimitedBlockingDeque} with the given (fixed) capacity.
*
* @param capacity the capacity of this deque
* @throws IllegalArgumentException if {@code capacity} is less than 1
*/
public RateLimitedBlockingDeque(int capacity)
{
this.capacity = capacity;
this.delegate = new LinkedBlockingDeque<>(capacity);
}
/**
* @return the capacity of the deque
*/
public int getCapacity()
{
return capacity;
}
/**
* Indicates the rate at which elements may be taken from the queue.
*
* @param elements the number of elements that may be taken per {@code period}
* @param period indicates how often elements may be taken
* @throws NullPointerException if {@code period} is null
* @throws IllegalArgumentException if the requested rate is greater than element per nanosecond
*/
public void setRate(long elements, Duration period)
{
synchronized (rateLimit)
{
Limit newLimit = new Limit(elements, period, 0, Long.MAX_VALUE);
if (rateLimit.getLimits().isEmpty())
rateLimit.addLimit(newLimit);
else
{
Limit oldLimit = rateLimit.getLimits().iterator().next();
rateLimit.replaceLimit(oldLimit, newLimit);
}
}
}
/**
* Allows consumption of elements without limit.
*/
public void removeRate()
{
synchronized (rateLimit)
{
rateLimit.removeAllLimits();
}
}
@Override
public void addFirst(E e)
{
delegate.addFirst(e);
}
@Override
public void addLast(E e)
{
delegate.addLast(e);
}
@Override
public boolean offerFirst(E e)
{
return delegate.offerFirst(e);
}
@Override
public boolean offerLast(E e)
{
return delegate.offerLast(e);
}
@Override
public void putFirst(E e) throws InterruptedException
{
delegate.putFirst(e);
}
@Override
public void putLast(E e) throws InterruptedException
{
delegate.putLast(e);
}
@Override
public boolean offerFirst(E e, long timeout, TimeUnit unit) throws InterruptedException
{
return delegate.offerFirst(e, timeout, unit);
}
@Override
public boolean offerLast(E e, long timeout, TimeUnit unit) throws InterruptedException
{
return delegate.offerLast(e, timeout, unit);
}
@Override
public E removeFirst()
{
if (rateLimit.tryConsume())
return delegate.removeFirst();
throw new NoSuchElementException();
}
@Override
public E removeLast()
{
if (rateLimit.tryConsume())
return delegate.removeLast();
throw new NoSuchElementException();
}
@Override
public E pollFirst()
{
if (rateLimit.tryConsume())
return delegate.pollFirst();
return null;
}
@Override
public E pollLast()
{
if (rateLimit.tryConsume())
return delegate.pollLast();
return null;
}
@Override
public E takeFirst() throws InterruptedException
{
rateLimit.consume();
return delegate.takeFirst();
}
@Override
public E takeLast() throws InterruptedException
{
rateLimit.consume();
return delegate.takeLast();
}
@Override
public E pollFirst(long timeout, TimeUnit unit) throws InterruptedException
{
if (rateLimit.consume(1, timeout, unit))
return delegate.pollFirst(timeout, unit);
return null;
}
@Override
public E pollLast(long timeout, TimeUnit unit) throws InterruptedException
{
if (rateLimit.consume(1, timeout, unit))
return delegate.pollLast(timeout, unit);
return null;
}
@Override
public E getFirst()
{
return delegate.getFirst();
}
@Override
public E getLast()
{
return delegate.getLast();
}
@Override
public E peekFirst()
{
return delegate.peekFirst();
}
@Override
public E peekLast()
{
return delegate.peekLast();
}
@Override
public boolean removeFirstOccurrence(Object o)
{
return delegate.removeFirstOccurrence(o);
}
@Override
public boolean removeLastOccurrence(Object o)
{
return delegate.removeLastOccurrence(o);
}
@Override
public boolean add(E e)
{
return delegate.add(e);
}
@Override
public boolean offer(E e)
{
return delegate.offer(e);
}
@Override
public void put(E e) throws InterruptedException
{
putLast(e);
}
@Override
public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException
{
return delegate.offer(e, timeout, unit);
}
@Override
public E remove()
{
return removeFirst();
}
@Override
public E poll()
{
return pollFirst();
}
@Override
public E take() throws InterruptedException
{
return takeFirst();
}
@Override
public E poll(long timeout, TimeUnit unit) throws InterruptedException
{
return pollFirst(timeout, unit);
}
@Override
public E element()
{
return getFirst();
}
@Override
public E peek()
{
return peekFirst();
}
@Override
public int remainingCapacity()
{
return delegate.remainingCapacity();
}
@Override
public int drainTo(Collection<? super E> c)
{
int result = 0;
while (true)
{
E next = pollFirst();
if (next == null)
break;
c.add(next);
}
return result;
}
@Override
public int drainTo(Collection<? super E> c, int maxElements)
{
int result = 0;
do
{
E next = pollFirst();
if (next == null)
break;
c.add(next);
}
while (result < maxElements);
return result;
}
@Override
public void push(E e)
{
addFirst(e);
}
@Override
public E pop()
{
return removeFirst();
}
@Override
public boolean remove(Object o)
{
return removeFirstOccurrence(o);
}
@Override
public int size()
{
return delegate.size();
}
@Override
public boolean contains(Object o)
{
return delegate.contains(o);
}
@Override
public Object[] toArray()
{
return delegate.toArray();
}
@Override
public <T> T[] toArray(T[] a)
{
return delegate.toArray(a);
}
@Override
public String toString()
{
return delegate.toString();
}
@Override
public void clear()
{
delegate.clear();
}
@Override
public Iterator<E> iterator()
{
return wrap(delegate.iterator());
}
/**
* @param delegateIterator the iterator to delegate to
* @return an iterator that respects the rate-limit
*/
private Iterator<E> wrap(Iterator<E> delegateIterator)
{
return new Iterator<E>()
{
private E previousElement = null;
@Override
public boolean hasNext()
{
return delegateIterator.hasNext();
}
@Override
public E next()
{
return delegateIterator.next();
}
@Override
public void remove()
{
if (previousElement == null)
throw new IllegalStateException("next() not invoked, or remove() already invoked");
try
{
rateLimit.consume();
}
catch (InterruptedException e)
{
throw new IllegalStateException(e);
}
delegateIterator.remove();
previousElement = null;
}
};
}
@Override
public Iterator<E> descendingIterator()
{
return wrap(delegate.descendingIterator());
}
@Override
public boolean addAll(Collection<? extends E> c)
{
requireThat("c", c).isNotNull().isNotEqualTo("this", this);
boolean modified = false;
for (E e: c)
if (add(e))
modified = true;
return modified;
}
@Override
public boolean isEmpty()
{
return delegate.isEmpty();
}
@Override
public boolean containsAll(Collection<?> c)
{
return delegate.containsAll(c);
}
@Override
public boolean removeAll(Collection<?> c)
{
Iterator<E> i = iterator();
boolean modified = true;
while (i.hasNext())
{
E element = i.next();
if (c.contains(element))
{
i.remove();
modified = true;
}
}
return modified;
}
@Override
public boolean retainAll(Collection<?> c)
{
Iterator<E> i = iterator();
boolean modified = true;
while (i.hasNext())
{
E element = i.next();
if (!c.contains(element))
{
i.remove();
modified = true;
}
}
return modified;
}
@Override
public int hashCode()
{
return delegate.hashCode();
}
@Override
public boolean equals(Object obj)
{
return delegate.equals(obj);
}
}
