private Bucket GetCurrentBucket()
{
long currentTime = time.CurrentTimeInMillis;
/* a shortcut to try and get the most common result of immediately finding the current bucket */
/*
* Retrieve the latest bucket if the given time is BEFORE the end of the bucket window, otherwise it returns NULL.
* NOTE: This is thread-safe because it's accessing 'buckets' which is a LinkedBlockingDeque
*/
Bucket currentBucket = _buckets.PeekLast;
if (currentBucket != null && currentTime < currentBucket._windowStart + _bucketSizeInMilliseconds)
{
// if we're within the bucket 'window of time' return the current one
// NOTE: We do not worry if we are BEFORE the window in a weird case of where thread scheduling causes that to occur,
// we'll just use the latest as long as we're not AFTER the window
return(currentBucket);
}
/* if we didn't find the current bucket above, then we have to create one */
/*
* The following needs to be synchronized/locked even with a synchronized/thread-safe data structure such as LinkedBlockingDeque because
* the logic involves multiple steps to check existence, create an object then insert the object. The 'check' or 'insertion' themselves
* are thread-safe by themselves but not the aggregate algorithm, thus we put this entire block of logic inside synchronized.
* I am using a tryLock if/then (https://download.oracle.com/javase/6/docs/api/java/util/concurrent/locks/Lock.html#tryLock())
* so that a single thread will get the lock and as soon as one thread gets the lock all others will go the 'else' block
* and just return the currentBucket until the newBucket is created. This should allow the throughput to be far higher
* and only slow down 1 thread instead of blocking all of them in each cycle of creating a new bucket based on some testing
* (and it makes sense that it should as well).
* This means the timing won't be exact to the millisecond as to what data ends up in a bucket, but that's acceptable.
* It's not critical to have exact precision to the millisecond, as long as it's rolling, if we can instead reduce the impact synchronization.
* More importantly though it means that the 'if' block within the lock needs to be careful about what it changes that can still
* be accessed concurrently in the 'else' block since we're not completely synchronizing access.
* For example, we can't have a multi-step process to add a bucket, remove a bucket, then update the sum since the 'else' block of code
* can retrieve the sum while this is all happening. The trade-off is that we don't maintain the rolling sum and let readers just iterate
* bucket to calculate the sum themselves. This is an example of favoring write-performance instead of read-performance and how the tryLock
* versus a synchronized block needs to be accommodated.
*/
bool lockTaken = false;
Monitor.TryEnter(newBucketLock, ref lockTaken);
if (lockTaken)
{
currentTime = time.CurrentTimeInMillis;
try
{
if (_buckets.PeekLast == null)
{
// the list is empty so create the first bucket
Bucket newBucket = new Bucket(currentTime, _bucketDataLength);
_buckets.AddLast(newBucket);
return(newBucket);
}
else
{
// We go into a loop so that it will create as many buckets as needed to catch up to the current time
// as we want the buckets complete even if we don't have transactions during a period of time.
for (int i = 0; i < _numberOfBuckets; i++)
{
// we have at least 1 bucket so retrieve it
Bucket lastBucket = _buckets.PeekLast;
if (currentTime < lastBucket._windowStart + _bucketSizeInMilliseconds)
{
// if we're within the bucket 'window of time' return the current one
// NOTE: We do not worry if we are BEFORE the window in a weird case of where thread scheduling causes that to occur,
// we'll just use the latest as long as we're not AFTER the window
return(lastBucket);
}
else if (currentTime - (lastBucket._windowStart + _bucketSizeInMilliseconds) > _timeInMilliseconds)
{
// the time passed is greater than the entire rolling counter so we want to clear it all and start from scratch
Reset();
// recursively call getCurrentBucket which will create a new bucket and return it
// return GetCurrentBucket();
Bucket newBucket = new Bucket(currentTime, _bucketDataLength);
_buckets.AddLast(newBucket);
return(newBucket);
}
else
{
// we're past the window so we need to create a new bucket
Bucket[] allBuckets = _buckets.Array;
// create a new bucket and add it as the new 'last' (once this is done other threads will start using it on subsequent retrievals)
_buckets.AddLast(new Bucket(lastBucket._windowStart + _bucketSizeInMilliseconds, _bucketDataLength));
// we created a new bucket so let's re-generate the PercentileSnapshot (not including the new bucket)
currentPercentileSnapshot = new PercentileSnapshot(allBuckets);
}
}
// we have finished the for-loop and created all of the buckets, so return the lastBucket now
return(_buckets.PeekLast);
}
}
finally
{
Monitor.Exit(newBucketLock);
}
}
else
{
currentBucket = _buckets.PeekLast;
if (currentBucket != null)
{
// we didn't get the lock so just return the latest bucket while another thread creates the next one
return(currentBucket);
}
else
{
// the rare scenario where multiple threads raced to create the very first bucket
// wait slightly and then use recursion while the other thread finishes creating a bucket
if (Time.WaitUntil(() => { return(_buckets.PeekLast != null); }, 500))
{
return(_buckets.PeekLast);
}
else
{
return(null);
}
}
}
}
