In a Hadoop cluster, Zookeeper Nodes are crucial for maintaining continuous operation and data accessibility. Zookeeper is a distributed coordination service that helps manage and synchronize distributed systems, ensuring the coordination of tasks and maintaining cluster stability.

Impala's in-memory processing feature enables it to store and analyze data in memory, providing faster query performance and real-time data analysis capabilities in Hadoop clusters.

Data Serialization is a critical factor in Hadoop Streaming API when dealing with streaming data from various sources. Proper serialization ensures that the data is efficiently encoded and decoded, enhancing the performance of data processing in Hadoop Streaming.

Apache Flume facilitates building data pipelines in Hadoop by serving as a reliable and scalable data ingestion tool. It efficiently collects, aggregates, and moves large amounts of log data from various sources to Hadoop storage, making it a valuable component in data pipeline construction.

For ensuring high availability in Hadoop, an administrator must configure NameNode High Availability (NameNode HA) effectively. This involves setting up multiple NameNodes and ensuring seamless failover in case of a NameNode failure, enhancing the reliability of the Hadoop cluster.

To address the 'Small Files Problem,' a Hadoop administrator can use CombineFileInputFormat. This technique allows the efficient processing of small files by combining them into larger input splits, reducing the overhead associated with managing numerous small files and improving overall processing efficiency.

Capacity Planning is used to estimate the processing capacity required for a Hadoop cluster based on data processing needs. It involves analyzing factors like data volume, processing speed, and storage requirements to ensure optimal cluster performance.

Hadoop's ResourceManager is responsible for resource allocation and management in the cluster. It assists in monitoring cluster performance by efficiently allocating resources to applications, ensuring optimal utilization and performance. This includes managing memory, CPU, and other resources for running tasks.

Apache Spark performs computations in-memory, which is a key improvement over the MapReduce model. This in-memory processing reduces the need for intermediate disk storage, resulting in faster data processing and analysis.

Stream processing is the most effective for immediate fraud detection in a financial institution. It enables the continuous analysis of incoming data in real-time, allowing for swift identification and response to fraudulent activities as they occur.