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SimpleQueueCache.cs
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SimpleQueueCache.cs
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using System;
using System.Collections.Generic;
using Microsoft.Extensions.Logging;
using Orleans.Runtime;
using Orleans.Streams;
namespace Orleans.Providers.Streams.Common
{
internal class CacheBucket
{
// For backpressure detection we maintain a histogram of 10 buckets.
// Every bucket records how many items are in the cache in that bucket
// and how many cursors are poinmting to an item in that bucket.
// We update the NumCurrentItems when we add and remove cache item (potentially opening or removing a bucket)
// We update NumCurrentCursors every time we move a cursor
// If the first (most outdated bucket) has at least one cursor pointing to it, we say we are under back pressure (in a full cache).
internal int NumCurrentItems { get; private set; }
internal int NumCurrentCursors { get; private set; }
internal void UpdateNumItems(int val)
{
NumCurrentItems = NumCurrentItems + val;
}
internal void UpdateNumCursors(int val)
{
NumCurrentCursors = NumCurrentCursors + val;
}
}
internal class SimpleQueueCacheItem
{
internal IBatchContainer Batch;
internal bool DeliveryFailure;
internal StreamSequenceToken SequenceToken;
internal CacheBucket CacheBucket;
}
/// <summary>
/// A queue cache that keeps items in memory
/// </summary>
public class SimpleQueueCache : IQueueCache
{
private readonly LinkedList<SimpleQueueCacheItem> cachedMessages;
private readonly int maxCacheSize;
private readonly ILogger logger;
private readonly List<CacheBucket> cacheCursorHistogram; // for backpressure detection
private const int NUM_CACHE_HISTOGRAM_BUCKETS = 10;
private readonly int CACHE_HISTOGRAM_MAX_BUCKET_SIZE;
/// <summary>
/// Number of items in the cache
/// </summary>
public int Size => cachedMessages.Count;
/// <summary>
/// The limit of the maximum number of items that can be added
/// </summary>
public int GetMaxAddCount()
{
return CACHE_HISTOGRAM_MAX_BUCKET_SIZE;
}
/// <summary>
/// SimpleQueueCache Constructor
/// </summary>
/// <param name="cacheSize"></param>
/// <param name="logger"></param>
public SimpleQueueCache(int cacheSize, ILogger logger)
{
cachedMessages = new LinkedList<SimpleQueueCacheItem>();
maxCacheSize = cacheSize;
this.logger = logger;
cacheCursorHistogram = new List<CacheBucket>();
CACHE_HISTOGRAM_MAX_BUCKET_SIZE = Math.Max(cacheSize / NUM_CACHE_HISTOGRAM_BUCKETS, 1); // we have 10 buckets
}
/// <summary>
/// Returns true if this cache is under pressure.
/// </summary>
public virtual bool IsUnderPressure()
{
return cacheCursorHistogram.Count >= NUM_CACHE_HISTOGRAM_BUCKETS;
}
/// <summary>
/// Ask the cache if it has items that can be purged from the cache
/// (so that they can be subsequently released them the underlying queue).
/// </summary>
/// <param name="purgedItems"></param>
public virtual bool TryPurgeFromCache(out IList<IBatchContainer> purgedItems)
{
purgedItems = null;
if (cachedMessages.Count == 0) return false; // empty cache
if (cacheCursorHistogram.Count == 0) return false; // no cursors yet - zero consumers basically yet.
if (cacheCursorHistogram[0].NumCurrentCursors > 0) return false; // consumers are still active in the oldest bucket - fast path
var allItems = new List<IBatchContainer>();
while (cacheCursorHistogram.Count > 0 && cacheCursorHistogram[0].NumCurrentCursors == 0)
{
List<IBatchContainer> items = DrainBucket(cacheCursorHistogram[0]);
allItems.AddRange(items);
cacheCursorHistogram.RemoveAt(0); // remove the last bucket
}
purgedItems = allItems;
Log(logger, "TryPurgeFromCache: purged {PurgedItemsCount} items from cache.", purgedItems.Count);
return true;
}
private List<IBatchContainer> DrainBucket(CacheBucket bucket)
{
var itemsToRelease = new List<IBatchContainer>(bucket.NumCurrentItems);
// walk all items in the cache starting from last
// and remove from the cache the oness that reside in the given bucket until we jump to a next bucket
while (bucket.NumCurrentItems > 0)
{
SimpleQueueCacheItem item = cachedMessages.Last.Value;
if (item.CacheBucket.Equals(bucket))
{
if (!item.DeliveryFailure)
{
itemsToRelease.Add(item.Batch);
}
bucket.UpdateNumItems(-1);
cachedMessages.RemoveLast();
}
else
{
// this item already points into the next bucket, so stop.
break;
}
}
return itemsToRelease;
}
/// <summary>
/// Add a list of message to the cache
/// </summary>
/// <param name="msgs"></param>
public virtual void AddToCache(IList<IBatchContainer> msgs)
{
if (msgs == null) throw new ArgumentNullException(nameof(msgs));
Log(logger, "AddToCache: added {ItemCount} items to cache.", msgs.Count);
foreach (var message in msgs)
{
Add(message, message.SequenceToken);
}
}
/// <summary>
/// Acquire a stream message cursor. This can be used to retrieve messages from the
/// cache starting at the location indicated by the provided token.
/// </summary>
/// <param name="streamIdentity"></param>
/// <param name="token"></param>
/// <returns></returns>
public virtual IQueueCacheCursor GetCacheCursor(IStreamIdentity streamIdentity, StreamSequenceToken token)
{
var cursor = new SimpleQueueCacheCursor(this, streamIdentity, logger);
InitializeCursor(cursor, token);
return cursor;
}
internal void InitializeCursor(SimpleQueueCacheCursor cursor, StreamSequenceToken sequenceToken)
{
Log(logger, "InitializeCursor: {Cursor} to sequenceToken {SequenceToken}", cursor, sequenceToken);
// Nothing in cache, unset token, and wait for more data.
if (cachedMessages.Count == 0)
{
UnsetCursor(cursor, sequenceToken);
return;
}
// if no token is provided, set token to item at end of cache
sequenceToken = sequenceToken ?? cachedMessages.First?.Value?.SequenceToken;
// If sequenceToken is too new to be in cache, unset token, and wait for more data.
if (sequenceToken.Newer(cachedMessages.First.Value.SequenceToken))
{
UnsetCursor(cursor, sequenceToken);
return;
}
LinkedListNode<SimpleQueueCacheItem> lastMessage = cachedMessages.Last;
// Check to see if offset is too old to be in cache
if (sequenceToken.Older(lastMessage.Value.SequenceToken))
{
throw new QueueCacheMissException(sequenceToken, cachedMessages.Last.Value.SequenceToken, cachedMessages.First.Value.SequenceToken);
}
// Now the requested sequenceToken is set and is also within the limits of the cache.
// Find first message at or below offset
// Events are ordered from newest to oldest, so iterate from start of list until we hit a node at a previous offset, or the end.
LinkedListNode<SimpleQueueCacheItem> node = cachedMessages.First;
while (node != null && node.Value.SequenceToken.Newer(sequenceToken))
{
// did we get to the end?
if (node.Next == null) // node is the last message
break;
// if sequenceId is between the two, take the higher
if (node.Next.Value.SequenceToken.Older(sequenceToken))
break;
node = node.Next;
}
// return cursor from start.
SetCursor(cursor, node);
}
internal void RefreshCursor(SimpleQueueCacheCursor cursor, StreamSequenceToken sequenceToken)
{
Log(logger, "RefreshCursor: {RefreshCursor} to sequenceToken {SequenceToken}", cursor, sequenceToken);
// set if unset
if (!cursor.IsSet)
{
InitializeCursor(cursor, cursor.SequenceToken ?? sequenceToken);
}
}
/// <summary>
/// Acquires the next message in the cache at the provided cursor
/// </summary>
/// <param name="cursor"></param>
/// <param name="batch"></param>
/// <returns></returns>
internal bool TryGetNextMessage(SimpleQueueCacheCursor cursor, out IBatchContainer batch)
{
Log(logger, "TryGetNextMessage: {Cursor}", cursor);
batch = null;
if (!cursor.IsSet) return false;
// If we are at the end of the cache unset cursor and move offset one forward
if (cursor.Element == cachedMessages.First)
{
UnsetCursor(cursor, null);
}
else // Advance to next:
{
AdvanceCursor(cursor, cursor.Element.Previous);
}
batch = cursor.Element?.Value.Batch;
return (batch != null);
}
private void AdvanceCursor(SimpleQueueCacheCursor cursor, LinkedListNode<SimpleQueueCacheItem> item)
{
Log(logger, "UpdateCursor: {Cursor} to item {Item}", cursor, item.Value.Batch);
cursor.Element.Value.CacheBucket.UpdateNumCursors(-1); // remove from prev bucket
cursor.Set(item);
cursor.Element.Value.CacheBucket.UpdateNumCursors(1); // add to next bucket
}
internal void SetCursor(SimpleQueueCacheCursor cursor, LinkedListNode<SimpleQueueCacheItem> item)
{
Log(logger, "SetCursor: {Cursor} to item {Item}", cursor, item.Value.Batch);
cursor.Set(item);
cursor.Element.Value.CacheBucket.UpdateNumCursors(1); // add to next bucket
}
internal void UnsetCursor(SimpleQueueCacheCursor cursor, StreamSequenceToken token)
{
Log(logger, "UnsetCursor: {Cursor}", cursor);
if (cursor.IsSet)
{
cursor.Element.Value.CacheBucket.UpdateNumCursors(-1);
}
cursor.UnSet(token);
}
private void Add(IBatchContainer batch, StreamSequenceToken sequenceToken)
{
if (batch == null) throw new ArgumentNullException(nameof(batch));
// this should never happen, but just in case
if (Size >= maxCacheSize) throw new CacheFullException();
CacheBucket cacheBucket;
if (cacheCursorHistogram.Count == 0)
{
cacheBucket = new CacheBucket();
cacheCursorHistogram.Add(cacheBucket);
}
else
{
cacheBucket = cacheCursorHistogram[cacheCursorHistogram.Count - 1]; // last one
}
if (cacheBucket.NumCurrentItems == CACHE_HISTOGRAM_MAX_BUCKET_SIZE) // last bucket is full, open a new one
{
cacheBucket = new CacheBucket();
cacheCursorHistogram.Add(cacheBucket);
}
// Add message to linked list
var item = new SimpleQueueCacheItem
{
Batch = batch,
SequenceToken = sequenceToken,
CacheBucket = cacheBucket
};
cachedMessages.AddFirst(new LinkedListNode<SimpleQueueCacheItem>(item));
cacheBucket.UpdateNumItems(1);
}
internal static void Log(ILogger logger, string format, params object[] args)
{
if (logger.IsEnabled(LogLevel.Debug)) logger.LogDebug(format, args);
//if(logger.IsInfo) logger.LogInformation(format, args);
}
}
}