TM - Distributed Systems

Question 1

What is a Distributed System

Answer 1

Several Distributed components under a single system.

Question 2

Features of a Distributed System

Answer 2

Can communicate with a message. Typically executed concurrently and autonomously. Coordinated through a computer network. Physically close or geographically distant. Memory can be shared or individual. Communication slow and unreliable.

Question 3

Examples of a Distributed System

Answer 3

The WWW, Online Games, An Office Network.

Question 4

Why build a Distributed System

Answer 4

Some problems are distributed by nature. Cost, Performance, Availability (Fault tolerant), Physical/Geographic Reasons.

Question 5

Multithreading

Answer 5

Multiple threads of execution in a single app. Threads =/= cores.

Question 6

Example of why use multithreading

Answer 6

UI apps can block while waiting for user input, this blocked work can execute on another thread.

Also video games use multithreading to render frames.

Question 7

Fork-Join Model

Answer 7

Parallel Programming Paradigm. Threads can be executed separately but logic is not always independent and can cause interleavings.

Question 8

Fork (Fork-Join)

Answer 8

Divides the task into smaller subtasks, creating parallel threads or processes to handle them

Question 9

Join (Fork-Join)

Answer 9

Gathers results from parallel threads or processes and combines them for the final result.

Question 10

Restricting possible interleavings

Answer 10

Race Condition. Atomic Actions. Critical Sections. Mutual Exclusion.

Question 11

Race Condition

Answer 11

Occurs when the app behaviour depends on the timing of execution sequences in a multithreaded environment.

Question 12

Atomic Action

Answer 12

An action to be executed as one unit

Question 13

Critical Section

Answer 13

Group of instructions to be executed atomically. Executed by a single process at a time (Mutually exclusive).

Question 14

Mutual Exclusion

Answer 14

One thread of execution never enters its critical section at the same time that another thread in execution is entering its critical section.

Question 15

Lock

Answer 15

A synchronization tool used to control resource access from multiple threads of execution. Relies on using lock() and unlock().

Question 16

Issues with Lock

Answer 16

Deadlock, Livelock, Starvation

Question 17

Deadlock

Answer 17

No process can progress because each is waiting for a resource held by another.

Question 18

Livelock

Answer 18

No process can progress because process is continuously changing state in response to changes in other processes without doing useful work.

Question 19

Starvation

Answer 19

Some process cannot get its chance to progress (overlooked by scheduler due to bad luck or low priority).

Question 20

Creating a Parallel Program (4 Steps)

Answer 20

• Decompose computation into tasks
• Assign tasks to processes
• Orchestrate data access, communicate, synchronize
• Map processes to processors

Question 21

Speedup

Answer 21

improvement in the overall performance or execution time of a task when processing is distributed

Question 22

Reasons against programs being able to execute in parallel

Answer 22

Performing Initialization
Communicating amongst processes
Resource Contention
Load Imbalance

Question 23

Client-Server Pattern

Answer 23

Server running program waiting for client request.
Client process submitting requests and getting response from server.
Both can work on a single machine or a distributed system.

Question 24

Two Main Classes of Servers

Answer 24

Iterative
Concurrent

Question 25

Iterative Class of Server

Answer 25

Clients are blocked while a client is being served

Question 26

Concurrent Class of Server

Answer 26

Time slicing applied by CPU, each processor given time to a client.

Question 27

Client Server Challenges

Answer 27

Interoperability on development platforms.
Address space and referencing remote objects.

Question 28

Java remote method invocation

Answer 28

Allows object to invoke a method from another JVM. Limited in cross platform combatibility as only works from JVM to JVM

Question 29

COM/OLE

Answer 29

Component Object Model/Object Linking and Embedding.
Standard interface for Microsoft software components. COM is base of OLE.

Question 30

Web Services

Answer 30

Use of existing infrastructure to enable development of client apps composed of web services (i.e services of remote objects) implemented on remote hosts.

Question 31

Remote Procedure Call (RPC)

Answer 31

Procedure executed in a different address space than the one invoking it (used in client-server).

Question 32

Execution order

Answer 32

Synchronous (Thread executed block until it’s complete)

Asynchronous (Thread executing current task proceeds with execution even if task is not complete). Completion Notified through call block.

Question 33

Microservices architecture

Answer 33

Organize around business processes. Decentralized. Infrastructure support + Continuous Delivery. Operate despite failure, not necessarily innovative. Challenging to understand complex sequences across certain microservices.

Question 34

Serialization

Answer 34

Transform state of objects into stream of bytes for consumption such that it can be rebuilt as an object with the same stream.

Question 35

Formats of serialization

Answer 35

XML - human readable don’t need to know schema + more verbose than JSON

JSON - human readable don’t need to know schema + less verbose than XML

Protobuf, Thrift.. - Not human readable, smaller output, faster processing, schema known in advance.

Question 36

Multiple clients working with same resource

Answer 36

Stale Data, Pessimistic Locking, Optimistic Locking

Question 37

Stale Data

Answer 37

Data which has changed since being retrieved by the current process

Question 38

Pessimistic Locking

Answer 38

Resource locked as soon as it is accessed and released as soon as changes committed. Prevents conflicts.

Question 39

Optimistic Locking

Answer 39

Resources can be read and changed freely. Check for conflict when committing changed result and act according to specified conflict resolution protocol.

Avoids overhead of locking resource for a long period of time.