CSE5306 Distributed Systems

Mohan Kumar

Preliminaries and Models: Week 2

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System models

Architectural models Client-server model Peer-to-peer model

Functional models Interaction model Failure model Security model

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Architecture

Structural organization of various components Simple of abstraction of components Two main objectives

Placements Network topology Data distribution

Interrelationships Patterns of communications Relationships between data objects Data access patterns, dependencies

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Basic Processes

Server Accepts inputs from other processes Performs a service Returns outcomes

Client User/application level Makes requests, receives results

The roles of server and client may change with time

Peer All are equal

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Layered representation

Applications and services

Middleware

Operating SystemCommunications Network

Hardware

Mask HeterogeneityProvide abstraction, transparencyUniformity

PLATFORM

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Middleware

Application Requirements•Quality of

service parameters

•Context

DynamicsMobility

Changing DataChanging Context

Challenges• Uncertainties• Lack of communication path• Lack of resources

Managing Resources

ServicesContext

Remote method invocation

Shared Objects

Negotiation

Resource allocation

Task scheduling

Load balancing

Task migration

Service Composition

Security, privacy and Trust

Client/Server

Client Server

Client Server Server

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Peer-to Peer and Client/server variations

Authors’ slides for figures Peer-top-peer

No distinction among peers Excellent scalability compared to C-S Resources are utilized in a distributed network, and more efficiently.

Minimize bottleneck points Variations

Multiple servers Each server specializes in a providing a particular service

E.g., web servers, DNS server, authentication etc.

Proxy servers Enhance availability Reduce latency

Caches Objects cached to reduce latency

Mobile code and mobile agents Mobile code (e.g., applet) downloaded to client’s site

Local interactions, fast response as there are no communication delays Mobile agents include code and data

Go around execute on different processors1/24/2011 CSE5306 SP 2011 M KUMAR

Multiple servers

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Basic applications Remote login

Keyboard and display interface Virtual terminal support

telnet, rlogin File transfer

File, file structures, file attributes E.g., FTP

Messaging Send and receive Email, SMTP

Browsing Information retrieval

Remote execution Execute a program on a remote server

E.g, MIME – multipurpose Internet mail extension1/24/2011 CSE5306 SP 2011 M KUMAR

Goals Efficiency

Propagation delays, communications Overlapped computation/communication Efficient distributed processing and load sharing

Flexibility User friendly Ability to evolve and migrate

Modularity, scalability, portability, and interoperability Consistency

Predictability and uniformity in system behavior Integrity in concurrency control, failure handling and failure handling

Robustness Ability to handle exceptional situations and errors

Change in topology, lost message, crashed system etc. Reliability, protection and access control

Secure and privacy preserving1/24/2011 CSE5306 SP 2011 M KUMAR

Design requirements

Performance Responsiveness

Access to shared resources Communication delays Server loads, scheduling, wait periods Control switching Load balancing Caching and replication Combined computation/communication

scheduling

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Transparency

Ability to hide/mask all system details from users/application developers System details are irrelevant to

users/developers System details are very relevant to

system managers Creation of an illusion of a model that

it is supposed to beThis is in contrast to the meaning of transparency in English – open, visible, see through etc.1/24/2011 CSE5306 SP 2011 M

KUMAR

Goals and transparencies Efficiency

Concurrency Parallelism Performance

Flexibility Access Location Migration Size

Consistency Access Replication performance

Robustness Failure Replication Size

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Major issues Communication Synchronization Distributed algorithms Process scheduling Deadlock handling Load balancing Resource scheduling File sharing Concurrency control Failure handling Configuration Redundancy

Interaction and control transparency

Performance transparency

Resource transparency

Failure transparency

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Services Primitive

Send, receive, synchronize System servers

Name server, directory server, network server, time server, file server, print server, security server

Value-added Web server, conferencing server,

Distributed A service is composed of several subservices

distributed in the network Services are invoked one-by-one

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Processes A process is a program in

execution Sequential

A single control block regulates the execution

A control block contains state information – program counters, register contents, stack pointers, communication ports, file descriptors etc.

Process control block (PCB) Concurrent

Simultaneously interacting sequential processes are said to be concurrent

Asynchronous Separate address space and

PCBs Components may interact

through communication/synchronization

Process

PCB

Process

PCB

Process

PCB

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Threads A lightweight process

Threads of a process share the same address space, but have their own registers

A thread control block (or TCB) is local to a thread

Typically, Threads have their own PC, SP and

register set. Threads share address space,

communication ports and file descriptors

Multiple threads are spawned by a process

A PCB is shared among interacting threads

Context switching among threads is lightweight compared to context switching among processes

PCB

Thre

ad

PCB

Thre

ad

Thre

ad

Thre

ad

Thre

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ad

TCB| TCB| TCB

PCBTCB| TCB

Thread run-time library supportOperating System Support

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Graph models

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Time-space model for interaction

P1

P2

P3

P4TIME

Processes

Event Communications1/24/2011 CSE5306 SP 2011 M KUMAR

Communication in C-S

Client and server communication model

RPC Communication

Message PassingCommunication

TCP/UDP Transport Service

Client Server

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Functional models Why?

Main entities Interaction among

entities Characteristics –

individual and collective

Purpose? Assumptions Generalizations

Algorithms Logical analysis Mathematical analysis

Models Interaction

Message passing Shared objects

Failure Faults

Detection Recovery

Security Security vs.

openness Privacy vs. efficiency

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Interaction model Process interact ions

C-S, P2P, message passing, shared space, synchronous, asynchronous

Single process/thread, multiple threads Distributed algorithms

Behavior of multiple processes Includes message transmissions Each process

Has own its PCB and inaccessible by other processes Likely on different systems in the network Difficult to coordinate

Two significant factors Communication performance Maintenance of global state

Computer locks drift Clock drifts differ from one another

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Performance of communication channels

Latency Time taken for the first string of bits to arrive at

the destination Delay in accessing the network Delays (processing times at)OS communication

services at both ends Bandwidth

Frequency, interference, number of channels sharing

Jitter Variation in times to deliver different components

of a message1/24/2011 CSE5306 SP 2011 M KUMAR

Two variants

Synchronous Process execution time is bounded Message latency over a channel is bounded Process’ local clock drift is bounded Though difficult to build, very useful as a

model Time outs Detect failures

Asynchronous Blue bullets (Assumptions) above are NOT true Most systems are asynchronous

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Event ordering

See author’s slide Lamport’s logical ordering

X sends m1 before Y receives m1 Y sends m2 before X receives m2 Because we know replies are sent after

receiving messages That is m2 is a reply to m1 Y receives m1 before sending m2

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Failure model Omission failures

Processor/process crash Communication failure/message drops

Arbitrary failures Process setting wrong values in data Data corruption during transmission

Timing failures Synchronous systems Real-time systems Clock, process, channel

Masking failures Replication Service to mask failures

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Security model

Protecting objects Who is allowed to access to access what data

Check access rights, verify identity Securing process and interactions

Processes Server, client, peer

Communication channel Copy/alter messages; inject harmful messages Encryption, authentication, time stamping

Denial of service Mobile code, mobile agents

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01/26/11

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Time services

Global time consensus is needed to Coordinate distributed activities

File backup Expiration time of a received message/data

Event related activities When an event occurs or occured How long did it take Which event occurred first

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Clocks

Physical clock Approximation of realtime

Logical clock Preserves ordering of events

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Physical Clocks

Physical clocks in computers synchronize and schedule hardware activities

Software timers depend on physical clock interrupts to manage software activities

In distributed systems There are many such clocks Need for a time server to synchronize clocks Delay associated with receiving time server

inputs

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Time server

Signal propagation delay Network communication delay Noise and other delays (TCP) Suppose

Ts: Time for client-to-server communication Tr: time for server-to-client communication and Tp: time to process the request at the time

server Time= Timeserver(time) + (Ts+Tr+Tp)/2

Tp/2 is included to account for service to transport layer chores

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Logical Clocks

Leslie Lamport’s logical clock is the basis for Ordering processes and events in distributed

systems Process Pi maintains a logical clock Ci

is the happens before relation Two Rules:

Rule1 : If event a occurs before event b within a single process then logical clocks C(a) and C(b) are assigned such that C(a) < C(b).

Rule 2: Suppose - a is a sending event of Pi and b is the corresponding receiving event of Pj then Ci(a) < Cj(b)

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Logical clocks (contd.)

Rule 1 is easy to enforce as both a and b are on the same process

How to enforce Rule 2

describes the causality between two events and it is transitive

If a b and b c, then a c. If neither a b nor b a then the two events are

disjoint and can be run concurrently.

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C(b) = C(a) + Cj(b) = max [TSa+ , Cj(b)]Tsa is the timestamp of the sending event and is a positive number

Happens-before relation

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= 1

a,25

b,35

c,24

27

2838

39

40

41

d,36

e,44

f,27 g,42

Synchronization

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Networking review

Please read up chapter 4 or a networking book

I will cover only mobile and wireless networking

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jec

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