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yuIntroduction to Distributed SystemsChapter twoArchitectures

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Introduction

Distributed systems are often complex pieces of software of which the components are by definition dispersed across multiple machines.

Organization of distributed systems is mostly about the software components that constitute the system.

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Introduction cont…

Organization • Logical organization• Physical organization

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Introduction cont…

Software architecture• how the various software components are

to be organized and how they should interact.

system architecture• instantiate and place software components

on real machines.

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Introduction cont…

Centralized architecture •Single server implements software components•Remote clients access the server with any communication means

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Sample centralized architecture

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Centralized architecture cont…

Drawback •Not easily scalable• Limitation in the number of CPUs.• Entire system needs to be upgraded or

replaced

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Introduction cont…

Decentralized architecture •Two or more machines•Plays more or less equal role

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Architectural styles

Formulated in terms of componentsThe way that components are connected

to each otherThe data exchanged between componentsHow these elements are jointly configured

into a system

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Architectural styles cont…

Layered architecture Object – based architectureData – centered architectureEvent – based architecture

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Layered architecture

Components are organized in a layered fashion

Component at layer L; is allowed to call components at the underlying layer Li

But not vise versa

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Layered architecture cont…

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Layered architecture cont…

Key observation is that control generally flows from layer to layer

Requests go down the hierarchy whereas the results flow upward

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Object – based architecture

Each object corresponds to what is called a component

Components are connected through a (remote) procedure call mechanism

Matches the client-server system architecture.

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Sample object – based arch

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Data – centric architecture

Processes communicate through a common repository.

Processes communicate through shared data

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Data – centric architecture cont…

Shared distributed file system in which virtually all communication takes place through files

Web-based distributed systems, are largely data-centric

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Event – based architectures

Processes essentially communicate through the propagation of events, which optionally also carry data.

Associated with what are known as publish/subscribe systems.

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Event – based architectures cont…

Processes publish events after middleware ensures that only those processes that subscribed to those events will receive them

Advantage :•Processes are Decoupled in space,

or referentially decoupled.

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Advantage cont…

Referentially decoupled implies processes need not explicitly refer to each other

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System architectures

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yuWhat is System Architecture?

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System Architecture

CentralizedDecentralized Hybrid

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Centralized Architecture

Basic client-server modelA server is a process (machine) implementing a specific service

for example, a file system service or a database service.

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Centralized Architecture cont…

A client is a process that requests a service from a server by sending it a request and subsequently waiting for the server's reply.

client-server interaction is also known as request-reply.

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Centralized Architecture cont…

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Centralized Architecture cont…

The communication can be either of the following:•Connection less protocol•Connection - oriented protocol

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Centralized Architecture cont…

Connectionless protocolIf the underlying network is fairly reliable as in many local-area networks

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How Connectionless protocol works

How client requests a service:•Packages a message for the server• Identifying the service it wants• Include necessary input data•The message is then sent to the server.

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Cont…

How servers works:•Always wait for an incoming request•Subsequently process requests•Package the results in a reply message•Send replies to the client

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Centralized Architecture cont…

Connection - oriented protocolIs not entirely appropriate in a local-area network due to relatively low performance

Works perfectly fine in wide-area systems in which communication is inherently unreliable

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Cont…

For example, All Internet application protocols are based on reliable TCP/IP connections.

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How connection – oriented protocol works

When client requests a service:•Sets up a connection to the server before sending the request•Server generally uses that same connection to send the reply message

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Cons of connection – oriented protocol

Setting up and tearing down a connection is relatively costly, especially when the request and reply messages are small.

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Centralized Architecture cont…Application Layering•The user-interface level•A part that handles interaction with a user•The processing level•A part that operates on a database or file

system•The data level•A middle part that generally contains the

core functionality of an application

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Cont.… Search Engine Example

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Decentralized Architecture

Vertical distribution• Is achieved by placing different

components on different machines• functions are logically and physically

split across multiple machines• each machine is tailored to a specific

group of functions

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Decentralized Architecture

Horizontal distribution•Client or server may be physically split up into logically equivalent parts, but•Each part is operating on its own share of the complete data set•Balance the load

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Structured peer – to – peer Arch

Network is constructed using a deterministic procedure like Distributed Hash Table (DHT)

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Structured peer to peer cont …

Distributed Hash Table (DHT)• Data items are assigned a random key

(128-bit or 160-bit identifier)• Nodes assigned a random number from

the same identifier space• DHT Maps the key of a data item to the

identifier of a node based on some distance metric

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Unstructured peer –to – peer Arch

Rely on randomized algorithms for constructing an overlay network

Each node maintains a list of neighbors

Data items are randomly placed on nodes

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Cont..

When a node needs to locate a specific data item, it floods the network with a search query.

Alternative way:•Super-peers

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Super - peers

Make use of special nodes that maintain an index of data items

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Super – peer cont

Problem •select the nodes that are eligible to become super peer•Leader-election problem

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Hybrid Architecture

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Hybrid Architecture

Client-server solutions are combined with decentralized architectures

•Edge-Server Systems•Collaborative Distributed Systems

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Edge – Server Systems

Deployed on the InternetServers are placed "at the edge" of the network

Boundary between enterprise networks and the actual Internet

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Edge – Server Systems

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Collaborative Distributed Systems

Once a node has joined the system, it can use a fully decentralized scheme for collaboration.

BitTorrent

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BitTorrent implementation

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System Models

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System Models

Provide an abstract, simplified but consistent description of a relevant aspect of distributed system design.

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System models cont…

Physical modelsArchitectural modelsFundamental models

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Physical models

Hardware composition of a system in terms of:

ComputersMobilesAnd their interconnecting

network

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Physical models cont…

Three generations of distributed systems:

1. Early Distributed Systems2. Internet – Scale Distributed Systems3. Contemporary Distributed Systems

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Generation of Distributed Sys.

Early Distributed Systemslate 1970s and early 1980s in response

to the emergence of local area networking technology, usually Ethernet

10 to 100 nodes interconnected by a LANSmall rage of services like:

Local printer sharing and file servers

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Early Distributed Systems cont…

Homogenous individual systemsOpenness not implemented largelyQoS didn’t metNodes are typically Desktop PCs:• Static• Autonomous•Discrete

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Generation cont…

Internet – scale distributed systems1990s in response to growth of the Internet

Large number of nodesNodes interconnected by network of networks (Internet)

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Internet – Scale Distributed Systems cont…Highly heterogeneous:

• Network, PC architecture, OS, implementation language and etc.

Nodes are typically Desktop PCs:• Static• Autonomous• Discrete

Google (1996)About 16 or 17 years before

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Generation cont…

Contemporary Distributed SystemsMobile computing (laptops and

smartphones capabilities)Ubiquitous computing (embedded

computers in everyday life)Cloud computing highly not

autonomous

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Fundamental models

Takes an abstract perspective in order to examine individual aspects of a distributed system• Interaction models•Failure models•Security models

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Fundamental models cont…

Interaction modelsConsider the structure and sequencing of the communication between the elements of the system

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Fundamental models cont…

Failure modelsConsider the ways in which a system may fail to operate correctly

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Fundamental models cont…

Security modelsConsider how the system is protected against attempts to interfere with its correct operation or to steal its data

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Architectural models

System structure in terms of separately specified components and their interrelationships

Architectural elementsCommunicating entitiesCommunication paradigms

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Communicating entities

Are the entities that are communicating in the distributed system•Objects• Components•Web services

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Communicating entities cont…

ObjectsEnable and encourage the use of object-oriented approaches in distributed systems (including both object-oriented design and OOP)

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Communicating entities cont…

Components Interconnected to form the systemAccessed through interfacesDependency between components

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Communicating entities cont…

Web servicesClosely related to objects and components

Are intrinsically integrated into the World Wide Web

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Communication paradigms

How entities communicate in a distributed system

Three paradigms:• Inter process communication• Remote invocation • Indirect communication

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Inter process communication

Low-level support for communication between processes

Including message-passing primitives, direct access to the API offered by Internet protocols (socket programming) and support for multicast communication.

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Remote invocation

Based on a two-way exchange between communicating entities in a distributed system and resulting in the calling of a remote operation, procedure or method.

Request-reply protocol• Remote procedure call• Remote method invocation

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Remote procedure call (RPC)

Procedures in processes on remote computers can be called as if they are procedures in the local address space

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Remote Method Invocation (RMI)

A calling object can invoke a method in a remote object

The underlying details are generally hidden from the user