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Hadoop High Availability(Hadoop HA) is an essential feature in the Hadoop ecosystem that ensures the continuous availability of Hadoop services even in the case of failures. In this article, we will explore the concept of Hadoop HA and understand how it works.

## What is Hadoop High Availability?

Hadoop High Availability is a technique that is used to eliminate single points of failure in the Hadoop cluster and make it resilient to failures. It ensures that the Hadoop services like NameNode, ResourceManager, and others are always available and can tolerate failures without any downtime.

## Understanding Hadoop HA Architecture

The Hadoop HA architecture consists of multiple components that work together to achieve high availability. The key components are:

### Active NameNode

The Active NameNode is the primary node that handles the metadata operations in the Hadoop cluster. It manages the file system namespace and keeps track of the blocks in the cluster. It is responsible for handling client requests and coordinating with the DataNodes.

### Standby NameNode

The Standby NameNode is a backup node that constantly updates itself with the changes happening on the Active NameNode. It keeps a copy of the namespace and block information, so it can take over as the Active NameNode in case of a failure.

### JournalNodes

The JournalNodes are a set of daemons that host the shared edit log of the NameNodes. They store the changes made to the file system namespace. Both the Active NameNode and the Standby NameNode read and write to the JournalNodes, which allows the Standby NameNode to stay synchronized with the Active NameNode.

### Quorum

The Quorum is the minimum number of JournalNodes that need to be available for the cluster to function properly. It is typically set to an odd number to ensure a majority can agree on the updates.

## How Does Hadoop HA Work?

To provide high availability, the Active NameNode and Standby NameNode continuously exchange heartbeat signals. The Standby NameNode monitors the health of the Active NameNode by checking for the heartbeat and sequential edits in the shared edit log.

In the event of a failure, the Standby NameNode detects the absence of heartbeat signals and considers the Active NameNode as dead. It initiates the failover process by requesting a set of finalized edit logs from the JournalNodes. The Standby NameNode then transitions to the Active state and starts serving client requests.

During the failover process, the DataNodes are notified about the new Active NameNode, and they start sending heartbeat signals to it. Once the old Active NameNode recovers, it becomes the new Standby NameNode and starts syncing with the new Active NameNode.

## Conclusion

Hadoop High Availability is a critical aspect of any Hadoop cluster that ensures uninterrupted service in the face of failures. By employing the Active-Standby architecture and utilizing JournalNodes for synchronization, Hadoop HA provides a reliable and fault-tolerant environment for big data processing.