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The AWS Java SDK for Amazon DynamoDB module holds the client classes that are used for communicating with Amazon DynamoDB Service
/*
* Copyright 2011-2024 Amazon.com, Inc. or its affiliates. All Rights Reserved.
*
* Licensed 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://aws.amazon.com/apache2.0
*
* This file 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 com.amazonaws.services.dynamodbv2.datamodeling;
import com.amazonaws.SdkClientException;
import com.amazonaws.AmazonClientException;
import com.amazonaws.annotation.SdkTestInternalApi;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDB;
import com.amazonaws.services.dynamodbv2.model.ScanRequest;
import com.amazonaws.services.dynamodbv2.model.ScanResult;
import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class ParallelScanTask {
/**
* The list of hard copies of ScanRequest with different segment number.
*/
private final List parallelScanRequests;
private final int totalSegments;
/**
* Cache all the future tasks, so that we can extract the exception when
* we see failed segment scan.
*/
private final List> segmentScanFutureTasks;
/**
* Cache all the most recent ScanResult on each segment.
*/
private final List segmentScanResults;
/**
* The current state of the scan on each segment.
* Used as the monitor for synchronization.
*/
private final List segmentScanStates;
private ExecutorService executorService;
private final AmazonDynamoDB dynamo;
@Deprecated
public ParallelScanTask(DynamoDBMapper mapper, AmazonDynamoDB dynamo, List parallelScanRequests) {
this(dynamo, parallelScanRequests);
}
ParallelScanTask(AmazonDynamoDB dynamo, List parallelScanRequests) {
this(dynamo, parallelScanRequests, Executors.newCachedThreadPool());
}
@SdkTestInternalApi
ParallelScanTask(AmazonDynamoDB dynamo, List parallelScanRequests,
ExecutorService executorService) {
this.dynamo = dynamo;
this.parallelScanRequests = parallelScanRequests;
this.totalSegments = parallelScanRequests.size();
this.executorService = executorService;
// Create synchronized views of the list to guarantee any changes are visible across all threads.
segmentScanFutureTasks = Collections
.synchronizedList(new ArrayList>(totalSegments));
segmentScanResults = Collections.synchronizedList(new ArrayList(totalSegments));
segmentScanStates = Collections
.synchronizedList(new ArrayList(totalSegments));
initSegmentScanStates();
}
String getTableName() {
return parallelScanRequests.get(0).getTableName();
}
public boolean isAllSegmentScanFinished() {
synchronized(segmentScanStates) {
for (int segment = 0; segment < totalSegments; segment++) {
if (segmentScanStates.get(segment) != SegmentScanState.SegmentScanCompleted)
return false;
}
// Shut down if all data have been scanned and loaded.
executorService.shutdown();
return true;
}
}
public List getNextBatchOfScanResults() throws SdkClientException {
/**
* Kick-off all the parallel scan tasks.
*/
startScanNextPages();
/**
* Wait till all the tasks have finished.
*/
synchronized(segmentScanStates) {
while (segmentScanStates.contains(SegmentScanState.Waiting)
|| segmentScanStates.contains(SegmentScanState.Scanning)) {
try {
segmentScanStates.wait();
} catch (InterruptedException ie) {
throw new SdkClientException("Parallel scan interrupted by other thread.", ie);
}
}
/**
* Keep the lock on segmentScanStates until all the cached results are marshaled and returned.
*/
return marshalParallelScanResults();
}
}
private void startScanNextPages() {
for (int segment = 0; segment < totalSegments; segment++) {
final int currentSegment = segment;
final SegmentScanState currentSegmentState = segmentScanStates.get(currentSegment);
/**
* Assert: Should never see any task in state of "Scanning" when starting a new batch.
*/
if (currentSegmentState == SegmentScanState.Scanning){
throw new SdkClientException("Should never see a 'Scanning' state when starting parallel scans.");
}
/**
* Skip any failed or completed segment, and clear the corresponding cached result.
*/
else if (currentSegmentState == SegmentScanState.Failed
|| currentSegmentState == SegmentScanState.SegmentScanCompleted) {
segmentScanResults.set(currentSegment, null);
continue;
}
/**
* Otherwise, submit a new future task and save it in segmentScanFutureTasks.
*/
else {
// Update the state to "Scanning" and notify any waiting thread.
synchronized(segmentScanStates) {
segmentScanStates.set(currentSegment, SegmentScanState.Scanning);
segmentScanStates.notifyAll();
}
Future futureTask = executorService.submit(new Callable() {
@Override
public ScanResult call() throws Exception {
try {
if (currentSegmentState == SegmentScanState.HasNextPage) {
return scanNextPageOfSegment(currentSegment, true);
}
else if (currentSegmentState == SegmentScanState.Waiting) {
return scanNextPageOfSegment(currentSegment, false);
}
else {
throw new SdkClientException("Should not start a new future task");
}
} catch (Exception e) {
synchronized (segmentScanStates) {
segmentScanStates.set(currentSegment, SegmentScanState.Failed);
segmentScanStates.notifyAll();
executorService.shutdown();
}
throw e;
}
}
});
// Cache the future task (for getting the Exceptions in the working thread).
segmentScanFutureTasks.set(currentSegment, futureTask);
}
}
}
private List marshalParallelScanResults() {
List scanResults = new LinkedList();
for (int segment = 0; segment < totalSegments; segment++) {
SegmentScanState currentSegmentState = segmentScanStates.get(segment);
/**
* Rethrow the exception from any failed segment scan.
*/
if (currentSegmentState == SegmentScanState.Failed) {
try {
segmentScanFutureTasks.get(segment).get();
throw new SdkClientException("No Exception found in the failed scan task.");
} catch (ExecutionException ee) {
if ( ee.getCause() instanceof AmazonClientException) {
throw (SdkClientException) (ee.getCause());
} else {
throw new SdkClientException("Internal error during the scan on segment #" + segment + ".",
ee.getCause());
}
} catch (Exception e) {
throw new SdkClientException("Error during the scan on segment #" + segment + ".", e);
}
}
/**
* Get the ScanResult from cache if the segment scan has finished.
*/
else if (currentSegmentState == SegmentScanState.HasNextPage
|| currentSegmentState == SegmentScanState.SegmentScanCompleted) {
ScanResult scanResult = segmentScanResults.get(segment);
scanResults.add(scanResult);
}
else if (currentSegmentState == SegmentScanState.Waiting
|| currentSegmentState == SegmentScanState.Scanning){
throw new SdkClientException("Should never see a 'Scanning' or 'Waiting' state when marshalling parallel scan results.");
}
}
return scanResults;
}
private ScanResult scanNextPageOfSegment(int currentSegment, boolean checkLastEvaluatedKey) {
ScanRequest segmentScanRequest = parallelScanRequests.get(currentSegment);
if (checkLastEvaluatedKey) {
ScanResult lastScanResult = segmentScanResults.get(currentSegment);
segmentScanRequest.setExclusiveStartKey(lastScanResult.getLastEvaluatedKey());
} else {
segmentScanRequest.setExclusiveStartKey(null);
}
ScanResult scanResult = dynamo.scan(DynamoDBMapper.applyUserAgent(segmentScanRequest));
/**
* Cache the scan result in segmentScanResults.
* We should never try to get these scan results by calling get() on the cached future tasks.
*/
segmentScanResults.set(currentSegment, scanResult);
/**
* Update the state and notify any waiting thread.
*/
synchronized(segmentScanStates) {
if (null == scanResult.getLastEvaluatedKey())
segmentScanStates.set(currentSegment, SegmentScanState.SegmentScanCompleted);
else
segmentScanStates.set(currentSegment, SegmentScanState.HasNextPage);
segmentScanStates.notifyAll();
}
return scanResult;
}
private void initSegmentScanStates() {
for (int segment = 0; segment < totalSegments; segment++) {
segmentScanFutureTasks.add(null);
segmentScanResults.add(null);
segmentScanStates.add(SegmentScanState.Waiting);
}
}
/**
* Enumeration of the possible states of the scan on a segment.
*/
private static enum SegmentScanState {
/** The scan on the segment is waiting for resources to execute and has not started yet. */
Waiting,
/** The scan is in process, and hasn't finished yet. */
Scanning,
/** The scan has already failed. */
Failed,
/** The scan on the current page has finished, but there are more pages in the segment to be scanned. */
HasNextPage,
/** The scan on the whole segment has completed. */
SegmentScanCompleted,
}
}