ndimiduk commented on a change in pull request #954: HBASE-23305: Master based registry implementation URL: https://github.com/apache/hbase/pull/954#discussion_r362657347
########## File path: hbase-client/src/main/java/org/apache/hadoop/hbase/ipc/HedgedRpcChannel.java ########## @@ -0,0 +1,266 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you 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 org.apache.hadoop.hbase.ipc; + +import java.io.IOException; +import java.net.InetSocketAddress; +import java.util.ArrayList; +import java.util.Collections; +import java.util.List; +import java.util.concurrent.CompletableFuture; +import java.util.concurrent.CountDownLatch; +import java.util.concurrent.atomic.AtomicInteger; +import java.util.concurrent.atomic.AtomicReference; +import org.apache.hadoop.hbase.security.User; +import org.apache.hadoop.hbase.util.PrettyPrinter; +import org.slf4j.Logger; +import org.slf4j.LoggerFactory; +import org.apache.hbase.thirdparty.com.google.common.base.Preconditions; +import org.apache.hbase.thirdparty.com.google.protobuf.Descriptors; +import org.apache.hbase.thirdparty.com.google.protobuf.Message; +import org.apache.hbase.thirdparty.com.google.protobuf.RpcCallback; +import org.apache.hbase.thirdparty.com.google.protobuf.RpcChannel; +import org.apache.hbase.thirdparty.com.google.protobuf.RpcController; + +/** + * A non-blocking implementation of RpcChannel that hedges requests to multiple service end points. + * First received response is returned to the caller. This abstracts out the logic needed to batch + * requests to multiple end points underneath and presents itself as a single logical RpcChannel to + * the client. + * + * Hedging Details: + * --------------- + * - Hedging of RPCs happens in multiple batches. In each iteration, we select a 'batch' of address + * end points to make the call to. We do multiple iterations until we get a proper response to the + * rpc call or all the service addresses are exhausted, which ever happens first. + * + * - We randomize the addresses up front so that the batch order per client is non deterministic. + * This avoids hot spots on the service side. The size of each batch is controlled via 'fanOutSize'. + * Higher fanOutSize implies we make more rpc calls in a single batch. One needs to mindful of the + * load on the client and server side when configuring the fan out. + * + * - In a happy case, once we receive a response from one end point, we cancel all the + * other inflight rpcs in the same batch and return the response to the caller. If we do not get a + * valid response from any address end point, we propagate the error back to the caller. + * + * - Rpc timeouts are applied to every hedged rpc. + * + * - Callers need to be careful about what rpcs they are trying to hedge. Not every kind of call can + * be hedged (for example: cluster state changing rpcs). + * + * (TODO) Retries and Adaptive hedging policy: + * ------------------------------------------ + * + * - No retries are handled at the channel level. Retries can be built in upper layers. However the + * question is, do we even need retries? Hedging in fact is a substitute for retries. + * + * - Clearly hedging puts more load on the service side. To mitigate this, we can make the hedging + * policy more adaptive. In most happy cases, the rpcs from the first few end points should return + * right away (especially short lived rpcs, that do not take up much time). In such cases, hedging + * is not needed. So, the idea is to make this request pattern pluggable so that the requests are + * hedged only when needed. + */ +class HedgedRpcChannel implements RpcChannel { + private static final Logger LOG = LoggerFactory.getLogger(HedgedRpcChannel.class); + + private final AbstractRpcClient rpcClient; + // List of service addresses to hedge the requests to. + private final List<InetSocketAddress> addrs; + private final User ticket; + private final int rpcTimeout; + // Controls the size of request fan out (number of rpcs per a single batch). + private final int fanOutSize; + + /** + * A simple rpc call back implementation to notify the batch context if any rpc is successful. + */ + private static class BatchRpcCtxCallBack implements RpcCallback<Message> { + private final BatchRpcCtx batchRpcCtx; + private final HBaseRpcController rpcController; + BatchRpcCtxCallBack(BatchRpcCtx batchRpcCtx, HBaseRpcController rpcController) { + this.batchRpcCtx = batchRpcCtx; + this.rpcController = rpcController; + } + @Override + public void run(Message result) { + batchRpcCtx.setResultIfNotSet(result, rpcController); + } + } + + /** + * A shared RPC context between a batch of hedged RPCs. Tracks the state and helpers needed to + * synchronize on multiple RPCs to different end points fetching the result. All the methods are + * thread-safe. + */ + private static class BatchRpcCtx { + // Result set by the thread finishing first. Set only once. + private final AtomicReference<Message> result = new AtomicReference<>(); + // Caller waits on this latch being set. + // We set this to 1, so that the first successful RPC result is returned to the client. + private CountDownLatch resultsReady = new CountDownLatch(1); + // Failed rpc book-keeping. + private AtomicInteger failedRpcCount = new AtomicInteger(); + // All the call handles for this batch. + private final List<Call> callsInFlight = Collections.synchronizedList(new ArrayList<>()); + + // Target addresses. + private final List<InetSocketAddress> addresses; + // Called when the result is ready. + private final RpcCallback<Message> callBack; + // Last failed rpc's exception. Used to propagate the reason to the controller. + private IOException lastFailedRpcReason; + + + BatchRpcCtx(List<InetSocketAddress> addresses, RpcCallback<Message> callBack) { + this.addresses = addresses; + this.callBack = Preconditions.checkNotNull(callBack); + } + + /** + * Sets the result only if it is not already set by another thread. Thread that successfully + * sets the result also count downs the latch. + * @param result Result to be set. + */ + public void setResultIfNotSet(Message result, HBaseRpcController rpcController) { + if (result == null) { + incrementFailedRpcs(rpcController.getFailed()); + return; + } + if (this.result.compareAndSet(null, result)) { + resultsReady.countDown(); + // Cancel all pending in flight calls. + for (Call call: callsInFlight) { + // It is ok to do it for all calls as it is a no-op if the call is already done. + call.setException(new CallCancelledException("Hedged call succeeded.")); Review comment: Perhaps something more descriptive like "Canceled because sibling hedged call succeeded". It's odd to see the message explaining an exception describe a successful result. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. 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