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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.cassandra.hints;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicLong;
import java.util.zip.CRC32;
import org.apache.cassandra.io.util.DataOutputBuffer;
import org.apache.cassandra.io.util.DataOutputBufferFixed;
import org.apache.cassandra.io.util.FileUtils;
import org.apache.cassandra.net.MessagingService;
import org.apache.cassandra.utils.AbstractIterator;
import org.apache.cassandra.utils.concurrent.OpOrder;
import static org.apache.cassandra.utils.FBUtilities.updateChecksum;
import static org.apache.cassandra.utils.FBUtilities.updateChecksumInt;
/**
* A shared buffer that temporarily holds the serialized hints before they are flushed to disk.
*
* Consists of :
* - a ByteBuffer holding the serialized hints (length, length checksum and total checksum included)
* - a pointer to the current allocation offset
* - an {@link OpOrder} appendOrder for {@link HintsWriteExecutor} to wait on for all writes completion
* - a map of (host id -> offset queue) for the hints written
*
* It's possible to write a single hint for two or more hosts at the same time, in which case the same offset will be put
* into two or more offset queues.
*/
final class HintsBuffer
{
// hint entry overhead in bytes (int length, int length checksum, int body checksum)
static final int ENTRY_OVERHEAD_SIZE = 12;
private final ByteBuffer slab; // the underlying backing ByteBuffer for all the serialized hints
private final AtomicLong position; // the position in the slab that we currently allocate from
private final ConcurrentMap> offsets;
private final OpOrder appendOrder;
private HintsBuffer(ByteBuffer slab)
{
this.slab = slab;
position = new AtomicLong();
offsets = new ConcurrentHashMap<>();
appendOrder = new OpOrder();
}
static HintsBuffer create(int slabSize)
{
return new HintsBuffer(ByteBuffer.allocateDirect(slabSize));
}
boolean isClosed()
{
return position.get() < 0;
}
int capacity()
{
return slab.capacity();
}
int remaining()
{
long pos = position.get();
return (int) (pos < 0 ? 0 : Math.max(0, capacity() - pos));
}
HintsBuffer recycle()
{
slab.clear();
return new HintsBuffer(slab);
}
void free()
{
FileUtils.clean(slab);
}
/**
* Wait for any appends started before this method was called.
*/
void waitForModifications()
{
appendOrder.awaitNewBarrier(); // issue a barrier and wait for it
}
Set hostIds()
{
return offsets.keySet();
}
/**
* Coverts the queue of offsets for the selected host id into an iterator of hints encoded as ByteBuffers.
*/
Iterator consumingHintsIterator(UUID hostId)
{
final Queue bufferOffsets = offsets.get(hostId);
if (bufferOffsets == null)
return Collections.emptyIterator();
return new AbstractIterator()
{
private final ByteBuffer flyweight = slab.duplicate();
protected ByteBuffer computeNext()
{
Integer offset = bufferOffsets.poll();
if (offset == null)
return endOfData();
int totalSize = slab.getInt(offset) + ENTRY_OVERHEAD_SIZE;
return (ByteBuffer) flyweight.clear().position(offset).limit(offset + totalSize);
}
};
}
@SuppressWarnings("resource")
Allocation allocate(int hintSize)
{
int totalSize = hintSize + ENTRY_OVERHEAD_SIZE;
if (totalSize > slab.capacity() / 2)
{
throw new IllegalArgumentException(String.format("Hint of %s bytes is too large - the maximum size is %s",
hintSize,
slab.capacity() / 2));
}
OpOrder.Group opGroup = appendOrder.start(); // will eventually be closed by the receiver of the allocation
try
{
return allocate(totalSize, opGroup);
}
catch (Throwable t)
{
opGroup.close();
throw t;
}
}
private Allocation allocate(int totalSize, OpOrder.Group opGroup)
{
int offset = allocateBytes(totalSize);
if (offset < 0)
{
opGroup.close();
return null;
}
return new Allocation(offset, totalSize, opGroup);
}
// allocate bytes in the slab, or return negative if not enough space
private int allocateBytes(int totalSize)
{
long prev = position.getAndAdd(totalSize);
if (prev < 0) // the slab has been 'closed'
return -1;
if ((prev + totalSize) > slab.capacity())
{
position.set(Long.MIN_VALUE); // mark the slab as no longer allocating if we've exceeded its capacity
return -1;
}
return (int)prev;
}
private void put(UUID hostId, int offset)
{
// we intentionally don't just return offsets.computeIfAbsent() because it's expensive compared to simple get(),
// and the method is on a really hot path
Queue queue = offsets.get(hostId);
if (queue == null)
queue = offsets.computeIfAbsent(hostId, (id) -> new ConcurrentLinkedQueue<>());
queue.offer(offset);
}
/**
* A placeholder for hint serialization. Should always be used in a try-with-resources block.
*/
final class Allocation implements AutoCloseable
{
private final Integer offset;
private final int totalSize;
private final OpOrder.Group opGroup;
Allocation(int offset, int totalSize, OpOrder.Group opGroup)
{
this.offset = offset;
this.totalSize = totalSize;
this.opGroup = opGroup;
}
void write(Iterable hostIds, Hint hint)
{
write(hint);
for (UUID hostId : hostIds)
put(hostId, offset);
}
public void close()
{
opGroup.close();
}
private void write(Hint hint)
{
ByteBuffer buffer = (ByteBuffer) slab.duplicate().position(offset).limit(offset + totalSize);
CRC32 crc = new CRC32();
int hintSize = totalSize - ENTRY_OVERHEAD_SIZE;
try (DataOutputBuffer dop = new DataOutputBufferFixed(buffer))
{
dop.writeInt(hintSize);
updateChecksumInt(crc, hintSize);
dop.writeInt((int) crc.getValue());
Hint.serializer.serialize(hint, dop, MessagingService.current_version);
updateChecksum(crc, buffer, buffer.position() - hintSize, hintSize);
dop.writeInt((int) crc.getValue());
}
catch (IOException e)
{
throw new AssertionError(); // cannot happen
}
}
}
}