Debian Hadoop 怎樣實現高可用性

Prerequisites for Debian Hadoop HA
Before configuring high availability (HA) for Hadoop on Debian, ensure the following prerequisites are met:

All cluster nodes (NameNodes, DataNodes, JournalNodes, ResourceManager/NodeManagers) run Debian with consistent system versions and package updates.
Java (OpenJDK 8 or 11) is installed and configured with JAVA_HOME set in /etc/environment.
Hadoop (version 2.7+ recommended) is installed on all nodes, with environment variables (HADOOP_HOME, PATH) properly configured.
Passwordless SSH is set up between all nodes (using ssh-keygen and ssh-copy-id) to enable seamless communication for ZKFC and other services.

1. Install and Configure ZooKeeper Cluster
ZooKeeper is critical for distributed coordination in Hadoop HA, providing leader election and cluster state management. For fault tolerance, deploy an odd number of ZooKeeper nodes (3 or 5).

Install ZooKeeper: On each ZooKeeper node, run:

sudo apt-get update && sudo apt-get install -y zookeeper zookeeperd

Configure ZooKeeper: Edit /etc/zookeeper/conf/zoo.cfg on all nodes to include cluster members:
```
server.1=zookeeper1:2888:3888
server.2=zookeeper2:2888:3888
server.3=zookeeper3:2888:3888
```
Create a myid file in /var/lib/zookeeper/ on each node with a unique ID (e.g., 1 for zookeeper1).

Start ZooKeeper: Launch the service on all nodes:

sudo systemctl start zookeeper && sudo systemctl enable zookeeper

Verify ZooKeeper status with echo stat | nc zookeeper1 2181 (replace with your node name).

2. Configure HDFS High Availability (NameNode HA)
HDFS HA eliminates the single point of failure (SPOF) of the NameNode by using Active/Standby nodes synchronized via JournalNodes.

Edit core-site.xml: Add HDFS cluster and ZooKeeper configurations:

<property>
  <name>fs.defaultFS</name>
  <value>hdfs://mycluster</value> <!-- Logical cluster name -->
</property>
<property>
  <name>ha.zookeeper.quorum</name>
  <value>zookeeper1:2181,zookeeper2:2181,zookeeper3:2181</value> <!-- ZooKeeper quorum -->
</property>

Edit hdfs-site.xml: Define NameNode roles, shared storage, and failover settings:

<property>
  <name>dfs.nameservices</name>
  <value>mycluster</value> <!-- Must match fs.defaultFS -->
</property>
<property>
  <name>dfs.ha.namenodes.mycluster</name>
  <value>nn1,nn2</value> <!-- Active and Standby NameNode IDs -->
</property>
<property>
  <name>dfs.namenode.rpc-address.mycluster.nn1</name>
  <value>namenode1:8020</value> <!-- RPC address for nn1 -->
</property>
<property>
  <name>dfs.namenode.rpc-address.mycluster.nn2</name>
  <value>namenode2:8020</value> <!-- RPC address for nn2 -->
</property>
<property>
  <name>dfs.namenode.shared.edits.dir</name>
  <value>qjournal://journalnode1:8485;journalnode2:8485;journalnode3:8485/mycluster</value> <!-- JournalNode quorum for shared edits -->
</property>
<property>
  <name>dfs.journalnode.edits.dir</name>
  <value>/var/lib/hadoop/hdfs/journalnode</value> <!-- Local edit log directory on JournalNodes -->
</property>
<property>
  <name>dfs.ha.automatic-failover.enabled</name>
  <value>true</value> <!-- Enable automatic failover -->
</property>
<property>
  <name>dfs.client.failover.proxy.provider.mycluster</name>
  <value>org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider</value> <!-- Client-side proxy for failover -->
</property>
<property>
  <name>dfs.ha.fencing.methods</name>
  <value>sshfence</value> <!-- Prevent split-brain with SSH fencing -->
</property>
<property>
  <name>dfs.ha.fencing.ssh.private-key-files</name>
  <value>/root/.ssh/id_rsa</value> <!-- Path to SSH private key for fencing -->
</property>

Format NameNode: On the Active NameNode (e.g., namenode1), format the NameNode for the first time:
```
hdfs namenode -format
```
Start JournalNodes: On all JournalNodes, start the service:
```
hadoop-daemon.sh start journalnode
```

Start NameNodes: Initialize the shared edits directory on the Active NameNode and start both NameNodes:

hdfs namenode -bootstrapStandby  # Sync metadata to Standby NameNode
start-dfs.sh  # Start all HDFS daemons (NameNodes, DataNodes, JournalNodes)

Verify NameNode status with hdfs haadmin -report (should show one Active and one Standby NameNode).

3. Configure YARN High Availability (ResourceManager HA)
YARN HA ensures the ResourceManager (RM) remains available by running multiple RMs in Active/Standby mode, coordinated by ZooKeeper.

Edit yarn-site.xml: Add ResourceManager HA configurations:

<property>
  <name>yarn.resourcemanager.ha.enabled</name>
  <value>true</value> <!-- Enable HA -->
</property>
<property>
  <name>yarn.resourcemanager.cluster-id</name>
  <value>yarn-cluster</value> <!-- Logical cluster name -->
</property>
<property>
  <name>yarn.resourcemanager.ha.rm-ids</name>
  <value>rm1,rm2</value> <!-- RM IDs -->
</property>
<property>
  <name>yarn.resourcemanager.zk-address</name>
  <value>zookeeper1:2181,zookeeper2:2181,zookeeper3:2181</value> <!-- ZooKeeper quorum -->
</property>
<property>
  <name>yarn.resourcemanager.ha.id</name>
  <value>rm1</value> <!-- Set to rm1 for the first RM, rm2 for the second -->
</property>
<property>
  <name>yarn.nodemanager.aux-services</name>
  <value>mapreduce_shuffle</value> <!-- Shuffle service for MapReduce -->
</property>

Start ResourceManagers: On each ResourceManager node, start the service:

start-yarn.sh  # Start all YARN daemons (ResourceManagers, NodeManagers)

Verify ResourceManager status with yarn node -list (should show the Active RM handling requests).

4. Enable Automatic Failover with ZKFC
The ZK Failover Controller (ZKFC) monitors NameNode health and triggers automatic failover if the Active NameNode fails.

Start ZKFC: On all NameNode nodes, start the ZKFC daemon:
```
hadoop-daemon.sh start zkfc
```

ZKFC uses ZooKeeper to manage the Active/Standby state:

Each ZKFC runs a “fencing” mechanism (SSH by default) to kill the old Active NameNode before promoting the Standby.
The Standby NameNode takes over as Active within seconds of failure.

5. Validate HA Configuration
Test the HA setup to ensure it works as expected:

Check Cluster Status: Run hdfs dfsadmin -report to verify NameNode status (Active/Standby) and DataNode health.
Simulate NameNode Failure: Stop the Active NameNode (hadoop-daemon.sh stop namenode on namenode1) and check if the Standby promotes to Active (hdfs haadmin -report).
Access HDFS: Create a test file (hdfs dfs -put /local/file /hdfs/path) and verify it’s accessible after failover.
Check YARN Status: Run yarn node -list to ensure the ResourceManager is serving requests.

6. Monitor and Maintain the Cluster
Proactive monitoring is essential for long-term HA reliability:

Use Monitoring Tools: Deploy tools like Prometheus (with Hadoop exporters) and Grafana to track cluster metrics (NameNode heap usage, DataNode disk space, ResourceManager queue status).
Set Up Alerts: Configure alerts for critical failures (e.g., NameNode downtime, ZooKeeper quorum loss, DataNode disconnections).
Regular Backups: Backup NameNode metadata (edit logs, fsimage) to a secure location to recover from catastrophic failures.
Update Configurations: Periodically review and update Hadoop configurations to align with new versions and cluster growth.

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