Hadoop HDFS Automatic failover between Namenodes is not functioning correctly.

What is Hadoop HDFS Automatic failover between Namenodes is not functioning correctly.

Understanding Hadoop HDFS

Hadoop Distributed File System (HDFS) is a distributed file system designed to run on commodity hardware. It is highly fault-tolerant and is designed to be deployed on low-cost hardware. HDFS provides high throughput access to application data and is suitable for applications that have large data sets.

Identifying the Symptom

In a high-availability (HA) setup of Hadoop HDFS, you might encounter an issue where the automatic failover between Namenodes is not functioning correctly. This can manifest as a lack of response from the standby Namenode when the active Namenode fails, leading to potential data access issues.

Common Error Messages

When this issue occurs, you might see error messages in the logs such as:

FailoverController: Failed to failover to standby Namenode Connection refused to Zookeeper

Exploring the Issue

The issue, identified as HDFS-015: Namenode Failover Not Working, arises when the automatic failover mechanism between Namenodes is not functioning as expected. This is often due to misconfigurations in the HA setup or issues with Zookeeper, which is responsible for managing the state of the Namenodes.

Root Causes

Incorrect HA configuration in the hdfs-site.xml file. Zookeeper ensemble not running or misconfigured. Network issues preventing communication between Namenodes and Zookeeper.

Steps to Resolve the Issue

To resolve the Namenode failover issue, follow these steps:

1. Verify HA Configuration

Ensure that the HA configuration in hdfs-site.xml is correct. Check for the following properties:

<property> <name>dfs.nameservices</name> <value>mycluster</value></property><property> <name>dfs.ha.namenodes.mycluster</name> <value>namenode1,namenode2</value></property>

Ensure that the nameservice ID and Namenode IDs are correctly specified.

2. Check Zookeeper Status

Verify that the Zookeeper ensemble is running and accessible. Use the following command to check the status of Zookeeper:

zkServer.sh status

Ensure that all Zookeeper nodes are in a healthy state. For more information on Zookeeper setup, refer to the Zookeeper Getting Started Guide.

3. Review Network Configuration

Ensure that there are no network issues preventing communication between the Namenodes and Zookeeper. Check firewall settings and ensure that the necessary ports are open.

4. Restart Namenode Services

After verifying the configuration and network settings, restart the Namenode services to apply the changes:

hadoop-daemon.sh stop namenodehadoop-daemon.sh start namenode

Conclusion

By following these steps, you should be able to resolve the Namenode failover issue in your Hadoop HDFS setup. Proper configuration and monitoring of both the Namenodes and Zookeeper are crucial for maintaining a robust and reliable HDFS environment. For further reading, check the HDFS High Availability Guide.