in gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/HttpGremlinEndpointHandler.java [413:551]
private void handleIterator(final Context context, final Iterator itty, final MessageSerializer<?> serializer, final boolean bulking) throws InterruptedException {
final ChannelHandlerContext nettyContext = context.getChannelHandlerContext();
final RequestMessage msg = context.getRequestMessage();
final Settings settings = context.getSettings();
// used to limit warnings for when netty fills the buffer and hits the high watermark - prevents
// over-logging of the same message.
long lastWarningTime = 0;
int warnCounter = 0;
// we have an empty iterator - happens on stuff like: g.V().iterate()
if (!itty.hasNext()) {
ByteBuf chunk = null;
try {
chunk = makeChunk(context, serializer, new ArrayList<>(), false, bulking);
nettyContext.writeAndFlush(new DefaultHttpContent(chunk));
} catch (Exception ex) {
// Bytebuf is a countable release - if it does not get written downstream
// it needs to be released here
if (chunk != null) chunk.release();
}
sendTrailingHeaders(nettyContext, HttpResponseStatus.OK, "");
return;
}
// the batch size can be overridden by the request
final int resultIterationBatchSize = (Integer) msg.optionalField(Tokens.ARGS_BATCH_SIZE)
.orElse(settings.resultIterationBatchSize);
List<Object> aggregate = new ArrayList<>(resultIterationBatchSize);
// use an external control to manage the loop as opposed to just checking hasNext() in the while. this
// prevent situations where auto transactions create a new transaction after calls to commit() withing
// the loop on calls to hasNext().
boolean hasMore = itty.hasNext();
while (hasMore) {
if (Thread.interrupted()) throw new InterruptedException();
// have to check the aggregate size because it is possible that the channel is not writeable (below)
// so iterating next() if the message is not written and flushed would bump the aggregate size beyond
// the expected resultIterationBatchSize. Total serialization time for the response remains in
// effect so if the client is "slow" it may simply timeout.
//
// there is a need to check hasNext() on the iterator because if the channel is not writeable the
// previous pass through the while loop will have next()'d the iterator and if it is "done" then a
// NoSuchElementException will raise its head. also need a check to ensure that this iteration doesn't
// require a forced flush which can be forced by sub-classes.
//
// this could be placed inside the isWriteable() portion of the if-then below but it seems better to
// allow iteration to continue into a batch if that is possible rather than just doing nothing at all
// while waiting for the client to catch up
if (aggregate.size() < resultIterationBatchSize && itty.hasNext()) {
if (bulking) {
Traverser traverser = (Traverser) itty.next();
aggregate.add(traverser.get());
aggregate.add(traverser.bulk());
} else {
aggregate.add(itty.next());
}
}
// Don't keep executor busy if client has already given up; there is no way to catch up if the channel is
// not active, and hence we should break the loop.
if (!nettyContext.channel().isActive()) {
break;
}
// send back a page of results if batch size is met or if it's the end of the results being iterated.
// also check writeability of the channel to prevent OOME for slow clients.
//
// clients might decide to close the Netty channel to the server with a CloseWebsocketFrame after errors
// like CorruptedFrameException. On the server, although the channel gets closed, there might be some
// executor threads waiting for watermark to clear which will not clear in these cases since client has
// already given up on these requests. This leads to these executors waiting for the client to consume
// results till the timeout. checking for isActive() should help prevent that.
if (nettyContext.channel().isActive() && nettyContext.channel().isWritable()) {
if (aggregate.size() == resultIterationBatchSize || !itty.hasNext()) {
ByteBuf chunk = null;
try {
chunk = makeChunk(context, serializer, aggregate, itty.hasNext(), bulking);
} catch (Exception ex) {
// Bytebuf is a countable release - if it does not get written downstream
// it needs to be released here
if (chunk != null) chunk.release();
// exception is handled in makeFrame() - serialization error gets written back to driver
// at that point
break;
}
// track whether there is anything left in the iterator because it needs to be accessed after
// the transaction could be closed - in that case a call to hasNext() could open a new transaction
// unintentionally
hasMore = itty.hasNext();
try {
// only need to reset the aggregation list if there's more stuff to write
if (hasMore) {
aggregate = new ArrayList<>(resultIterationBatchSize);
}
} catch (Exception ex) {
// Bytebuf is a countable release - if it does not get written downstream
// it needs to be released here
if (chunk != null) chunk.release();
throw ex;
}
nettyContext.writeAndFlush(new DefaultHttpContent(chunk));
if (!hasMore) {
sendTrailingHeaders(nettyContext, HttpResponseStatus.OK, "");
}
}
} else {
final long currentTime = System.currentTimeMillis();
// exponential delay between warnings. don't keep triggering this warning over and over again for the
// same request. totalPendingWriteBytes is volatile so it is possible that by the time this warning
// hits the log the low watermark may have been hit
long interval = (long) Math.pow(2, warnCounter) * 1000;
if (currentTime - lastWarningTime >= interval) {
final Channel ch = context.getChannelHandlerContext().channel();
logger.warn("Warning {}: Outbound buffer size={}, pausing response writing as writeBufferHighWaterMark exceeded on request {} for channel {} - writing will continue once client has caught up",
warnCounter,
ch.unsafe().outboundBuffer().totalPendingWriteBytes(),
ch.attr(StateKey.REQUEST_ID),
ch.id());
lastWarningTime = currentTime;
warnCounter++;
}
// since the client is lagging we can hold here for a period of time for the client to catch up.
// this isn't blocking the IO thread - just a worker.
TimeUnit.MILLISECONDS.sleep(WRITE_PAUSE_TIME_MS);
writePausesMeter.mark();
}
}
}