In an HDFS High Availability (HA) architecture, the ZooKeeper Failover Controller (ZKFC) and the Standby NameNode have distinct, critical roles. The ZKFC is a daemon that runs on each NameNode machine, responsible for monitoring the health of the active NameNode and initiating an automatic failover to the standby if the active one fails. The Standby NameNode constantly consumes transaction logs (edits) from the shared JournalNodes to maintain a synchronized namespace image. It also performs the checkpointing process, which involves merging the current namespace image (fsimage) with the recent edit logs. This metadata file merge operation is offloaded from the active NameNode to improve its performance.

B. The Primary (Active) NameNode serves client requests; it does not manage the failover process itself, which is the ZKFC's role.

C. The master (Active) NameNode does not perform the metadata merge in an HA configuration; this task is handled by the Standby NameNode.

D. JournalNodes are daemons that store edit logs from the Active NameNode for the Standby to consume; they do not perform the merge or manage failover.

1. Apache Hadoop 3.3.6 Documentation, "HDFS High Availability Using the Quorum Journal Manager (QJM)":

ZKFC Role: In the "Architecture" section, the subsection "ZKFailoverController (ZKFC)" states, "The ZKFC is a ZooKeeper client which is also responsible for managing the state of the NameNode... Each of the machines which runs a NameNode also runs a ZKFC, and this ZKFC is responsible for: Health monitoring... ZooKeeper leader election."

Standby NameNode Role (Metadata Merge/Checkpointing): In the "Architecture" section, the description of the Standby NameNode's function explains that it reads edits from JournalNodes to stay synchronized and performs checkpoints of the namespace state, effectively merging the metadata.

2. Huawei FusionInsight HD 6.5.1 Administrator Guide, "HDFS Architecture":

Section 3.1.1, "HDFS Architecture (HA Mode)": This section describes the components of an HDFS HA cluster. It details that the ZKFC component is responsible for monitoring NameNode status and performing active/standby switchovers. It also clarifies that the Standby NameNode is responsible for synchronizing metadata from the Active NameNode and performing checkpoint operations (merging fsimage and edits).

3. Hadoop: The Definitive Guide, 4th Edition by Tom White (O'Reilly Media):

Chapter 3, "The Hadoop Distributed Filesystem", Section "High Availability": This chapter details the HDFS HA architecture. It explicitly describes the role of the ZKFC in managing failover and the role of the standby NameNode in performing checkpoints, which is the process of merging the fsimage and edits log. (Note: While a book, it is widely considered an authoritative reference in the field, often used in university curricula).