MapReduce and Distributed Systems Overview

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Questions and Answers

What is a primary benefit of using a distributed system?

Lower initial setup costs

Reduced power consumption

Easier management of resources

Improved reliability even if one node fails (correct)

Which of the following is a challenge faced in managing distributed systems?

Centralized load balancing

Uniform data security

Communication reliability (correct)

Simplified fault tolerance

What does the CAP theorem address in distributed systems?

The security risks associated with data transmission

The trade-offs among Consistency, Availability, and Partition tolerance (correct)

The impact of latency on system performance

The relation between memory usage and processing speed

What does the reducer do in the example of the Word Count MapReduce job?

Sums the values associated with each word key Signup and view all the answers

Which mechanism is commonly used to achieve fault tolerance in distributed systems?

Data replication Signup and view all the answers

What is one reason why latency might increase in a distributed system?

Network communication failures Signup and view all the answers

What is the final output format of the Word Count example?

A list containing words and their counts Signup and view all the answers

During the Map phase of the Maximum Temperature MapReduce job, what key-value pair is emitted for the record 2022-08-25, 30?

(2022, 30) Signup and view all the answers

Which of the following describes a performance improvement benefit of distributed systems?

Parallel processing across multiple nodes reduces latency Signup and view all the answers

What is a significant complexity introduced by distributed systems?

Managing data consistency across nodes Signup and view all the answers

What is the size of the input records for the Maximum Temperature job in terms of field count?

2 fields Signup and view all the answers

In the Shuffle and Sort phase of the Maximum Temperature job, how are the records grouped?

By the year as key Signup and view all the answers

Why is security a concern in distributed systems?

They increase the attack surface area Signup and view all the answers

What type of data does the Map phase of the Maximum Temperature job process?

Weather station records Signup and view all the answers

What type of operation does the reducer perform in the Maximum Temperature job?

Determines the maximum temperature for each year Signup and view all the answers

What approximate output is expected after the Reduce phase in the Maximum Temperature job?

A list of years with their corresponding maximum temperatures Signup and view all the answers

What function does the Interface Definition Language (IDL) serve in RPC systems?

It specifies the interface between client and server. Signup and view all the answers

Which of the following is NOT a key feature of the Interface Definition Language?

Network Connection Management Signup and view all the answers

What does the term 'language agnostic' imply in the context of IDL?

It allows for communication across different programming languages. Signup and view all the answers

In what scenario are Interface Definition Languages particularly beneficial?

In distributed systems with multiple languages. Signup and view all the answers

Which statement best describes how IDL interacts with transport protocols?

IDL determines the structure of messages, regardless of the transport protocol. Signup and view all the answers

What is one key advantage of generating 'stubs' from an IDL file?

It simplifies coding by providing a template for calls. Signup and view all the answers

Which framework is known for utilizing IDL in RPC systems?

Google RPC (gRPC) Signup and view all the answers

Which of the following best describes interoperability in the context of IDL?

It allows systems using different data types to communicate seamlessly. Signup and view all the answers

What is the central issue addressed by the Byzantine Generals Problem?

Reaching agreement in the presence of faulty or malicious components. Signup and view all the answers

Who were the first to introduce the Byzantine Generals Problem?

Leslie Lamport, Robert Shostak, and Marshall Pease Signup and view all the answers

What is a characteristic of the generals who are considered traitors in the Byzantine Generals Problem?

They actively send conflicting or false messages. Signup and view all the answers

What problem does the two generals scenario demonstrate regarding communication?

Reliable communication and agreement cannot be achieved over an unreliable channel. Signup and view all the answers

What term describes the issues of being excessively complicated and bureaucratic, as used in relation to the Byzantine Empire?

Byzantine Signup and view all the answers

What major challenge does the Two Generals Problem illustrate?

Reliable communication over an unreliable network Signup and view all the answers

Which of the following best describes the outcome of the Two Generals Problem?

It is considered unsolvable in its most general form Signup and view all the answers

What is one of the fundamental issues addressed by models of distributed systems?

Communication over unreliable networks Signup and view all the answers

What decision does General 1 face in the Two Generals Problem?

To attack only with a positive response from General 2 Signup and view all the answers

What does common knowledge imply in the context of the Two Generals Problem?

Both generals have the same information about the attack Signup and view all the answers

Which problem serves as the basis for the Two Generals Problem in distributed systems?

The Byzantine Generals Problem Signup and view all the answers

How do failures influence the design of distributed systems according to models?

Systems should tolerate or recover from failures Signup and view all the answers

What is a key characteristic of the communication channel in the Two Generals Problem?

It is prone to message loss Signup and view all the answers

Study Notes

MapReduce Example: Word Count

The reducer function sums values for each key.
Example output: ("Hello", 3), ("world", 2)
Final output is a list of words and their counts.

MapReduce Example: Maximum Temperature

Problem: Find the maximum temperature for each year.
Input: A dataset with Date (YYYY-MM-DD) & Temperature.
Map Phase: Extracts year and temperature, emits (Year, Temperature) pairs.
Shuffle and Sort: Groups pairs with same key (Year).
Reduce Phase: Processes grouped pairs and finds the maximum value for each year.

Distributed Systems: Why Not Make a System Distributed?

Increased Complexity: Designing, implementing, and managing a distributed system is more complex than centralized systems.
Increased Latency and Synchronization: Network communication can introduce latency and maintaining consistency across nodes can be challenging.
Increased Security Risks: Distributing resources increases the attack surface, requiring careful management of security protocols and access controls.

Distributed Systems: Challenges

Communication: Reliable communication between nodes is difficult due to network failures, latency, and bandwidth constraints.
Synchronization: Synchronizing clocks, data, and operations across distributed nodes is complex.
Consistency: It's challenging to achieve data consistency.
Fault Tolerance: Detecting and recovering from node failures is essential for maintaining system reliability.

Client-Server Example: Online Payments

Client interacts with the server to make payments.
Server processes payments and interacts with other services (e.g., payment gateways).
Communication between client and server is essential.

RPC in Enterprise Systems

Service-oriented Architecture (SOA) / Microservices: Splitting a large software application into multiple services that communicate via RPC.
Interoperability: Different services can be implemented in different languages.
Interface Definition Language (IDL): Provides a language-independent API specification for communication between services.

Interface Definition Language (IDL)

IDL defines the interface between a client and server in RPC systems.
Allows different programming languages and systems to communicate.
Useful in distributed systems and RPC frameworks (e.g., gRPC, CORBA, Thrift).

Key Features of IDL

Language Agnostic: IDL allows different languages to communicate by defining a common interface.
Protocol Agnostic: IDL doesn't specify the communication method, only the structure of messages and services.
Service and Data Type Definition: IDL specifies the operations (methods) that can be called remotely and the data types (messages) exchanged.

Why Use IDL?

Enables communication between services implemented in different programming languages.
Provides a consistent interface for developers.
Simplifies the development of distributed systems.

Models of Distributed Systems

Fundamental problems arise in distributed systems due to decentralized control, unreliable networks, and the possibility of failures.
Two widely recognized problems are:
- The Two Generals Problem
- The Byzantine Generals Problem

The Two Generals Problem

Illustrates the challenges of achieving coordination and reliable communication between two parties over an unreliable network.
It is considered unsolvable in its most general form due to the impossibility of guaranteeing message delivery and achieving consensus.

The Two Generals Problem - Explanation

Two generals must decide whether to attack or retreat.
They communicate through unreliable messengers.
If a response is not received, the sender is unsure if the message was received.

Summary: The Two Generals Problem

The Two Generals Problem involves two parties trying to coordinate an action.
They communicate over an unreliable channel where messages can be lost.
The goal is to reach an agreement, but message delivery cannot be guaranteed, making absolute certainty impossible.
It demonstrates that reliable communication and agreement are impossible over an unreliable channel.

The Two Generals Problem - Applied

Applying the problem to real-world scenarios:
- Two parties trying to coordinate over a network connection.
- Systems relying on distributed databases.

Byzantine Generals Problem

Deals with achieving consensus in the presence of faulty or malicious components (Byzantine faults).
Introduced by Leslie Lamport, Robert Shostak, and Marshall Pease in 1982.
Models systems with participants who might behave incorrectly or maliciously, yet still need to reach an agreement.

The Byzantine Empire (650 CE)

"Byzantine" has long been used for "excessively complicated, bureaucratic, devious".

Byzantine Generals Problem - Overview

A group of generals must decide on a common strategy (e.g., attack or retreat).
Some generals may be traitors (Byzantine generals) who send conflicting or false messages.
The challenge is for the loyal generals to reach a consensus despite the presence of traitors.

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Description

This quiz explores key concepts in MapReduce, including examples like Word Count and Maximum Temperature. It also highlights challenges in building distributed systems such as complexity and security risks. Test your understanding of these critical topics in computer science.