1. Distributed Systems CS 15-440
Hadoop
Lecture 13, October 25, 2017
Mohammad Hammoud

2. Today Last Session: Today’s Session: Announcements: MPI (Concluded)
Hadoop Distributed File System and MapReduce
Announcements:
P2 grades are out
PS4 is out. It is due on Nov 1st by midnight
P3 is due on Nov 12th by midnight

3. We Live in a World of Data…

4. What Do We Do With Big Data?
Store
Share
Access
Process
Encrypt
…. and more!
We want to do all these seamlessly...

5. Where to Store Big Data?
The underlying storage system is a key component for enabling Big Data querying/mining/analytics
Typically, the storage system would “partition” and “distribute” Big Data, using striping (or partitioning) and placement techniques
This allows for concurrent accesses to data
as well as improves fault-tolerance
Logical File
Striping Unit
Stripe Size 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Server 1
Server 2
Server 3
Server 4 4 8 12 1 5 9 13 2 6 10 14 3 7 11 15

6. Example: The Google File System
GFS paritions large files into fixed-size blocks and distributes them randomly across cluster machines
Large File
Blk 0
Blk 1
Blk 2
Blk 3
Blk 4
Blk 5
Blk 6
Server 0
(Writer)
Server 1
Server 2
Server 3 0M
Blk 0
Blk 0
Blk 1
Blk 0
64M
Blk 1
Blk 2
Blk 2
Blk 1
128M
Blk 2
Blk 3
Blk 4
Blk 4
Blk 3
Blk 3
Blk 5
Blk 5
192M
Blk 6
256M
Blk 4
Blk 6
Blk 5
320M
Blk 6
384M

7. Example: The Google File System
GFS adopts a master-slave architecture
File name
GFS client
Master
Contact address
Chunk Id, range
Chunk Server
Chunk Server
Chunk Server
Chunk data
Linux File System
Linux File System
Linux File System

8. How to Process Big Data?
One alternative: Create a custom distributed system (or program) for each new algorithm
Cumbersome!
Another alternative: utilize modern distributed analytics frameworks, which:
Relieve programmers from concerns with many of the difficult aspects of developing distributed programs
Allow programmers to focus on ONLY the sequential parts of their programs
Examples:
Hadoop MapReduce
Google’s Pregel
CMU’s Distributed GraphLab

9. Distributed Analytics Frameworks
Hadoop MapReduce
Introduction
Programming Model
Execution Model
Architectural & Scheduling
Models

10. Hadoop
Hadoop is one of the most successful realizations of large-scale “data-parallel” distributed analytics frameworks
Hadoop MapReduce is an open source implementation of Google’s MapReduce
Hadoop uses Hadoop Distributed File System (HDFS) as a storage layer
HDFS is an open source implementation of GFS

11. Hadoop MapReduce: A Bird’s Eye View
Hadoop MapReduce incorporates two phases, Map and Reduce phases, which encompass multiple Map and Reduce tasks
Map Task
HDFS BLK
Split 0
Partition
Partition
Reduce Task
Partition
Partition
Map Task
Partition
Partition
Split 1
HDFS BLK
Dataset
Partition
Reduce Task
Partition
Partition
Partition
To HDFS
Map Task
Split 2
HDFS BLK
HDFS
Partition
Partition
Reduce Task
Partition
Partition
Map Task
Partition
Split 3
HDFS BLK
Partition
Partition
Merge & Sort Stage
Shuffle Stage
Reduce Stage
Map Phase
Reduce Phase

12. Distributed Analytics Frameworks
Hadoop MapReduce
Introduction
Programming Model
Execution Model
Architectural & Scheduling
Models

13. The Programming Model
Hadoop MapReduce employs a shared-based programming model, which entails that:
Tasks can interact (if needed) via reading and writing to a shared space
HDFS provides the shared space for all Map and Reduce tasks
Programmers write only sequential code, without defining functions that send/receive messages between tasks
A Shared Address Space (Provided by HDFS)
MT1
MT2
MT3
MT4
MT5
MT6
“Implicit” communication (provided by the MapReduce Engine)- Programmers do not write or call any communication routines
RT1
RT2
RT3
A Shared Address Space (Provided by HDFS)

14. Example: Word Count A Text File A Map Function A Map Function A Reduce
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15. Distributed Analytics Frameworks
Hadoop MapReduce
Introduction
Programming Model
Execution Model
Architectural & Scheduling
Models

16. The Execution Model
Hadoop MapReduce adopts a synchronous execution model
A distributed program (or system) is said to be synchronous if and only if its constituent tasks operate in a lock-step mode
No two tasks can run concurrently under two different iterations
In MapReduce:
Each iteration is treated as a MapReduce job
A job can encompass 1 or many Map tasks and 0 or many Reduce tasks
Programs with multiple iterations (i.e., iterative programs) are executed using multiple chained MapReduce jobs
When all Reduce tasks within job i are committed, a new job i + 1 is started (if any)
Hence, two different tasks cannot run in parallel under two different jobs (or iterations)

17. Distributed Analytics Frameworks
Hadoop MapReduce
Introduction
Programming Model
Execution Model
Architectural & Scheduling
Models

18. The Architectural and Scheduling Models
Hadoop MapReduce employs a master-slave architecture
A pull-based task scheduling strategy is used, whereby:
Map tasks are scheduled in proximity of HDFS blocks
Reduce tasks are scheduled anywhere
Core Switch
A slave
The master
Rack Switch 1
Rack Switch 2
TaskTracker1
TaskTracker2
TaskTracker3
TaskTracker4
TaskTracker5
JobTracker
MT2
MT3
MT1
MT2
MT3
Request a Map Task
Schedule a Map Task at an Empty Map Slot on TaskTracker1

19. The Architectural and Scheduling Models
Hadoop MapReduce employs a master-slave architecture
With the above setup, how many Map tasks can run in parallel?
Each TaskTracker has by default two Map slots, thus can run two Map tasks concurrently
With 4 TaskTrackers and 2 Map slots on each TaskTracker, 8 Map tasks can be executed in parallel
The maximum number of Map tasks that can run in parallel is denoted as Map wave
Core Switch
A slave
The master
Rack Switch 1
Rack Switch 2
TaskTracker1
TaskTracker2
TaskTracker3
TaskTracker4
TaskTracker5
JobTracker
MT1
MT2
MT3
MT2
MT3
Request a Map Task
Schedule a Map Task at an Empty Map Slot on TaskTracker1

20. The Architectural and Scheduling Models
Hadoop MapReduce employs a master-slave architecture
For a dataset with a size of 1024MB, how many Map waves are needed?
The size of each HDFS block is by default 64MB and each split encompasses by default 1 HDFS block
Hence, there will be a total of 1024/64 = 16 HDFS blocks or 16 splits
The input to each Map task is a single split, thus there will be a total of 16 Map tasks
Therefore, 16 tasks/8 slots = 2 Map waves will be needed
Core Switch
A slave
The master
Rack Switch 1
Rack Switch 2
TaskTracker1
TaskTracker2
TaskTracker3
TaskTracker4
TaskTracker5
JobTracker
MT1
MT2
MT3
MT2
MT3
Request a Map Task
Schedule a Map Task at an Empty Map Slot on TaskTracker1

21. Hadoop MapReduce: Summary
Aspect
Hadoop MapReduce
Programming Model
Shared-Based
Execution Model
Synchronous
Architectural Model
Master-Slave
Scheduling Model
Pull-Based
Suitable Applications
Loosely-Connected/Embarrassingly-Parallel Applications

22. Hadoop MapReduce: Summary
Aspect
Hadoop MapReduce
Programming Model
Shared-Based
Execution Model
Synchronous
Architectural Model
Master-Slave
Scheduling Model
Pull-Based
Suitable Applications
Loosely-Connected/Embarrassingly-Parallel Applications

23. Next Class
Pregel and GraphLab