Examples with CopyOnWriteArrayList java.util.concurrent.CopyOnWriteArrayList used on opensource projects

Example 16 with CopyOnWriteArrayList

use of java.util.concurrent.CopyOnWriteArrayList in project elasticsearch by elastic.

the class RestClientMultipleHostsIntegTests method testAsyncRequests.

public void testAsyncRequests() throws Exception {
    int numRequests = randomIntBetween(5, 20);
    final CountDownLatch latch = new CountDownLatch(numRequests);
    final List<TestResponse> responses = new CopyOnWriteArrayList<>();
    for (int i = 0; i < numRequests; i++) {
        final String method = RestClientTestUtil.randomHttpMethod(getRandom());
        //we don't test status codes that are subject to retries as they interfere with hosts being stopped
        final int statusCode = randomBoolean() ? randomOkStatusCode(getRandom()) : randomErrorNoRetryStatusCode(getRandom());
        restClient.performRequestAsync(method, "/" + statusCode, new ResponseListener() {

            @Override
            public void onSuccess(Response response) {
                responses.add(new TestResponse(method, statusCode, response));
                latch.countDown();
            }

            @Override
            public void onFailure(Exception exception) {
                responses.add(new TestResponse(method, statusCode, exception));
                latch.countDown();
            }
        });
    }
    assertTrue(latch.await(5, TimeUnit.SECONDS));
    assertEquals(numRequests, responses.size());
    for (TestResponse testResponse : responses) {
        Response response = testResponse.getResponse();
        assertEquals(testResponse.method, response.getRequestLine().getMethod());
        assertEquals(testResponse.statusCode, response.getStatusLine().getStatusCode());
        assertEquals((pathPrefix.length() > 0 ? pathPrefix : "") + "/" + testResponse.statusCode, response.getRequestLine().getUri());
    }
}

Example 17 with CopyOnWriteArrayList

use of java.util.concurrent.CopyOnWriteArrayList in project crate by crate.

the class BlobRecoverySourceHandler method phase1.

/**
     * Perform phase1 of the recovery operations. Once this {@link SnapshotIndexCommit}
     * snapshot has been performed no commit operations (files being fsync'd)
     * are effectively allowed on this index until all recovery phases are done
     * <p/>
     * Phase1 examines the segment files on the target node and copies over the
     * segments that are missing. Only segments that have the same size and
     * checksum can be reused
     */
public void phase1(final SnapshotIndexCommit snapshot, final Translog.View translogView) {
    cancellableThreads.checkForCancel();
    // Total size of segment files that are recovered
    long totalSize = 0;
    // Total size of segment files that were able to be re-used
    long existingTotalSize = 0;
    final Store store = shard.store();
    store.incRef();
    try {
        // CRATE CHANGE
        if (blobRecoveryHandler != null) {
            blobRecoveryHandler.phase1();
        }
        StopWatch stopWatch = new StopWatch().start();
        final Store.MetadataSnapshot recoverySourceMetadata;
        try {
            recoverySourceMetadata = store.getMetadata(snapshot);
        } catch (CorruptIndexException | IndexFormatTooOldException | IndexFormatTooNewException ex) {
            shard.engine().failEngine("recovery", ex);
            throw ex;
        }
        for (String name : snapshot.getFiles()) {
            final StoreFileMetaData md = recoverySourceMetadata.get(name);
            if (md == null) {
                logger.info("Snapshot differs from actual index for file: {} meta: {}", name, recoverySourceMetadata.asMap());
                throw new CorruptIndexException("Snapshot differs from actual index - maybe index was removed metadata has " + recoverySourceMetadata.asMap().size() + " files", name);
            }
        }
        // Generate a "diff" of all the identical, different, and missing
        // segment files on the target node, using the existing files on
        // the source node
        String recoverySourceSyncId = recoverySourceMetadata.getSyncId();
        String recoveryTargetSyncId = request.metadataSnapshot().getSyncId();
        final boolean recoverWithSyncId = recoverySourceSyncId != null && recoverySourceSyncId.equals(recoveryTargetSyncId);
        if (recoverWithSyncId) {
            final long numDocsTarget = request.metadataSnapshot().getNumDocs();
            final long numDocsSource = recoverySourceMetadata.getNumDocs();
            if (numDocsTarget != numDocsSource) {
                throw new IllegalStateException("try to recover " + request.shardId() + " from primary shard with sync id but number of docs differ: " + numDocsTarget + " (" + request.sourceNode().getName() + ", primary) vs " + numDocsSource + "(" + request.targetNode().getName() + ")");
            }
            // we shortcut recovery here because we have nothing to copy. but we must still start the engine on the target.
            // so we don't return here
            logger.trace("[{}][{}] skipping [phase1] to {} - identical sync id [{}] found on both source and target", indexName, shardId, request.targetNode(), recoverySourceSyncId);
        } else {
            final Store.RecoveryDiff diff = recoverySourceMetadata.recoveryDiff(request.metadataSnapshot());
            for (StoreFileMetaData md : diff.identical) {
                response.phase1ExistingFileNames.add(md.name());
                response.phase1ExistingFileSizes.add(md.length());
                existingTotalSize += md.length();
                if (logger.isTraceEnabled()) {
                    logger.trace("[{}][{}] recovery [phase1] to {}: not recovering [{}], exists in local store and has checksum [{}], size [{}]", indexName, shardId, request.targetNode(), md.name(), md.checksum(), md.length());
                }
                totalSize += md.length();
            }
            for (StoreFileMetaData md : Iterables.concat(diff.different, diff.missing)) {
                if (request.metadataSnapshot().asMap().containsKey(md.name())) {
                    logger.trace("[{}][{}] recovery [phase1] to {}: recovering [{}], exists in local store, but is different: remote [{}], local [{}]", indexName, shardId, request.targetNode(), md.name(), request.metadataSnapshot().asMap().get(md.name()), md);
                } else {
                    logger.trace("[{}][{}] recovery [phase1] to {}: recovering [{}], does not exists in remote", indexName, shardId, request.targetNode(), md.name());
                }
                response.phase1FileNames.add(md.name());
                response.phase1FileSizes.add(md.length());
                totalSize += md.length();
            }
            response.phase1TotalSize = totalSize;
            response.phase1ExistingTotalSize = existingTotalSize;
            logger.trace("[{}][{}] recovery [phase1] to {}: recovering_files [{}] with total_size [{}], reusing_files [{}] with total_size [{}]", indexName, shardId, request.targetNode(), response.phase1FileNames.size(), new ByteSizeValue(totalSize), response.phase1ExistingFileNames.size(), new ByteSizeValue(existingTotalSize));
            cancellableThreads.execute(new Interruptable() {

                @Override
                public void run() throws InterruptedException {
                    RecoveryFilesInfoRequest recoveryInfoFilesRequest = new RecoveryFilesInfoRequest(request.recoveryId(), request.shardId(), response.phase1FileNames, response.phase1FileSizes, response.phase1ExistingFileNames, response.phase1ExistingFileSizes, translogView.totalOperations());
                    transportService.submitRequest(request.targetNode(), RecoveryTarget.Actions.FILES_INFO, recoveryInfoFilesRequest, TransportRequestOptions.builder().withTimeout(recoverySettings.internalActionTimeout()).build(), EmptyTransportResponseHandler.INSTANCE_SAME).txGet();
                }
            });
            // This latch will be used to wait until all files have been transferred to the target node
            final CountDownLatch latch = new CountDownLatch(response.phase1FileNames.size());
            final CopyOnWriteArrayList<Throwable> exceptions = new CopyOnWriteArrayList<>();
            final AtomicReference<Throwable> corruptedEngine = new AtomicReference<>();
            int fileIndex = 0;
            ThreadPoolExecutor pool;
            // How many bytes we've copied since we last called RateLimiter.pause
            final AtomicLong bytesSinceLastPause = new AtomicLong();
            for (final String name : response.phase1FileNames) {
                long fileSize = response.phase1FileSizes.get(fileIndex);
                // separately.
                if (fileSize > RecoverySettings.SMALL_FILE_CUTOFF_BYTES) {
                    pool = recoverySettings.concurrentStreamPool();
                } else {
                    pool = recoverySettings.concurrentSmallFileStreamPool();
                }
                pool.execute(new AbstractRunnable() {

                    @Override
                    public void onFailure(Throwable t) {
                        // we either got rejected or the store can't be incremented / we are canceled
                        logger.debug("Failed to transfer file [" + name + "] on recovery");
                    }

                    @Override
                    public void onAfter() {
                        // Signify this file has completed by decrementing the latch
                        latch.countDown();
                    }

                    @Override
                    protected void doRun() {
                        cancellableThreads.checkForCancel();
                        store.incRef();
                        final StoreFileMetaData md = recoverySourceMetadata.get(name);
                        try (final IndexInput indexInput = store.directory().openInput(name, IOContext.READONCE)) {
                            // at least one!
                            final int BUFFER_SIZE = (int) Math.max(1, recoverySettings.fileChunkSize().getBytes());
                            final byte[] buf = new byte[BUFFER_SIZE];
                            boolean shouldCompressRequest = recoverySettings.compress();
                            if (CompressorFactory.isCompressed(indexInput)) {
                                shouldCompressRequest = false;
                            }
                            final long len = indexInput.length();
                            long readCount = 0;
                            final TransportRequestOptions requestOptions = TransportRequestOptions.builder().withCompress(shouldCompressRequest).withType(TransportRequestOptions.Type.RECOVERY).withTimeout(recoverySettings.internalActionTimeout()).build();
                            while (readCount < len) {
                                if (shard.state() == IndexShardState.CLOSED) {
                                    // check if the shard got closed on us
                                    throw new IndexShardClosedException(shard.shardId());
                                }
                                int toRead = readCount + BUFFER_SIZE > len ? (int) (len - readCount) : BUFFER_SIZE;
                                final long position = indexInput.getFilePointer();
                                // Pause using the rate limiter, if desired, to throttle the recovery
                                RateLimiter rl = recoverySettings.rateLimiter();
                                long throttleTimeInNanos = 0;
                                if (rl != null) {
                                    long bytes = bytesSinceLastPause.addAndGet(toRead);
                                    if (bytes > rl.getMinPauseCheckBytes()) {
                                        // Time to pause
                                        bytesSinceLastPause.addAndGet(-bytes);
                                        throttleTimeInNanos = rl.pause(bytes);
                                        shard.recoveryStats().addThrottleTime(throttleTimeInNanos);
                                    }
                                }
                                indexInput.readBytes(buf, 0, toRead, false);
                                final BytesArray content = new BytesArray(buf, 0, toRead);
                                readCount += toRead;
                                final boolean lastChunk = readCount == len;
                                final RecoveryFileChunkRequest fileChunkRequest = new RecoveryFileChunkRequest(request.recoveryId(), request.shardId(), md, position, content, lastChunk, translogView.totalOperations(), throttleTimeInNanos);
                                cancellableThreads.execute(new Interruptable() {

                                    @Override
                                    public void run() throws InterruptedException {
                                        // Actually send the file chunk to the target node, waiting for it to complete
                                        transportService.submitRequest(request.targetNode(), RecoveryTarget.Actions.FILE_CHUNK, fileChunkRequest, requestOptions, EmptyTransportResponseHandler.INSTANCE_SAME).txGet();
                                    }
                                });
                            }
                        } catch (Throwable e) {
                            final Throwable corruptIndexException;
                            if ((corruptIndexException = ExceptionsHelper.unwrapCorruption(e)) != null) {
                                if (store.checkIntegrityNoException(md) == false) {
                                    // we are corrupted on the primary -- fail!
                                    logger.warn("{} Corrupted file detected {} checksum mismatch", shard.shardId(), md);
                                    if (corruptedEngine.compareAndSet(null, corruptIndexException) == false) {
                                        // if we are not the first exception, add ourselves as suppressed to the main one:
                                        corruptedEngine.get().addSuppressed(e);
                                    }
                                } else {
                                    // corruption has happened on the way to replica
                                    RemoteTransportException exception = new RemoteTransportException("File corruption occurred on recovery but checksums are ok", null);
                                    exception.addSuppressed(e);
                                    // last exception first
                                    exceptions.add(0, exception);
                                    logger.warn("{} Remote file corruption on node {}, recovering {}. local checksum OK", corruptIndexException, shard.shardId(), request.targetNode(), md);
                                }
                            } else {
                                // last exceptions first
                                exceptions.add(0, e);
                            }
                        } finally {
                            store.decRef();
                        }
                    }
                });
                fileIndex++;
            }
            cancellableThreads.execute(new Interruptable() {

                @Override
                public void run() throws InterruptedException {
                    // Wait for all files that need to be transferred to finish transferring
                    latch.await();
                }
            });
            if (corruptedEngine.get() != null) {
                shard.engine().failEngine("recovery", corruptedEngine.get());
                throw corruptedEngine.get();
            } else {
                ExceptionsHelper.rethrowAndSuppress(exceptions);
            }
            cancellableThreads.execute(new Interruptable() {

                @Override
                public void run() throws InterruptedException {
                    // are deleted
                    try {
                        transportService.submitRequest(request.targetNode(), RecoveryTarget.Actions.CLEAN_FILES, new RecoveryCleanFilesRequest(request.recoveryId(), shard.shardId(), recoverySourceMetadata, translogView.totalOperations()), TransportRequestOptions.builder().withTimeout(recoverySettings.internalActionTimeout()).build(), EmptyTransportResponseHandler.INSTANCE_SAME).txGet();
                    } catch (RemoteTransportException remoteException) {
                        final IOException corruptIndexException;
                        //   - maybe due to old segments without checksums or length only checks
                        if ((corruptIndexException = ExceptionsHelper.unwrapCorruption(remoteException)) != null) {
                            try {
                                final Store.MetadataSnapshot recoverySourceMetadata = store.getMetadata(snapshot);
                                StoreFileMetaData[] metadata = Iterables.toArray(recoverySourceMetadata, StoreFileMetaData.class);
                                ArrayUtil.timSort(metadata, new Comparator<StoreFileMetaData>() {

                                    @Override
                                    public int compare(StoreFileMetaData o1, StoreFileMetaData o2) {
                                        // check small files first
                                        return Long.compare(o1.length(), o2.length());
                                    }
                                });
                                for (StoreFileMetaData md : metadata) {
                                    logger.debug("{} checking integrity for file {} after remove corruption exception", shard.shardId(), md);
                                    if (store.checkIntegrityNoException(md) == false) {
                                        // we are corrupted on the primary -- fail!
                                        shard.engine().failEngine("recovery", corruptIndexException);
                                        logger.warn("{} Corrupted file detected {} checksum mismatch", shard.shardId(), md);
                                        throw corruptIndexException;
                                    }
                                }
                            } catch (IOException ex) {
                                remoteException.addSuppressed(ex);
                                throw remoteException;
                            }
                            // corruption has happened on the way to replica
                            RemoteTransportException exception = new RemoteTransportException("File corruption occurred on recovery but checksums are ok", null);
                            exception.addSuppressed(remoteException);
                            logger.warn("{} Remote file corruption during finalization on node {}, recovering {}. local checksum OK", corruptIndexException, shard.shardId(), request.targetNode());
                            throw exception;
                        } else {
                            throw remoteException;
                        }
                    }
                }
            });
        }
        prepareTargetForTranslog(translogView);
        logger.trace("[{}][{}] recovery [phase1] to {}: took [{}]", indexName, shardId, request.targetNode(), stopWatch.totalTime());
        response.phase1Time = stopWatch.totalTime().millis();
    } catch (Throwable e) {
        throw new RecoverFilesRecoveryException(request.shardId(), response.phase1FileNames.size(), new ByteSizeValue(totalSize), e);
    } finally {
        store.decRef();
    }
}

Example 18 with CopyOnWriteArrayList

use of java.util.concurrent.CopyOnWriteArrayList in project elasticsearch by elastic.

the class ESIntegTestCase method indexRandom.

/**
     * Indexes the given {@link IndexRequestBuilder} instances randomly. It shuffles the given builders and either
     * indexes them in a blocking or async fashion. This is very useful to catch problems that relate to internal document
     * ids or index segment creations. Some features might have bug when a given document is the first or the last in a
     * segment or if only one document is in a segment etc. This method prevents issues like this by randomizing the index
     * layout.
     *
     * @param forceRefresh   if <tt>true</tt> all involved indices are refreshed once the documents are indexed.
     * @param dummyDocuments if <tt>true</tt> some empty dummy documents may be randomly inserted into the document list and deleted once
     *                       all documents are indexed. This is useful to produce deleted documents on the server side.
     * @param maybeFlush     if <tt>true</tt> this method may randomly execute full flushes after index operations.
     * @param builders       the documents to index.
     */
public void indexRandom(boolean forceRefresh, boolean dummyDocuments, boolean maybeFlush, List<IndexRequestBuilder> builders) throws InterruptedException, ExecutionException {
    Random random = random();
    Set<String> indicesSet = new HashSet<>();
    for (IndexRequestBuilder builder : builders) {
        indicesSet.add(builder.request().index());
    }
    Set<Tuple<String, String>> bogusIds = new HashSet<>();
    if (random.nextBoolean() && !builders.isEmpty() && dummyDocuments) {
        builders = new ArrayList<>(builders);
        final String[] indices = indicesSet.toArray(new String[indicesSet.size()]);
        // inject some bogus docs
        final int numBogusDocs = scaledRandomIntBetween(1, builders.size() * 2);
        final int unicodeLen = between(1, 10);
        for (int i = 0; i < numBogusDocs; i++) {
            String id = randomRealisticUnicodeOfLength(unicodeLen) + Integer.toString(dummmyDocIdGenerator.incrementAndGet());
            String index = RandomPicks.randomFrom(random, indices);
            bogusIds.add(new Tuple<>(index, id));
            builders.add(client().prepareIndex(index, RANDOM_BOGUS_TYPE, id).setSource("{}", XContentType.JSON));
        }
    }
    final String[] indices = indicesSet.toArray(new String[indicesSet.size()]);
    Collections.shuffle(builders, random());
    final CopyOnWriteArrayList<Tuple<IndexRequestBuilder, Exception>> errors = new CopyOnWriteArrayList<>();
    List<CountDownLatch> inFlightAsyncOperations = new ArrayList<>();
    // If you are indexing just a few documents then frequently do it one at a time.  If many then frequently in bulk.
    if (builders.size() < FREQUENT_BULK_THRESHOLD ? frequently() : builders.size() < ALWAYS_BULK_THRESHOLD ? rarely() : false) {
        if (frequently()) {
            logger.info("Index [{}] docs async: [{}] bulk: [{}]", builders.size(), true, false);
            for (IndexRequestBuilder indexRequestBuilder : builders) {
                indexRequestBuilder.execute(new PayloadLatchedActionListener<IndexResponse, IndexRequestBuilder>(indexRequestBuilder, newLatch(inFlightAsyncOperations), errors));
                postIndexAsyncActions(indices, inFlightAsyncOperations, maybeFlush);
            }
        } else {
            logger.info("Index [{}] docs async: [{}] bulk: [{}]", builders.size(), false, false);
            for (IndexRequestBuilder indexRequestBuilder : builders) {
                indexRequestBuilder.execute().actionGet();
                postIndexAsyncActions(indices, inFlightAsyncOperations, maybeFlush);
            }
        }
    } else {
        List<List<IndexRequestBuilder>> partition = eagerPartition(builders, Math.min(MAX_BULK_INDEX_REQUEST_SIZE, Math.max(1, (int) (builders.size() * randomDouble()))));
        logger.info("Index [{}] docs async: [{}] bulk: [{}] partitions [{}]", builders.size(), false, true, partition.size());
        for (List<IndexRequestBuilder> segmented : partition) {
            BulkRequestBuilder bulkBuilder = client().prepareBulk();
            for (IndexRequestBuilder indexRequestBuilder : segmented) {
                bulkBuilder.add(indexRequestBuilder);
            }
            BulkResponse actionGet = bulkBuilder.execute().actionGet();
            assertThat(actionGet.hasFailures() ? actionGet.buildFailureMessage() : "", actionGet.hasFailures(), equalTo(false));
        }
    }
    for (CountDownLatch operation : inFlightAsyncOperations) {
        operation.await();
    }
    final List<Exception> actualErrors = new ArrayList<>();
    for (Tuple<IndexRequestBuilder, Exception> tuple : errors) {
        if (ExceptionsHelper.unwrapCause(tuple.v2()) instanceof EsRejectedExecutionException) {
            // re-index if rejected
            tuple.v1().execute().actionGet();
        } else {
            actualErrors.add(tuple.v2());
        }
    }
    assertThat(actualErrors, emptyIterable());
    if (!bogusIds.isEmpty()) {
        // delete the bogus types again - it might trigger merges or at least holes in the segments and enforces deleted docs!
        for (Tuple<String, String> doc : bogusIds) {
            assertEquals("failed to delete a dummy doc [" + doc.v1() + "][" + doc.v2() + "]", DocWriteResponse.Result.DELETED, client().prepareDelete(doc.v1(), RANDOM_BOGUS_TYPE, doc.v2()).get().getResult());
        }
    }
    if (forceRefresh) {
        assertNoFailures(client().admin().indices().prepareRefresh(indices).setIndicesOptions(IndicesOptions.lenientExpandOpen()).execute().get());
    }
}

Example 19 with CopyOnWriteArrayList

use of java.util.concurrent.CopyOnWriteArrayList in project elasticsearch by elastic.

the class CacheTests method testDependentKeyDeadlock.

public void testDependentKeyDeadlock() throws BrokenBarrierException, InterruptedException {
    class Key {

        private final int key;

        Key(int key) {
            this.key = key;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o)
                return true;
            if (o == null || getClass() != o.getClass())
                return false;
            Key key1 = (Key) o;
            return key == key1.key;
        }

        @Override
        public int hashCode() {
            return key % 2;
        }
    }
    int numberOfThreads = randomIntBetween(2, 32);
    final Cache<Key, Integer> cache = CacheBuilder.<Key, Integer>builder().build();
    CopyOnWriteArrayList<ExecutionException> failures = new CopyOnWriteArrayList<>();
    CyclicBarrier barrier = new CyclicBarrier(1 + numberOfThreads);
    CountDownLatch deadlockLatch = new CountDownLatch(numberOfThreads);
    List<Thread> threads = new ArrayList<>();
    for (int i = 0; i < numberOfThreads; i++) {
        Thread thread = new Thread(() -> {
            try {
                try {
                    barrier.await();
                } catch (BrokenBarrierException | InterruptedException e) {
                    throw new AssertionError(e);
                }
                Random random = new Random(random().nextLong());
                for (int j = 0; j < numberOfEntries; j++) {
                    Key key = new Key(random.nextInt(numberOfEntries));
                    try {
                        cache.computeIfAbsent(key, k -> {
                            if (k.key == 0) {
                                return 0;
                            } else {
                                Integer value = cache.get(new Key(k.key / 2));
                                return value != null ? value : 0;
                            }
                        });
                    } catch (ExecutionException e) {
                        failures.add(e);
                        break;
                    }
                }
            } finally {
                // successfully avoided deadlock, release the main thread
                deadlockLatch.countDown();
            }
        });
        threads.add(thread);
        thread.start();
    }
    AtomicBoolean deadlock = new AtomicBoolean();
    assert !deadlock.get();
    // start a watchdog service
    ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
    scheduler.scheduleAtFixedRate(() -> {
        Set<Long> ids = threads.stream().map(t -> t.getId()).collect(Collectors.toSet());
        ThreadMXBean mxBean = ManagementFactory.getThreadMXBean();
        long[] deadlockedThreads = mxBean.findDeadlockedThreads();
        if (!deadlock.get() && deadlockedThreads != null) {
            for (long deadlockedThread : deadlockedThreads) {
                // ensure that we detected deadlock on our threads
                if (ids.contains(deadlockedThread)) {
                    deadlock.set(true);
                    // release the main test thread to fail the test
                    for (int i = 0; i < numberOfThreads; i++) {
                        deadlockLatch.countDown();
                    }
                    break;
                }
            }
        }
    }, 1, 1, TimeUnit.SECONDS);
    // everything is setup, release the hounds
    barrier.await();
    // wait for either deadlock to be detected or the threads to terminate
    deadlockLatch.await();
    // shutdown the watchdog service
    scheduler.shutdown();
    assertThat(failures, is(empty()));
    assertFalse("deadlock", deadlock.get());
}

Example 20 with CopyOnWriteArrayList

use of java.util.concurrent.CopyOnWriteArrayList in project elasticsearch by elastic.

the class CacheTests method testComputeIfAbsentCallsOnce.

public void testComputeIfAbsentCallsOnce() throws BrokenBarrierException, InterruptedException {
    int numberOfThreads = randomIntBetween(2, 32);
    final Cache<Integer, String> cache = CacheBuilder.<Integer, String>builder().build();
    AtomicReferenceArray flags = new AtomicReferenceArray(numberOfEntries);
    for (int j = 0; j < numberOfEntries; j++) {
        flags.set(j, false);
    }
    CopyOnWriteArrayList<ExecutionException> failures = new CopyOnWriteArrayList<>();
    CyclicBarrier barrier = new CyclicBarrier(1 + numberOfThreads);
    for (int i = 0; i < numberOfThreads; i++) {
        Thread thread = new Thread(() -> {
            try {
                barrier.await();
                for (int j = 0; j < numberOfEntries; j++) {
                    try {
                        cache.computeIfAbsent(j, key -> {
                            assertTrue(flags.compareAndSet(key, false, true));
                            return Integer.toString(key);
                        });
                    } catch (ExecutionException e) {
                        failures.add(e);
                        break;
                    }
                }
                barrier.await();
            } catch (BrokenBarrierException | InterruptedException e) {
                throw new AssertionError(e);
            }
        });
        thread.start();
    }
    // wait for all threads to be ready
    barrier.await();
    // wait for all threads to finish
    barrier.await();
    assertThat(failures, is(empty()));
}