szetszwo commented on code in PR #6613:
URL: https://github.com/apache/ozone/pull/6613#discussion_r2463035082
##########
hadoop-hdds/client/src/main/java/org/apache/hadoop/hdds/scm/XceiverClientGrpc.java:
##########
@@ -504,6 +516,65 @@ private XceiverClientReply sendCommandWithRetry(
}
}
+ /**
+ * Starts a streaming read operation, intended to read entire blocks from
the datanodes. This method expects a
+ * {@link StreamObserver} to be passed in, which will be used to receive the
streamed data from the datanode.
+ * Upon successfully starting the streaming read, a {@link
StreamingReadResponse} is returned, which contains
+ * information about the datanode used for the read, and the request
observer that can be used to manage the stream
+ * (e.g., to cancel it if needed). A semaphore is acquired to limit the
number of concurrent streaming reads so upon
+ * successful return of this method, the caller must ensure to call {@link
#completeStreamRead(StreamingReadResponse)}
+ * to release the semaphore once the streaming read is complete.
+ * @param request The container command request to initiate the streaming
read.
+ * @param streamObserver The observer that will handle the streamed
responses.
+ * @return A {@link StreamingReadResponse} containing details of the
streaming read operation.
+ * @throws IOException
+ * @throws InterruptedException
+ */
+ @Override
+ public StreamingReadResponse streamRead(ContainerCommandRequestProto request,
+ StreamObserver<ContainerCommandResponseProto> streamObserver) throws
IOException, InterruptedException {
+ List<DatanodeDetails> datanodeList = sortDatanodes(request);
+ IOException lastException = null;
+ for (DatanodeDetails dn : datanodeList) {
+ try {
+ checkOpen(dn);
+ semaphore.acquire();
+ XceiverClientProtocolServiceStub stub = asyncStubs.get(dn.getID());
+ if (stub == null) {
+ throw new IOException("Failed to get gRPC stub for DataNode: " + dn);
+ }
+ if (LOG.isDebugEnabled()) {
+ LOG.debug("Executing command {} on datanode {}",
processForDebug(request), dn);
+ }
+ StreamObserver<ContainerCommandRequestProto> requestObserver = stub
+ .withDeadlineAfter(timeout, TimeUnit.SECONDS)
+ .send(streamObserver);
+ requestObserver.onNext(request);
+ requestObserver.onCompleted();
+ return new StreamingReadResponse(dn,
(ClientCallStreamObserver<ContainerCommandRequestProto>) requestObserver);
Review Comment:
> The approach used is similar to the HDFS xceiver approach, ...
HDFS uses blocking API from Socket but we are using gRPC async API here. We
should not bring the HDFS xceiver approach to Ozone.
- When all the gRPC threads are blocked, any gRPC calls will be blocked.
Any commands sending to that datanode will be blocked. (HDFS does not have
such problem.)
Moreover, it is easy for user applications to get a deadlock due to the
problem above.
1. Suppose there are $n$ gRPC threads.
2. Open $n+1$ files to read at the same time.
3. Do not close the files until the all files are processed. // deadlock
here
See also
<img width="664" height="932" alt="image"
src="https://github.com/user-attachments/assets/556e144b-42f1-4ff0-816c-63debc30ac2f";
/>
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