2010 1

Overview

Distributed systems

2010 2

Layers•Communication is logically on the application layer•Only that has to be considered

•except for speed, reliability, security and cost

•Error correction (and security) might (will) be on application layer, but is usually also on lower layers

2010 3

Client-Server and Peer-to-PeerServer always on; Client and Peers not

P2P: Skype, BitTorrent; IM: partly, messages yes, setup+addr. not

2010 4

Problems•No throughput guaranties:

•problems with bandwidth-sensitive application, like many multimedia applications •some may use adaptive coding techniques (reducing quality) to match available throughput

•No timing (delay or jitter) guaranties•problems for real-time streaming multimedia

•like telephony, multi-layer games, teleconferencing•no solution for this except special networks•non-real time streaming multimedia (like a movie replay) can buffer at the receiver

•No security•this can be cured by SSL(secure socket layer)•also by network layer security methods

2010 5

Distributed systemsA collection of independent computersthat appears to its users (people or programs)as a single coherent system.

2010 6

Goals: sharing, transparent• making resources accessible

– economics: printers, storage systems, supercomputers– information exchange: mail, audio, video– collaboration: groupware, videoconferencing, virtual

organizations• distribution transparency

2010 7

Goals: openness• Openness

– offer services according standard rules• describing syntax and semantics of services• computer networks: protocols• distributed systems: interfaces described in an IDL

(Interface Definition Language)– to achieve interoperability and portability– extensible: add new components or replace existing

• collection of relatively small component• separate policies and mechanism

2010 8

• size: easily add more users and resources

• geographically: increasing distances

• administration: easy to manage if it increases limitations

• decentralized algorithm:

– no machine has complete information about the system state

– each machines makes decisions based only on local information

– failure of one machine does not ruin the algorithm

– no implicit assumption a global clock exists

Goals: scalable

2010 9

Distributing Computing SystemsCluster Computing Systems: high performance computing

2010 10

Grid Computing Systemshigh degree of heterogeneity: resource from different organizations

are brought together in a virtual organization.

2010 11

Distributed Information SystemsTransaction Processing System

•Atomic: To the outside world, the transaction happens indivisibly.•Consistent: The transaction does not violate system invariants.•Isolated: Concurrent transactions do not interfere with each other.•Durable: Once a transaction commits, the changes are permanent.

2010 12

Nested Transaction

2010 13

TP monitor

2010 14

Distributed Pervasive Systems• consisting of mobile and embedded computing devices

– small, battery-powered, mobile, wireless connections

2010 15

Sensor Networks

2010 16

Important topics1. architecture: software and system

2. processes: treads, virtual machines, client-server organization, code migration

3. communication: layered protocols, Remote Procedure Calls, Message Passing Interface

4. naming: names, identifiers, addresses

5. synchronization: (logical) clocks, mutual exclusion, election algorithms

6. consistency and replication

7. fault tolerance

8. security

2010 17

Architecture (1)• layered• object-based• data-centered• event-based

2010 18

Architecture (2)

Processes communicate through a common (passive or active) repository.

Events may carry dataPublish/subscribe systemsLoosely coupled processes

2010 19

Application layering• user-interface• processing• data

Using an Internet search engine

2010 20

Alternative client-server organization

Thin - fat clientseasier – difficult to manage

application and database on different servers Vertical distribution: placing logically different components on different machines

2010 21

Peer-to-peer systemsHorizontal distribution:

client or server physically

split up in equivalent parts,

operating on its own share of the data set

Distributed Hash Tables

data items with key k

mapped on node with

id: smallest id >= k

2010 22

Collaborative Distributed Systemsfor a node to join often a client-server scheme is used

an example is BitTorrent

a Tracker keeps an account of active nodes (currently downloading some file) having (chunks of) the requested file

the client node becomes than active, providing also (chunks of) files

2010 23

Processes and treads• a way to do more things at the same time• illusion that each one has it own virtual CPU• used in clients (e.g. browser to start downloading parts of a

website at the same time) and servers

2010 24

Virtual Machines• not only virtualization of CPU but also of other resources• many different OS’s working concurrently on 1 machine• old technique from the 1960’s

2010 25

Process virtual machinesame OS, different runtime-systems (with applications)

2010 26

Virtual machine managermultiple different OS’s concurrently on same hardware

System HardwareSystem Hardware

Host Operating SystemHost Operating System

Virtual Machine ManagerVirtual Machine Manager

Virtual MachineVirtual Machine

Guest OSGuest OS(Windows 2003)(Windows 2003)

ApplicationsApplications

Virtual MachineVirtual Machine Virtual MachineVirtual Machine

Guest OSGuest OS(Windows NT)(Windows NT)

Guest OSGuest OS(Windows 2000)(Windows 2000)

ApplicationsApplications ApplicationsApplications

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2010 27

Virtual private servers• bridge the gap between shared web hosting services and

dedicated hosting services• also for workstations• examples VMware, VirtualPC

2010 28

Communication

• send and receive over TCP streams using socket interface for networks

• message passing, higher level of abstraction– representation of integers, floats, structures, etc– usable for shared memory communication and high-

speed interconnect busses on parallel machines• RPC, Remote Procedure Call

2010 29

Naming

• Names are used to refer to entities (anything that can be operated on)

• The naming system may be itself be implemented in a distributed fashion.

• We need to resolve a name to the entity it refers to. • How to organize a human friendly name system?

E.g. files systems, World Wide Web• How to locate from a name the entity it refers to in a

way that is independent of their current location.• How to resolve names by means of entity attributes?• Internets Domain Name System as an example

2010 30

Synchronization• Synchronization of distributed processes is more difficult

than that of processes in uni/multi-processor systems.• using physical clocks on systems is not accurate enough,

need for logical clocks• distributed global states• distributed mutual exclusion• the bully election algorithm