/*
* 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.flink.runtime.io.disk.iomanager;
import java.io.IOException;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import org.apache.flink.core.memory.MemorySegment;
/**
* A writer that writes data in blocks to a file channel. The writer receives the data blocks in the form of
* {@link org.apache.flink.core.memory.MemorySegment}, which it writes entirely to the channel,
* regardless of how space in the segment is used. The writing happens in an asynchronous fashion. That is, a write
* request is not processed by the thread that issues it, but by an asynchronous writer thread. Once the request
* is done, the asynchronous writer adds the MemorySegment to a <i>return queue</i> where it can be popped by the
* worker thread, to be reused. The return queue is in this case a
* {@link java.util.concurrent.LinkedBlockingQueue}, such that the working thread blocks until the request has been served,
* if the request is still pending when the it requires the segment back.
* <p>
* Typical write behind is realized, by having a small set of segments in the return queue at all times. When a
* memory segment must be written, the request is issued to the writer and a new segment is immediately popped from
* the return queue. Once too many requests have been issued and the I/O thread cannot keep up, the working thread
* naturally blocks until another segment is available again.
*/
public class BlockChannelWriter extends BlockChannelAccess<WriteRequest, LinkedBlockingQueue<MemorySegment>>
{
/**
* Creates a new block channel writer for the given channel.
*
* @param channelID The ID of the channel to write to.
* @param requestQueue The request queue of the asynchronous writer thread, to which the I/O requests
* are added.
* @param returnSegments The return queue, to which the processed Memory Segments are added.
* @throws IOException Thrown, if the underlying file channel could not be opened exclusively.
*/
protected BlockChannelWriter(Channel.ID channelID, RequestQueue<WriteRequest> requestQueue,
LinkedBlockingQueue<MemorySegment> returnSegments, int numRequestsToBundle)
throws IOException
{
super(channelID, requestQueue, returnSegments, true);
}
/**
* Issues a asynchronous write request to the writer.
*
* @param segment The segment to be written.
* @throws IOException Thrown, when the writer encounters an I/O error. Due to the asynchronous nature of the
* writer, the exception thrown here may have been caused by an earlier write request.
*/
public void writeBlock(MemorySegment segment) throws IOException
{
// check the error state of this channel
checkErroneous();
// write the current buffer and get the next one
this.requestsNotReturned.incrementAndGet();
if (this.closed || this.requestQueue.isClosed()) {
// if we found ourselves closed after the counter increment,
// decrement the counter again and do not forward the request
this.requestsNotReturned.decrementAndGet();
throw new IOException("The writer has been closed.");
}
this.requestQueue.add(new SegmentWriteRequest(this, segment));
}
/**
* Gets the next memory segment that has been written and is available again.
* This method blocks until such a segment is available, or until an error occurs in the writer, or the
* writer is closed.
* <p>
* WARNING: If this method is invoked without any segment ever returning (for example, because the
* {@link #writeBlock(MemorySegment)} method has not been invoked appropriately), the method may block
* forever.
*
* @return The next memory segment from the writers's return queue.
* @throws IOException Thrown, if an I/O error occurs in the writer while waiting for the request to return.
*/
public MemorySegment getNextReturnedSegment() throws IOException
{
try {
while (true) {
final MemorySegment next = this.returnBuffers.poll(2000, TimeUnit.MILLISECONDS);
if (next != null) {
return next;
} else {
if (this.closed) {
throw new IOException("The writer has been closed.");
}
checkErroneous();
}
}
} catch (InterruptedException iex) {
throw new IOException("Writer was interrupted while waiting for the next returning segment.");
}
}
}