ndimiduk commented on a change in pull request #781: HBASE-18095: Zookeeper-less client connection implementation URL: https://github.com/apache/hbase/pull/781#discussion_r344420872
########## File path: hbase-server/src/main/java/org/apache/hadoop/hbase/master/MetaRegionLocationCache.java ########## @@ -0,0 +1,185 @@ +/** + * 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.master; + +import org.apache.hadoop.hbase.MetaRegionsNotAvailableException; +import org.apache.hadoop.hbase.shaded.protobuf.generated.HBaseProtos; +import org.apache.hadoop.hbase.shaded.protobuf.ProtobufUtil; + +import org.apache.hadoop.hbase.HRegionLocation; +import org.apache.hadoop.hbase.types.CopyOnWriteArrayMap; +import org.apache.hadoop.hbase.zookeeper.MetaTableLocator; +import org.apache.hadoop.hbase.zookeeper.ZKListener; +import org.apache.hadoop.hbase.zookeeper.ZKWatcher; +import org.apache.hadoop.hbase.zookeeper.ZNodePaths; +import org.apache.hbase.thirdparty.com.google.common.annotations.VisibleForTesting; +import org.apache.yetus.audience.InterfaceAudience; +import org.apache.zookeeper.KeeperException; +import org.slf4j.Logger; +import org.slf4j.LoggerFactory; + +import java.util.ArrayList; +import java.util.List; +import java.util.concurrent.ConcurrentNavigableMap; + +/** + * A cache of meta region location metadata. This cache is used to serve 'GetMetaLocations' RPCs + * from clients. Registers a listener on ZK to track changes to the meta table znodes. Clients + * are expected to retry if the meta information is stale. This class is thread-safe. + */ +@InterfaceAudience.Private +public class MetaRegionLocationCache extends ZKListener { + + private static final Logger LOG = LoggerFactory.getLogger(MetaRegionLocationCache.class); + + // Maximum number of times we retry when ZK operation times out. Should this be configurable? + private static final int MAX_ZK_META_FETCH_RETRIES = 10; + + private ZKWatcher watcher; + // Cached meta region locations indexed by replica ID. + // CopyOnWriteArrayMap ensures synchronization during updates and a consistent snapshot during + // client requests. Even though CopyOnWriteArrayMap copies the data structure for every write, + // that should be OK since the size of the list is often small and mutations are not too often + // and we do not need to block client requests while mutations are in progress. + CopyOnWriteArrayMap<Integer, HRegionLocation> cachedMetaLocations; + + private enum ZNodeOpType { + INIT, + CREATED, + CHANGED, + DELETED + }; + + public MetaRegionLocationCache(ZKWatcher zkWatcher) { + super(zkWatcher); + watcher = zkWatcher; + cachedMetaLocations = new CopyOnWriteArrayMap<>(); + watcher.registerListener(this); + // Populate the initial snapshot of data from meta znodes. + // This is needed because stand-by masters can potentially start after the initial znode + // creation. + populateInitialMetaLocations(); + } + + private void populateInitialMetaLocations() { + int retries = 0; + while (retries++ < MAX_ZK_META_FETCH_RETRIES) { + try { + List<String> znodes = watcher.getMetaReplicaNodes(); + for (String znode: znodes) { + String path = ZNodePaths.joinZNode(watcher.getZNodePaths().baseZNode, znode); + updateMetaLocation(path, ZNodeOpType.INIT); + } + break; + } catch (KeeperException.OperationTimeoutException e) { + LOG.warn("Timed out connecting to ZK cluster", e); + + } catch (KeeperException e) { + LOG.warn("Error populating initial meta locations", e); + break; + } + } + } + + private void updateMetaLocation(String path, ZNodeOpType opType) { + if (!isValidMetaZNode(path)) { + return; + } + LOG.info("Meta znode for path {}: {}", path, opType.name()); + int replicaId = watcher.getZNodePaths().getMetaReplicaIdFromPath(path); + if (opType == ZNodeOpType.DELETED) { + cachedMetaLocations.remove(replicaId); + return; + } + RegionState state = null; + int retries = 0; + while (retries++ < MAX_ZK_META_FETCH_RETRIES) { + try { + state = MetaTableLocator.getMetaRegionState(watcher, replicaId); + break; + } catch (KeeperException.OperationTimeoutException oe) { + // LOG and retry. + LOG.warn("Timed out fetching meta location information for path {}", path, oe); + } catch (KeeperException e) { + LOG.error("Error getting meta location for path {}", path, e); + break; + } + } + if (state == null) { + cachedMetaLocations.put(replicaId, null); + return; + } + cachedMetaLocations.put( + replicaId, new HRegionLocation(state.getRegion(), state.getServerName())); + } + + /** + * Converts the current cache snapshot into a GetMetaLocations() RPC return payload. + * @return Protobuf serialized list of cached meta HRegionLocations + * @throws MetaRegionsNotAvailableException if the cache is not populated. Review comment: I'm not a fan of exception-based flow control. How about an empty list or perhaps an `Optional.empty()`? ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org With regards, Apache Git Services
