/*
* 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 reader that reads data in blocks from a file channel. The reader reads the blocks into a
* {@link org.apache.flink.core.memory.MemorySegment} in an asynchronous fashion. That is, a read
* request is not processed by the thread that issues it, but by an asynchronous reader thread. Once the read request
* is done, the asynchronous reader adds the full MemorySegment to a <i>return queue</i> where it can be popped by the
* worker thread, once it needs the data. 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 data.
* <p>
* Typical pre-fetching reads are done by issuing the read requests early and popping the return queue once the data
* is actually needed.
* <p>
* The reader has no notion whether the size of the memory segments is actually the size of the blocks on disk,
* or even whether the file was written in blocks of the same size, or in blocks at all. Ensuring that the
* writing and reading is consistent with each other (same blocks sizes) is up to the programmer.
*/
public class BlockChannelReader extends BlockChannelAccess<ReadRequest, LinkedBlockingQueue<MemorySegment>>
{
/**
* Creates a new block channel reader for the given channel.
*
* @param channelID The ID of the channel to read.
* @param requestQueue The request queue of the asynchronous reader thread, to which the I/O requests
* are added.
* @param returnSegments The return queue, to which the full Memory Segments are added.
* @throws IOException Thrown, if the underlying file channel could not be opened.
*/
protected BlockChannelReader(Channel.ID channelID, RequestQueue<ReadRequest> requestQueue,
LinkedBlockingQueue<MemorySegment> returnSegments, int numRequestsToBundle)
throws IOException
{
super(channelID, requestQueue, returnSegments, false);
}
/**
* Issues a read request, which will asynchronously fill the given segment with the next block in the
* underlying file channel. Once the read request is fulfilled, the segment will be added to this reader's
* return queue.
*
* @param segment The segment to read the block into.
* @throws IOException Thrown, when the reader encounters an I/O error. Due to the asynchronous nature of the
* reader, the exception thrown here may have been caused by an earlier read request.
*/
public void readBlock(MemorySegment segment) throws IOException
{
// check the error state of this channel
checkErroneous();
// write the current buffer and get the next one
// the statements have to be in this order to avoid incrementing the counter
// after the channel has been closed
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 reader has been closed.");
}
this.requestQueue.add(new SegmentReadRequest(this, segment));
}
/**
* Gets the next memory segment that has been filled with data by the reader. This method blocks until
* such a segment is available, or until an error occurs in the reader, or the reader is closed.
* <p>
* WARNING: If this method is invoked without any segment ever returning (for example, because the
* {@link #readBlock(MemorySegment)} method has not been invoked appropriately), the method may block
* forever.
*
* @return The next memory segment from the reader's return queue.
* @throws IOException Thrown, if an I/O error occurs in the reader 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 reader has been asynchronously closed.");
}
checkErroneous();
}
}
} catch (InterruptedException iex) {
throw new IOException("Reader was interrupted while waiting for the next returning segment.");
}
}
}