---
title: "Hadoop daemons explained"
subject: "Big Data Technologies"
chapter: 6
key: "semester_8/big_data_technologies:hadoop-daemons"
source: https://www.bctengineer.com/q/semester_8/big_data_technologies/hadoop-daemons
site: BCT Engineering
---

# Hadoop daemons explained

Big Data Technologies · Chapter 6: Case Study: Hadoop

- Asked in: 2074 Bhadra, 8 marks; 2073 Magh, 3+7 marks (asked 2 times)
- Video: [5 Daemons of Hadoop, Name Node, Data Node, Secondary Name Node, Job Tracker, Task Tracker](https://www.youtube.com/watch?v=Dj9FncAS5XE) — Monalisa Meena
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/big_data_technologies/hadoop-daemons
- Whole chapter: https://www.bctengineer.com/md/semester_8/big_data_technologies/chapter-6

## Question

Explain in brief five daemons of Hadoop. (What are different daemons in HADOOP cluster? Explain each in details.)

## Short answer

Hadoop 1 has five daemons: NameNode stores HDFS metadata, DataNode stores data blocks, Secondary NameNode merges fsimage and edits for checkpointing, JobTracker schedules MapReduce jobs, and TaskTracker executes tasks. Hadoop 2 replaces JobTracker and TaskTracker with YARN's ResourceManager, NodeManager and ApplicationMaster.

## Answer

**Daemons** are background Java processes (JVMs) that run on the machines of a Hadoop cluster. Hadoop 1 has five main daemons: three for HDFS (storage) and two for MapReduce (processing). They follow a master/slave design.

```text
        MASTER                         SLAVES
 NameNode          <------>   DataNode (many)
 Secondary NameNode
 JobTracker        <------>   TaskTracker (many)
```

#### 1. NameNode (HDFS master)

- Stores the **metadata** of HDFS: the directory tree, file names, permissions, the blocks that make up each file, and the DataNodes that hold each block.
- Keeps the metadata in RAM and persists it as `fsimage` (a snapshot) plus `edits` (a log of changes).
- Receives heartbeats and block reports from the DataNodes, and directs re-replication when a node fails.
- It is a single point of failure in Hadoop 1 (fixed by the HA NameNode in Hadoop 2).

#### 2. DataNode (HDFS slave)

- Stores the actual data **blocks** (128 MB by default in Hadoop 2; 64 MB in Hadoop 1) on its local disks.
- Serves read and write requests from clients.
- Sends a heartbeat every 3 seconds and periodic block reports to the NameNode; creates, deletes and replicates blocks as instructed.

#### 3. Secondary NameNode

- It is not a hot standby. It periodically downloads `fsimage` and `edits` from the NameNode, merges them into a new fsimage (checkpointing) and sends it back.
- This keeps the edits log small and makes NameNode restarts fast.

#### 4. JobTracker (MapReduce master)

- Accepts jobs from clients, splits them into map and reduce tasks, and schedules the tasks on TaskTrackers close to the data (data locality).
- Monitors task progress and re-schedules failed tasks. There is one per cluster.

#### 5. TaskTracker (MapReduce slave)

- Runs on each slave node and executes map and reduce tasks in its task slots, as directed by the JobTracker.
- Sends heartbeats and progress reports to the JobTracker.

#### Hadoop 2 (YARN)

JobTracker and TaskTracker are replaced by the **ResourceManager** (cluster-wide resource scheduling), the **NodeManager** (per-node container management) and the **ApplicationMaster** (one per job).

#### Summary

| Daemon | Role | Type |
|---|---|---|
| NameNode | Metadata | Master |
| DataNode | Block storage | Slave |
| Secondary NameNode | Checkpointing | Helper |
| JobTracker | Job scheduling | Master |
| TaskTracker | Task execution | Slave |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the NameNode store?
   A. The actual data blocks of files
   B. Metadata: directory tree, file-to-block mapping and block locations
   C. The map and reduce tasks of running jobs
   D. Backups of all DataNode disks
   Answer: B. The NameNode keeps HDFS metadata (in RAM, persisted as fsimage plus edits) while DataNodes hold the blocks. (Key point: NameNode role)

2. What does the Secondary NameNode do?
   A. Acts as a hot standby that takes over instantly
   B. Stores replicas of data blocks
   C. Periodically merges fsimage and edits (checkpointing) so the edits log stays small
   D. Schedules MapReduce tasks
   Answer: C. It is not a hot standby; it checkpoints by merging fsimage and edits and sends back a new fsimage. (Key point: Secondary NameNode)

3. How does a DataNode let the NameNode know it is alive?
   A. It sends a heartbeat every 3 seconds plus periodic block reports
   B. It sends nothing; the NameNode polls the disks
   C. It writes to the edits log once a day
   D. It asks the JobTracker to report for it
   Answer: A. Heartbeats and block reports let the NameNode detect failures and re-replicate blocks. (Key point: DataNode heartbeat)

4. Which is correct for MapReduce daemons in Hadoop 1?
   A. TaskTracker schedules jobs; JobTracker runs tasks on each slave
   B. Both run only on the master
   C. JobTracker splits jobs into tasks and schedules them near data; TaskTrackers on slaves execute them and report progress
   D. JobTracker stores HDFS blocks
   Answer: C. The JobTracker is the single master that schedules and monitors; TaskTrackers run the tasks in slots. (Key point: JobTracker and TaskTracker)

## Related in Big Data Technologies

- [Components of Hadoop](https://www.bctengineer.com/question.md?key=semester_8%2Fbig_data_technologies%3Ahadoop-components) — Big Data Technologies, chapter 6

## Related in other subjects

- [Tractive resistance explained](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Atractive-resistance) — Transportation Engineering I, chapter 3