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AsyncResultSetImpl.java
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AsyncResultSetImpl.java
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/*
* Copyright 2020 Google LLC
*
* 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://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 com.google.cloud.spanner;
import com.google.api.core.ApiFuture;
import com.google.api.core.ApiFutures;
import com.google.api.core.ListenableFutureToApiFuture;
import com.google.api.core.SettableApiFuture;
import com.google.api.gax.core.ExecutorProvider;
import com.google.cloud.spanner.AbstractReadContext.ListenableAsyncResultSet;
import com.google.common.base.Function;
import com.google.common.base.Preconditions;
import com.google.common.base.Supplier;
import com.google.common.base.Suppliers;
import com.google.common.collect.ImmutableList;
import com.google.common.util.concurrent.ListeningScheduledExecutorService;
import com.google.common.util.concurrent.MoreExecutors;
import com.google.spanner.v1.ResultSetStats;
import java.util.Collection;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.BlockingDeque;
import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.logging.Level;
import java.util.logging.Logger;
/** Default implementation for {@link AsyncResultSet}. */
class AsyncResultSetImpl extends ForwardingStructReader implements ListenableAsyncResultSet {
private static final Logger log = Logger.getLogger(AsyncResultSetImpl.class.getName());
/** State of an {@link AsyncResultSetImpl}. */
private enum State {
INITIALIZED,
/** SYNC indicates that the {@link ResultSet} is used in sync pattern. */
SYNC,
CONSUMING,
RUNNING,
PAUSED,
CANCELLED(true),
DONE(true);
/** Does this state mean that the result set should permanently stop producing rows. */
private final boolean shouldStop;
State() {
shouldStop = false;
}
State(boolean shouldStop) {
this.shouldStop = shouldStop;
}
}
static final int DEFAULT_BUFFER_SIZE = 10;
private static final int MAX_WAIT_FOR_BUFFER_CONSUMPTION = 10;
private static final SpannerException CANCELLED_EXCEPTION =
SpannerExceptionFactory.newSpannerException(
ErrorCode.CANCELLED, "This AsyncResultSet has been cancelled");
private final Object monitor = new Object();
private boolean closed;
/**
* {@link ExecutorProvider} provides executor services that are used to fetch data from the
* backend and put these into the buffer for further consumption by the callback.
*/
private final ExecutorProvider executorProvider;
private final ListeningScheduledExecutorService service;
private final BlockingDeque<Struct> buffer;
private Struct currentRow;
/** Supplies the underlying synchronous {@link ResultSet} that will be producing the rows. */
private final Supplier<ResultSet> delegateResultSet;
/**
* Any exception that occurs while executing the query and iterating over the result set will be
* stored in this variable and propagated to the user through {@link #tryNext()}.
*/
private volatile SpannerException executionException;
/**
* Executor for callbacks. Regardless of the type of executor that is provided, the {@link
* AsyncResultSetImpl} will ensure that at most 1 callback call will be active at any one time.
*/
private Executor executor;
private ReadyCallback callback;
/**
* Listeners that will be called when the {@link AsyncResultSetImpl} has finished fetching all
* rows and any underlying transaction or session can be closed.
*/
private Collection<Runnable> listeners = new LinkedList<>();
private State state = State.INITIALIZED;
/**
* This variable indicates whether all the results from the underlying result set have been read.
*/
private volatile boolean finished;
private volatile ApiFuture<Void> result;
/**
* This variable indicates whether {@link #tryNext()} has returned {@link CursorState#DONE} or a
* {@link SpannerException}.
*/
private volatile boolean cursorReturnedDoneOrException;
/**
* This variable is used to pause the producer when the {@link AsyncResultSet} is paused. The
* production of rows that are put into the buffer is only paused once the buffer is full.
*/
private volatile CountDownLatch pausedLatch = new CountDownLatch(1);
/**
* This variable is used to pause the producer when the buffer is full and the consumer needs some
* time to catch up.
*/
private volatile CountDownLatch bufferConsumptionLatch = new CountDownLatch(0);
/**
* This variable is used to pause the producer when all rows have been put into the buffer, but
* the consumer (the callback) has not yet received and processed all rows.
*/
private volatile CountDownLatch consumingLatch = new CountDownLatch(0);
AsyncResultSetImpl(ExecutorProvider executorProvider, ResultSet delegate, int bufferSize) {
this(executorProvider, Suppliers.ofInstance(Preconditions.checkNotNull(delegate)), bufferSize);
}
AsyncResultSetImpl(
ExecutorProvider executorProvider, Supplier<ResultSet> delegate, int bufferSize) {
super(delegate);
this.executorProvider = Preconditions.checkNotNull(executorProvider);
this.delegateResultSet = Preconditions.checkNotNull(delegate);
this.service = MoreExecutors.listeningDecorator(executorProvider.getExecutor());
this.buffer = new LinkedBlockingDeque<>(bufferSize);
}
/**
* Closes the {@link AsyncResultSet}. {@link #close()} is non-blocking and may be called multiple
* times without side effects. An {@link AsyncResultSet} may be closed before all rows have been
* returned to the callback, and calling {@link #tryNext()} on a closed {@link AsyncResultSet} is
* allowed as long as this is done from within a {@link ReadyCallback}. Calling {@link #resume()}
* on a closed {@link AsyncResultSet} is also allowed.
*/
@Override
public void close() {
synchronized (monitor) {
if (this.closed) {
return;
}
if (state == State.INITIALIZED || state == State.SYNC) {
delegateResultSet.get().close();
}
this.closed = true;
}
}
/**
* Adds a listener that will be called when no more rows will be read from the underlying {@link
* ResultSet}, either because all rows have been read, or because {@link
* ReadyCallback#cursorReady(AsyncResultSet)} returned {@link CallbackResponse#DONE}.
*/
@Override
public void addListener(Runnable listener) {
Preconditions.checkState(state == State.INITIALIZED);
listeners.add(listener);
}
@Override
public void removeListener(Runnable listener) {
Preconditions.checkState(state == State.INITIALIZED);
listeners.remove(listener);
}
/**
* Tries to advance this {@link AsyncResultSet} to the next row. This method may only be called
* from within a {@link ReadyCallback}.
*/
@Override
public CursorState tryNext() throws SpannerException {
synchronized (monitor) {
if (state == State.CANCELLED) {
cursorReturnedDoneOrException = true;
throw CANCELLED_EXCEPTION;
}
if (buffer.isEmpty() && executionException != null) {
cursorReturnedDoneOrException = true;
throw executionException;
}
Preconditions.checkState(
this.callback != null, "tryNext may only be called after a callback has been set.");
Preconditions.checkState(
this.state == State.CONSUMING,
"tryNext may only be called from a DataReady callback. Current state: "
+ this.state.name());
if (finished && buffer.isEmpty()) {
cursorReturnedDoneOrException = true;
return CursorState.DONE;
}
}
if (!buffer.isEmpty()) {
// Set the next row from the buffer as the current row of the StructReader.
replaceDelegate(currentRow = buffer.pop());
synchronized (monitor) {
bufferConsumptionLatch.countDown();
}
return CursorState.OK;
}
return CursorState.NOT_READY;
}
private void closeDelegateResultSet() {
try {
delegateResultSet.get().close();
} catch (Throwable t) {
log.log(Level.FINE, "Ignoring error from closing delegate result set", t);
}
}
/**
* {@link CallbackRunnable} calls the {@link ReadyCallback} registered for this {@link
* AsyncResultSet}.
*/
private class CallbackRunnable implements Runnable {
@Override
public void run() {
try {
while (true) {
synchronized (monitor) {
if (cursorReturnedDoneOrException) {
break;
}
if (state == State.CANCELLED) {
// The callback should always get at least one chance to catch the CANCELLED
// exception. It is however possible that the callback does not call tryNext(), and
// instead directly returns PAUSE or DONE. In those cases, the callback runner should
// also stop, even though the callback has not seen the CANCELLED state.
cursorReturnedDoneOrException = true;
}
}
CallbackResponse response;
try {
response = callback.cursorReady(AsyncResultSetImpl.this);
} catch (Throwable e) {
synchronized (monitor) {
if (cursorReturnedDoneOrException
&& state == State.CANCELLED
&& e instanceof SpannerException
&& ((SpannerException) e).getErrorCode() == ErrorCode.CANCELLED) {
// The callback did not catch the cancelled exception (which it should have), but
// we'll keep the cancelled state.
return;
}
executionException = SpannerExceptionFactory.asSpannerException(e);
cursorReturnedDoneOrException = true;
}
return;
}
synchronized (monitor) {
if (state == State.CANCELLED) {
if (cursorReturnedDoneOrException) {
return;
}
} else {
switch (response) {
case DONE:
state = State.DONE;
cursorReturnedDoneOrException = true;
return;
case PAUSE:
state = State.PAUSED;
// Make sure no-one else is waiting on the current pause latch and create a new
// one.
pausedLatch.countDown();
pausedLatch = new CountDownLatch(1);
return;
case CONTINUE:
if (buffer.isEmpty()) {
// Call the callback once more if the entire result set has been processed but
// the callback has not yet received a CursorState.DONE or a CANCELLED error.
if (finished && !cursorReturnedDoneOrException) {
break;
}
state = State.RUNNING;
return;
}
break;
default:
throw new IllegalStateException("Unknown response: " + response);
}
}
}
}
} finally {
synchronized (monitor) {
// Count down all latches that the producer might be waiting on.
consumingLatch.countDown();
while (bufferConsumptionLatch.getCount() > 0L) {
bufferConsumptionLatch.countDown();
}
}
}
}
}
private final CallbackRunnable callbackRunnable = new CallbackRunnable();
/**
* {@link ProduceRowsCallable} reads data from the underlying {@link ResultSet}, places these in
* the buffer and dispatches the {@link CallbackRunnable} when data is ready to be consumed.
*/
private class ProduceRowsCallable implements Callable<Void> {
@Override
public Void call() throws Exception {
boolean stop = false;
boolean hasNext = false;
try {
hasNext = delegateResultSet.get().next();
} catch (Throwable e) {
synchronized (monitor) {
executionException = SpannerExceptionFactory.asSpannerException(e);
}
}
try {
while (!stop && hasNext) {
try {
synchronized (monitor) {
stop = state.shouldStop;
}
if (!stop) {
while (buffer.remainingCapacity() == 0 && !stop) {
waitIfPaused();
// The buffer is full and we should let the callback consume a number of rows before
// we proceed with producing any more rows to prevent us from potentially waiting on
// a full buffer repeatedly.
// Wait until at least half of the buffer is available, or if it's a bigger buffer,
// wait until at least 10 rows can be placed in it.
// TODO: Make this more dynamic / configurable?
startCallbackWithBufferLatchIfNecessary(
Math.min(
Math.min(buffer.size() / 2 + 1, buffer.size()),
MAX_WAIT_FOR_BUFFER_CONSUMPTION));
bufferConsumptionLatch.await();
synchronized (monitor) {
stop = state.shouldStop;
}
}
}
if (!stop) {
buffer.put(delegateResultSet.get().getCurrentRowAsStruct());
startCallbackIfNecessary();
hasNext = delegateResultSet.get().next();
}
} catch (Throwable e) {
synchronized (monitor) {
executionException = SpannerExceptionFactory.asSpannerException(e);
stop = true;
}
}
}
// We don't need any more data from the underlying result set, so we close it as soon as
// possible. Any error that might occur during this will be ignored.
closeDelegateResultSet();
// Ensure that the callback has been called at least once, even if the result set was
// cancelled.
synchronized (monitor) {
finished = true;
stop = cursorReturnedDoneOrException;
}
// Call the callback if there are still rows in the buffer that need to be processed.
while (!stop) {
waitIfPaused();
startCallbackIfNecessary();
// Make sure we wait until the callback runner has actually finished.
consumingLatch.await();
synchronized (monitor) {
stop = cursorReturnedDoneOrException;
}
}
} finally {
if (executorProvider.shouldAutoClose()) {
service.shutdown();
}
for (Runnable listener : listeners) {
listener.run();
}
synchronized (monitor) {
if (executionException != null) {
throw executionException;
}
if (state == State.CANCELLED) {
throw CANCELLED_EXCEPTION;
}
}
}
return null;
}
private void waitIfPaused() throws InterruptedException {
CountDownLatch pause;
synchronized (monitor) {
pause = pausedLatch;
}
pause.await();
}
private void startCallbackIfNecessary() {
startCallbackWithBufferLatchIfNecessary(0);
}
private void startCallbackWithBufferLatchIfNecessary(int bufferLatch) {
synchronized (monitor) {
if ((state == State.RUNNING || state == State.CANCELLED)
&& !cursorReturnedDoneOrException) {
consumingLatch = new CountDownLatch(1);
if (bufferLatch > 0) {
bufferConsumptionLatch = new CountDownLatch(bufferLatch);
}
if (state == State.RUNNING) {
state = State.CONSUMING;
}
executor.execute(callbackRunnable);
}
}
}
}
/** Sets the callback for this {@link AsyncResultSet}. */
@Override
public ApiFuture<Void> setCallback(Executor exec, ReadyCallback cb) {
synchronized (monitor) {
Preconditions.checkState(!closed, "This AsyncResultSet has been closed");
Preconditions.checkState(
this.state == State.INITIALIZED, "callback may not be set multiple times");
// Start to fetch data and buffer these.
this.result =
new ListenableFutureToApiFuture<>(this.service.submit(new ProduceRowsCallable()));
this.executor = MoreExecutors.newSequentialExecutor(Preconditions.checkNotNull(exec));
this.callback = Preconditions.checkNotNull(cb);
this.state = State.RUNNING;
pausedLatch.countDown();
return result;
}
}
Future<Void> getResult() {
return result;
}
@Override
public void cancel() {
synchronized (monitor) {
Preconditions.checkState(
state != State.INITIALIZED && state != State.SYNC,
"cannot cancel a result set without a callback");
state = State.CANCELLED;
pausedLatch.countDown();
}
}
@Override
public void resume() {
synchronized (monitor) {
Preconditions.checkState(
state != State.INITIALIZED && state != State.SYNC,
"cannot resume a result set without a callback");
if (state == State.PAUSED) {
state = State.RUNNING;
pausedLatch.countDown();
}
}
}
private static class CreateListCallback<T> implements ReadyCallback {
private final SettableApiFuture<List<T>> future;
private final Function<StructReader, T> transformer;
private final ImmutableList.Builder<T> builder = ImmutableList.builder();
private CreateListCallback(
SettableApiFuture<List<T>> future, Function<StructReader, T> transformer) {
this.future = future;
this.transformer = transformer;
}
@Override
public CallbackResponse cursorReady(AsyncResultSet resultSet) {
try {
while (true) {
switch (resultSet.tryNext()) {
case DONE:
future.set(builder.build());
return CallbackResponse.DONE;
case NOT_READY:
return CallbackResponse.CONTINUE;
case OK:
builder.add(transformer.apply(resultSet));
break;
}
}
} catch (Throwable t) {
future.setException(t);
return CallbackResponse.DONE;
}
}
}
@Override
public <T> ApiFuture<List<T>> toListAsync(
Function<StructReader, T> transformer, Executor executor) {
synchronized (monitor) {
Preconditions.checkState(!closed, "This AsyncResultSet has been closed");
Preconditions.checkState(
this.state == State.INITIALIZED, "This AsyncResultSet has already been used.");
final SettableApiFuture<List<T>> res = SettableApiFuture.create();
CreateListCallback<T> callback = new CreateListCallback<>(res, transformer);
ApiFuture<Void> finished = setCallback(executor, callback);
return ApiFutures.transformAsync(finished, ignored -> res, MoreExecutors.directExecutor());
}
}
@Override
public <T> List<T> toList(Function<StructReader, T> transformer) throws SpannerException {
ApiFuture<List<T>> future = toListAsync(transformer, MoreExecutors.directExecutor());
try {
return future.get();
} catch (ExecutionException e) {
throw SpannerExceptionFactory.asSpannerException(e.getCause());
} catch (Throwable e) {
throw SpannerExceptionFactory.asSpannerException(e);
}
}
@Override
public boolean next() throws SpannerException {
synchronized (monitor) {
Preconditions.checkState(
this.state == State.INITIALIZED || this.state == State.SYNC,
"Cannot call next() on a result set with a callback.");
this.state = State.SYNC;
}
boolean res = delegateResultSet.get().next();
currentRow = res ? delegateResultSet.get().getCurrentRowAsStruct() : null;
return res;
}
@Override
public ResultSetStats getStats() {
return delegateResultSet.get().getStats();
}
@Override
protected void checkValidState() {
synchronized (monitor) {
Preconditions.checkState(
state == State.SYNC || state == State.CONSUMING || state == State.CANCELLED,
"only allowed after a next() call or from within a ReadyCallback#cursorReady callback");
Preconditions.checkState(state != State.SYNC || !closed, "ResultSet is closed");
}
}
@Override
public Struct getCurrentRowAsStruct() {
checkValidState();
return currentRow;
}
}