[GitHub] [hadoop-ozone] nandakumar131 commented on a change in pull request #1340: HDDS-3188 Add failover proxy for SCM block location.

[GitBox <gi...@apache.org>]

nandakumar131 commented on a change in pull request #1340:
URL: https://github.com/apache/hadoop-ozone/pull/1340#discussion_r485108126



##########
File path: hadoop-hdds/framework/src/main/java/org/apache/hadoop/hdds/scm/proxy/SCMBlockLocationFailoverProxyProvider.java
##########
@@ -0,0 +1,281 @@
+/**
+ * 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.hdds.scm.proxy;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hdds.conf.ConfigurationSource;
+import org.apache.hadoop.hdds.scm.ScmConfigKeys;
+import org.apache.hadoop.hdds.scm.protocol.ScmBlockLocationProtocol;
+import org.apache.hadoop.hdds.scm.protocolPB.ScmBlockLocationProtocolPB;
+import org.apache.hadoop.hdds.utils.LegacyHadoopConfigurationSource;
+import org.apache.hadoop.io.retry.FailoverProxyProvider;
+import org.apache.hadoop.io.retry.RetryPolicy;
+import org.apache.hadoop.io.retry.RetryPolicy.RetryAction;
+import org.apache.hadoop.ipc.ProtobufRpcEngine;
+import org.apache.hadoop.ipc.RPC;
+import org.apache.hadoop.net.NetUtils;
+import org.apache.hadoop.security.UserGroupInformation;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.Closeable;
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+import static org.apache.hadoop.hdds.scm.ScmConfigKeys.OZONE_SCM_NAMES;
+import static org.apache.hadoop.hdds.scm.ScmConfigKeys.OZONE_SCM_SERVICE_IDS_KEY;
+import static org.apache.hadoop.hdds.HddsUtils.getScmAddressForBlockClients;
+import static org.apache.hadoop.hdds.HddsUtils.getPortNumberFromConfigKeys;
+import static org.apache.hadoop.hdds.HddsUtils.getHostName;
+
+/**
+ * Failover proxy provider for SCM.
+ */
+public class SCMBlockLocationFailoverProxyProvider implements
+    FailoverProxyProvider<ScmBlockLocationProtocolPB>, Closeable {
+  public static final Logger LOG =
+      LoggerFactory.getLogger(SCMBlockLocationFailoverProxyProvider.class);
+
+  private Map<String, ProxyInfo<ScmBlockLocationProtocolPB>> scmProxies;
+  private Map<String, SCMProxyInfo> scmProxyInfoMap;
+  private List<String> scmNodeIDList;
+
+  private String currentProxySCMNodeId;
+  private int currentProxyIndex;
+
+  private final ConfigurationSource conf;
+  private final long scmVersion;
+
+  private final String scmServiceId;
+
+  private String lastAttemptedLeader;
+
+  private final int maxRetryCount;
+  private final long retryInterval;
+
+  public static final String SCM_DUMMY_NODEID_PREFIX = "scm";
+
+  public SCMBlockLocationFailoverProxyProvider(ConfigurationSource conf) {
+    this.conf = conf;
+    this.scmVersion = RPC.getProtocolVersion(ScmBlockLocationProtocol.class);
+    this.scmServiceId = conf.getTrimmed(OZONE_SCM_SERVICE_IDS_KEY);
+    this.scmProxies = new HashMap<>();
+    this.scmProxyInfoMap = new HashMap<>();
+    this.scmNodeIDList = new ArrayList<>();
+    loadConfigs();
+
+    this.currentProxyIndex = 0;
+    currentProxySCMNodeId = scmNodeIDList.get(currentProxyIndex);
+
+    this.maxRetryCount = conf.getObject(SCMBlockClientConfig.class)
+        .getRetryCount();
+    this.retryInterval = conf.getObject(SCMBlockClientConfig.class)

Review comment:
       `conf#getObject` is a heavy call, we can avoid calling it twice.

##########
File path: hadoop-hdds/framework/src/main/java/org/apache/hadoop/hdds/scm/proxy/SCMBlockLocationFailoverProxyProvider.java
##########
@@ -0,0 +1,281 @@
+/**
+ * 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.hdds.scm.proxy;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hdds.conf.ConfigurationSource;
+import org.apache.hadoop.hdds.scm.ScmConfigKeys;
+import org.apache.hadoop.hdds.scm.protocol.ScmBlockLocationProtocol;
+import org.apache.hadoop.hdds.scm.protocolPB.ScmBlockLocationProtocolPB;
+import org.apache.hadoop.hdds.utils.LegacyHadoopConfigurationSource;
+import org.apache.hadoop.io.retry.FailoverProxyProvider;
+import org.apache.hadoop.io.retry.RetryPolicy;
+import org.apache.hadoop.io.retry.RetryPolicy.RetryAction;
+import org.apache.hadoop.ipc.ProtobufRpcEngine;
+import org.apache.hadoop.ipc.RPC;
+import org.apache.hadoop.net.NetUtils;
+import org.apache.hadoop.security.UserGroupInformation;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.Closeable;
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+import static org.apache.hadoop.hdds.scm.ScmConfigKeys.OZONE_SCM_NAMES;
+import static org.apache.hadoop.hdds.scm.ScmConfigKeys.OZONE_SCM_SERVICE_IDS_KEY;
+import static org.apache.hadoop.hdds.HddsUtils.getScmAddressForBlockClients;
+import static org.apache.hadoop.hdds.HddsUtils.getPortNumberFromConfigKeys;
+import static org.apache.hadoop.hdds.HddsUtils.getHostName;
+
+/**
+ * Failover proxy provider for SCM.
+ */
+public class SCMBlockLocationFailoverProxyProvider implements
+    FailoverProxyProvider<ScmBlockLocationProtocolPB>, Closeable {
+  public static final Logger LOG =
+      LoggerFactory.getLogger(SCMBlockLocationFailoverProxyProvider.class);
+
+  private Map<String, ProxyInfo<ScmBlockLocationProtocolPB>> scmProxies;
+  private Map<String, SCMProxyInfo> scmProxyInfoMap;
+  private List<String> scmNodeIDList;
+
+  private String currentProxySCMNodeId;
+  private int currentProxyIndex;
+
+  private final ConfigurationSource conf;
+  private final long scmVersion;
+
+  private final String scmServiceId;
+
+  private String lastAttemptedLeader;
+
+  private final int maxRetryCount;
+  private final long retryInterval;
+
+  public static final String SCM_DUMMY_NODEID_PREFIX = "scm";
+
+  public SCMBlockLocationFailoverProxyProvider(ConfigurationSource conf) {
+    this.conf = conf;
+    this.scmVersion = RPC.getProtocolVersion(ScmBlockLocationProtocol.class);
+    this.scmServiceId = conf.getTrimmed(OZONE_SCM_SERVICE_IDS_KEY);
+    this.scmProxies = new HashMap<>();
+    this.scmProxyInfoMap = new HashMap<>();
+    this.scmNodeIDList = new ArrayList<>();
+    loadConfigs();
+
+    this.currentProxyIndex = 0;
+    currentProxySCMNodeId = scmNodeIDList.get(currentProxyIndex);
+
+    this.maxRetryCount = conf.getObject(SCMBlockClientConfig.class)
+        .getRetryCount();
+    this.retryInterval = conf.getObject(SCMBlockClientConfig.class)
+        .getRetryInterval();
+  }
+
+  @VisibleForTesting
+  protected Collection<InetSocketAddress> getSCMAddressList() {
+    Collection<String> scmAddressList =
+        conf.getTrimmedStringCollection(OZONE_SCM_NAMES);
+    Collection<InetSocketAddress> resultList = new ArrayList<>();
+    if (!scmAddressList.isEmpty()) {
+      final int port = getPortNumberFromConfigKeys(conf,
+          ScmConfigKeys.OZONE_SCM_BLOCK_CLIENT_ADDRESS_KEY)
+          .orElse(ScmConfigKeys.OZONE_SCM_BLOCK_CLIENT_PORT_DEFAULT);
+      for (String scmAddress : scmAddressList) {
+        LOG.info("SCM Address for proxy is {}", scmAddress);
+
+        Optional<String> hostname = getHostName(scmAddress);
+        if (hostname.isPresent()) {
+          resultList.add(NetUtils.createSocketAddr(
+              hostname.get() + ":" + port));
+        }
+      }
+    }
+    if (resultList.isEmpty()) {
+      // fall back
+      resultList.add(getScmAddressForBlockClients(conf));
+    }
+    return resultList;
+  }
+
+  private void loadConfigs() {
+    Collection<InetSocketAddress> scmAddressList = getSCMAddressList();
+    int scmNodeIndex = 1;
+    for (InetSocketAddress scmAddress : scmAddressList) {
+      String nodeId = SCM_DUMMY_NODEID_PREFIX + scmNodeIndex;
+      if (scmAddress == null) {
+        LOG.error("Failed to create SCM proxy for {}.", nodeId);
+        continue;
+      }
+      scmNodeIndex++;
+      SCMProxyInfo scmProxyInfo = new SCMProxyInfo(
+          scmServiceId, nodeId, scmAddress);
+      ProxyInfo<ScmBlockLocationProtocolPB> proxy = new ProxyInfo<>(
+          null, scmProxyInfo.toString());
+      scmProxies.put(nodeId, proxy);

Review comment:
       Can we avoid putting `null` proxy?
   While reading, instead of checking if `proxyInfo.proxy == null`, we can do `proxyInfo#contains`.

##########
File path: hadoop-hdds/framework/src/main/java/org/apache/hadoop/hdds/scm/proxy/SCMBlockLocationFailoverProxyProvider.java
##########
@@ -0,0 +1,281 @@
+/**
+ * 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.hdds.scm.proxy;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hdds.conf.ConfigurationSource;
+import org.apache.hadoop.hdds.scm.ScmConfigKeys;
+import org.apache.hadoop.hdds.scm.protocol.ScmBlockLocationProtocol;
+import org.apache.hadoop.hdds.scm.protocolPB.ScmBlockLocationProtocolPB;
+import org.apache.hadoop.hdds.utils.LegacyHadoopConfigurationSource;
+import org.apache.hadoop.io.retry.FailoverProxyProvider;
+import org.apache.hadoop.io.retry.RetryPolicy;
+import org.apache.hadoop.io.retry.RetryPolicy.RetryAction;
+import org.apache.hadoop.ipc.ProtobufRpcEngine;
+import org.apache.hadoop.ipc.RPC;
+import org.apache.hadoop.net.NetUtils;
+import org.apache.hadoop.security.UserGroupInformation;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.Closeable;
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+import static org.apache.hadoop.hdds.scm.ScmConfigKeys.OZONE_SCM_NAMES;
+import static org.apache.hadoop.hdds.scm.ScmConfigKeys.OZONE_SCM_SERVICE_IDS_KEY;
+import static org.apache.hadoop.hdds.HddsUtils.getScmAddressForBlockClients;
+import static org.apache.hadoop.hdds.HddsUtils.getPortNumberFromConfigKeys;
+import static org.apache.hadoop.hdds.HddsUtils.getHostName;
+
+/**
+ * Failover proxy provider for SCM.
+ */
+public class SCMBlockLocationFailoverProxyProvider implements
+    FailoverProxyProvider<ScmBlockLocationProtocolPB>, Closeable {
+  public static final Logger LOG =
+      LoggerFactory.getLogger(SCMBlockLocationFailoverProxyProvider.class);
+
+  private Map<String, ProxyInfo<ScmBlockLocationProtocolPB>> scmProxies;
+  private Map<String, SCMProxyInfo> scmProxyInfoMap;
+  private List<String> scmNodeIDList;
+
+  private String currentProxySCMNodeId;
+  private int currentProxyIndex;
+
+  private final ConfigurationSource conf;
+  private final long scmVersion;
+
+  private final String scmServiceId;
+
+  private String lastAttemptedLeader;
+
+  private final int maxRetryCount;
+  private final long retryInterval;
+
+  public static final String SCM_DUMMY_NODEID_PREFIX = "scm";
+
+  public SCMBlockLocationFailoverProxyProvider(ConfigurationSource conf) {
+    this.conf = conf;
+    this.scmVersion = RPC.getProtocolVersion(ScmBlockLocationProtocol.class);
+    this.scmServiceId = conf.getTrimmed(OZONE_SCM_SERVICE_IDS_KEY);
+    this.scmProxies = new HashMap<>();
+    this.scmProxyInfoMap = new HashMap<>();
+    this.scmNodeIDList = new ArrayList<>();
+    loadConfigs();
+
+    this.currentProxyIndex = 0;
+    currentProxySCMNodeId = scmNodeIDList.get(currentProxyIndex);
+
+    this.maxRetryCount = conf.getObject(SCMBlockClientConfig.class)
+        .getRetryCount();
+    this.retryInterval = conf.getObject(SCMBlockClientConfig.class)
+        .getRetryInterval();
+  }
+
+  @VisibleForTesting
+  protected Collection<InetSocketAddress> getSCMAddressList() {
+    Collection<String> scmAddressList =
+        conf.getTrimmedStringCollection(OZONE_SCM_NAMES);
+    Collection<InetSocketAddress> resultList = new ArrayList<>();
+    if (!scmAddressList.isEmpty()) {
+      final int port = getPortNumberFromConfigKeys(conf,
+          ScmConfigKeys.OZONE_SCM_BLOCK_CLIENT_ADDRESS_KEY)
+          .orElse(ScmConfigKeys.OZONE_SCM_BLOCK_CLIENT_PORT_DEFAULT);
+      for (String scmAddress : scmAddressList) {
+        LOG.info("SCM Address for proxy is {}", scmAddress);
+
+        Optional<String> hostname = getHostName(scmAddress);
+        if (hostname.isPresent()) {
+          resultList.add(NetUtils.createSocketAddr(
+              hostname.get() + ":" + port));
+        }
+      }
+    }
+    if (resultList.isEmpty()) {
+      // fall back
+      resultList.add(getScmAddressForBlockClients(conf));
+    }
+    return resultList;
+  }
+
+  private void loadConfigs() {
+    Collection<InetSocketAddress> scmAddressList = getSCMAddressList();
+    int scmNodeIndex = 1;
+    for (InetSocketAddress scmAddress : scmAddressList) {
+      String nodeId = SCM_DUMMY_NODEID_PREFIX + scmNodeIndex;
+      if (scmAddress == null) {
+        LOG.error("Failed to create SCM proxy for {}.", nodeId);
+        continue;
+      }
+      scmNodeIndex++;
+      SCMProxyInfo scmProxyInfo = new SCMProxyInfo(
+          scmServiceId, nodeId, scmAddress);
+      ProxyInfo<ScmBlockLocationProtocolPB> proxy = new ProxyInfo<>(
+          null, scmProxyInfo.toString());
+      scmProxies.put(nodeId, proxy);
+      scmProxyInfoMap.put(nodeId, scmProxyInfo);
+      scmNodeIDList.add(nodeId);
+    }
+
+    if (scmProxies.isEmpty()) {
+      throw new IllegalArgumentException("Could not find any configured " +
+          "addresses for SCM. Please configure the system with "
+          + OZONE_SCM_NAMES);
+    }
+  }
+
+  @VisibleForTesting
+  public synchronized String getCurrentProxyOMNodeId() {
+    return currentProxySCMNodeId;
+  }
+
+  @Override
+  public synchronized ProxyInfo getProxy() {
+    ProxyInfo currentProxyInfo = scmProxies.get(currentProxySCMNodeId);
+    createSCMProxyIfNeeded(currentProxyInfo, currentProxySCMNodeId);
+    return currentProxyInfo;
+  }
+
+  @Override
+  public void performFailover(ScmBlockLocationProtocolPB newLeader) {
+    // Should do nothing here.

Review comment:
       What is the reason for not doing anything here?

##########
File path: hadoop-hdds/framework/src/main/java/org/apache/hadoop/hdds/scm/proxy/SCMBlockLocationFailoverProxyProvider.java
##########
@@ -0,0 +1,281 @@
+/**
+ * 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.hdds.scm.proxy;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hdds.conf.ConfigurationSource;
+import org.apache.hadoop.hdds.scm.ScmConfigKeys;
+import org.apache.hadoop.hdds.scm.protocol.ScmBlockLocationProtocol;
+import org.apache.hadoop.hdds.scm.protocolPB.ScmBlockLocationProtocolPB;
+import org.apache.hadoop.hdds.utils.LegacyHadoopConfigurationSource;
+import org.apache.hadoop.io.retry.FailoverProxyProvider;
+import org.apache.hadoop.io.retry.RetryPolicy;
+import org.apache.hadoop.io.retry.RetryPolicy.RetryAction;
+import org.apache.hadoop.ipc.ProtobufRpcEngine;
+import org.apache.hadoop.ipc.RPC;
+import org.apache.hadoop.net.NetUtils;
+import org.apache.hadoop.security.UserGroupInformation;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.Closeable;
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+import static org.apache.hadoop.hdds.scm.ScmConfigKeys.OZONE_SCM_NAMES;
+import static org.apache.hadoop.hdds.scm.ScmConfigKeys.OZONE_SCM_SERVICE_IDS_KEY;
+import static org.apache.hadoop.hdds.HddsUtils.getScmAddressForBlockClients;
+import static org.apache.hadoop.hdds.HddsUtils.getPortNumberFromConfigKeys;
+import static org.apache.hadoop.hdds.HddsUtils.getHostName;
+
+/**
+ * Failover proxy provider for SCM.
+ */
+public class SCMBlockLocationFailoverProxyProvider implements
+    FailoverProxyProvider<ScmBlockLocationProtocolPB>, Closeable {
+  public static final Logger LOG =
+      LoggerFactory.getLogger(SCMBlockLocationFailoverProxyProvider.class);
+
+  private Map<String, ProxyInfo<ScmBlockLocationProtocolPB>> scmProxies;
+  private Map<String, SCMProxyInfo> scmProxyInfoMap;
+  private List<String> scmNodeIDList;
+
+  private String currentProxySCMNodeId;
+  private int currentProxyIndex;
+
+  private final ConfigurationSource conf;
+  private final long scmVersion;
+
+  private final String scmServiceId;
+
+  private String lastAttemptedLeader;
+
+  private final int maxRetryCount;
+  private final long retryInterval;
+
+  public static final String SCM_DUMMY_NODEID_PREFIX = "scm";
+
+  public SCMBlockLocationFailoverProxyProvider(ConfigurationSource conf) {
+    this.conf = conf;
+    this.scmVersion = RPC.getProtocolVersion(ScmBlockLocationProtocol.class);
+    this.scmServiceId = conf.getTrimmed(OZONE_SCM_SERVICE_IDS_KEY);
+    this.scmProxies = new HashMap<>();
+    this.scmProxyInfoMap = new HashMap<>();
+    this.scmNodeIDList = new ArrayList<>();
+    loadConfigs();
+
+    this.currentProxyIndex = 0;
+    currentProxySCMNodeId = scmNodeIDList.get(currentProxyIndex);
+
+    this.maxRetryCount = conf.getObject(SCMBlockClientConfig.class)
+        .getRetryCount();
+    this.retryInterval = conf.getObject(SCMBlockClientConfig.class)
+        .getRetryInterval();
+  }
+
+  @VisibleForTesting
+  protected Collection<InetSocketAddress> getSCMAddressList() {
+    Collection<String> scmAddressList =
+        conf.getTrimmedStringCollection(OZONE_SCM_NAMES);
+    Collection<InetSocketAddress> resultList = new ArrayList<>();
+    if (!scmAddressList.isEmpty()) {
+      final int port = getPortNumberFromConfigKeys(conf,
+          ScmConfigKeys.OZONE_SCM_BLOCK_CLIENT_ADDRESS_KEY)
+          .orElse(ScmConfigKeys.OZONE_SCM_BLOCK_CLIENT_PORT_DEFAULT);
+      for (String scmAddress : scmAddressList) {
+        LOG.info("SCM Address for proxy is {}", scmAddress);
+
+        Optional<String> hostname = getHostName(scmAddress);
+        if (hostname.isPresent()) {
+          resultList.add(NetUtils.createSocketAddr(
+              hostname.get() + ":" + port));
+        }
+      }
+    }
+    if (resultList.isEmpty()) {
+      // fall back
+      resultList.add(getScmAddressForBlockClients(conf));
+    }
+    return resultList;
+  }
+
+  private void loadConfigs() {
+    Collection<InetSocketAddress> scmAddressList = getSCMAddressList();
+    int scmNodeIndex = 1;
+    for (InetSocketAddress scmAddress : scmAddressList) {
+      String nodeId = SCM_DUMMY_NODEID_PREFIX + scmNodeIndex;
+      if (scmAddress == null) {
+        LOG.error("Failed to create SCM proxy for {}.", nodeId);
+        continue;
+      }
+      scmNodeIndex++;
+      SCMProxyInfo scmProxyInfo = new SCMProxyInfo(
+          scmServiceId, nodeId, scmAddress);
+      ProxyInfo<ScmBlockLocationProtocolPB> proxy = new ProxyInfo<>(
+          null, scmProxyInfo.toString());
+      scmProxies.put(nodeId, proxy);
+      scmProxyInfoMap.put(nodeId, scmProxyInfo);
+      scmNodeIDList.add(nodeId);
+    }
+
+    if (scmProxies.isEmpty()) {
+      throw new IllegalArgumentException("Could not find any configured " +
+          "addresses for SCM. Please configure the system with "
+          + OZONE_SCM_NAMES);
+    }
+  }
+
+  @VisibleForTesting
+  public synchronized String getCurrentProxyOMNodeId() {
+    return currentProxySCMNodeId;
+  }
+
+  @Override
+  public synchronized ProxyInfo getProxy() {
+    ProxyInfo currentProxyInfo = scmProxies.get(currentProxySCMNodeId);
+    createSCMProxyIfNeeded(currentProxyInfo, currentProxySCMNodeId);
+    return currentProxyInfo;
+  }
+
+  @Override
+  public void performFailover(ScmBlockLocationProtocolPB newLeader) {
+    // Should do nothing here.
+    LOG.debug("Failing over to next proxy. {}", getCurrentProxyOMNodeId());
+  }
+
+  public void performFailoverToAssignedLeader(String newLeader) {
+    if (newLeader == null) {
+      // If newLeader is not assigned, it will fail over to next proxy.
+      nextProxyIndex();
+    } else {
+      if (!assignLeaderToNode(newLeader)) {
+        LOG.debug("Failing over OM proxy to nodeId: {}", newLeader);
+        nextProxyIndex();
+      }
+    }
+  }
+
+  @Override
+  public Class<ScmBlockLocationProtocolPB> getInterface() {
+    return ScmBlockLocationProtocolPB.class;
+  }
+
+  @Override
+  public synchronized void close() throws IOException {
+    for (ProxyInfo<ScmBlockLocationProtocolPB> proxy : scmProxies.values()) {
+      ScmBlockLocationProtocolPB scmProxy = proxy.proxy;
+      if (scmProxy != null) {
+        RPC.stopProxy(scmProxy);
+      }
+    }
+  }
+
+  public RetryAction getRetryAction(int failovers) {
+    if (failovers < maxRetryCount) {
+      return new RetryAction(RetryAction.RetryDecision.FAILOVER_AND_RETRY,
+          getRetryInterval());
+    } else {
+      return RetryAction.FAIL;
+    }
+  }
+
+  private synchronized long getRetryInterval() {
+    // TODO add exponential backup
+    return retryInterval;
+  }
+
+  private synchronized int nextProxyIndex() {
+    lastAttemptedLeader = currentProxySCMNodeId;
+
+    // round robin the next proxy
+    currentProxyIndex = (currentProxyIndex + 1) % scmProxies.size();
+    currentProxySCMNodeId =  scmNodeIDList.get(currentProxyIndex);
+    return currentProxyIndex;
+  }
+
+  synchronized boolean assignLeaderToNode(String newLeaderNodeId) {

Review comment:
       This can be made `private`.




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