part six: distributed systems natalia shatokhina ashwini mundya

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Part six: Distributed systems Natalia Shatokhina Ashwini Mundya

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Part six: Distributed systems

Natalia ShatokhinaAshwini Mundya

Outline• Reasons to build distributed system• Network OS vs Distributed OS• Data/process/communication migration in

distributed OS• Communication Structure• Communication Protocols• Robustness• Design Issues• An Example: Networking

Distributed system

client

Site B

Site A

server

Resources

Site C

Network

Why distributed systems?4 major reasons for building

distributed systems:• Resource Sharing• Computational speedup• Reliability• Communication

Reason 1: Resource sharing

• User on one site may be able to use the resources available at another site– Using a printer at remote site– Access a file– Access a database

Reason 2: Computational Speedup

If particular computation can be split into subcomputations that can run in parallel, the distributed system allows to distribute computations among various sites

Speedup can be achieved when running computations in parallel

Speedup= “serial computation time”/”parallel computation time”

In addition, load sharing helps to achieve speedup: move the jobs from the site overloaded with jobs to other with lighter load

Automated load sharing is not yet common

Reason 3: Reliability • If one site fails in distributed system

remaining sites can continue to operate and get the job done

• The function of failed site can be taken over by another site

To provide reliability:• The system must ensure the correct transfer

of function• Failure of a site must be detected by the

system and the services of that site should not be longer used

• Mechanisms must be available to integrate the recovered site back to the system

Reason 4: Communication• Sites can communicate over a

network by sending messages to one another.

• The advantage of distributed system is that the messages can be carried over a great distance (two geographically distant sites). That provides for efficient collaboration.

Types of network-based operating systems

• Network operating systems• Distributed operating systems• What is the difference?

Network operating systems (NOS)Definitions:

• From book: A network operating system provides an environment in which users, who are aware of the multiplicity of machines, can access remote resources by either logging in to the appropriate machine or transferring data from the remote machine to their own machines.

• From wikipedia: System designed to allow shared file and printer access among computers in a network, typically a local area network (LAN), a private network or Intranet.

Hardware architectures (source:

http://www.cs.jhu.edu/~yairamir/cs437/week7/sld005.htm):

cache

CPUMemory

cache

CPU

CPU

Memory

CPU

Memory

CPU

Memory

Parallel architecture

Distributed architecture

Multiprocessors

•Tightly coupled

•Shared memory

Multicomputers

•Loosely coupled

•Private memory

•Autonomous

Network OS vs Distributed OS (source: http://www.cs.jhu.edu/~yairamir/cs437/week7/sld005.htm):

• Network OS: Contains N copies of operating systems, communication between machines is via shared files.

• Distributed OS: Contains N copies of operating systems, communication between nodes is via messages over a network . These messages pass the necessary parameters for the task and on completion messages return the results. Its as if the computers email each other with a request and an answer.

Network Operating systemsRemote login:

• Network OS (for example Unix) provides an Internet service that allows the user to log in remotely

• telnet or ssh is used • After logging in user can compute on remote machine

just as any local user.• Ssh is used to establish encrypted connectionRemote file transfer (ftp):• Protocol allows to transfer a file from remote machine to

local computer• User must know the commands and the exact location of

the file he wants• Anonymous method provides access to files in directory

tree “anonymous” to anyone• Since ftp is not secure, scp is currently used for file

transfer

Distributed OS characteristics

• In a distributed OS users access remote resources in the same way they access local resources

• Data and process migration is under control of OS

Examples of distributed systems

(source: Wikipedia) • Network OS- UNIX; Windows 2000 Server; Netware.

• Distributed OS– Mosix2

• implemented as an OS virtualization layer that provides to users and applications a single system image with the Linux run-time environment

• Definition: single system image (SSI) cluster is a cluster of machines that appears to be one single system.

– Plan 9 (developed for research purposes, planned as UNIX successor);

– Amoeba (development is frozen). Project lead: Andrew Tannenbaum (MINIX author)

• Clusters: – a group of linked computers often connected in LAN,

working together closely thus in many respects forming a single computer

• Implementations of cluster systems (commercial and free) exists for many OS

• Categories of cluster systems: – high availability( Linux-HA, Lander Cluster), – load-balancing,– compute (Beowulf , OSCAR)– grid (Rocks).

• Web

Examples of distributed systems

Rocks cluster architecture

Cluster components• Front-end node• Compute node• Ethernet network• Application Message Passing Network

(optional)

Data migration approaches in distributed OS

• Transfer entire file (automated FTP system?). It was used in Andrew file system but found inefficient

• Transfer only portions of the file that are necessary for immediate task. Another portion requires another transfer. Protocols that are using this method: NFS, newer versions of Andrew, SMB (running on top of TCP/IP or Microsoft NetBEUI)

• The system may also perform data translation (if sites are using different character code representations)

Computation migration in distributed OS

• Example: to obtain a summary of large files that reside on different sites.

• Rather then copy files to one location and obtain a summary, it is more efficient to access the files at the sites where they reside and the return result to the initiating site– Process P wants to access file at site A. Access is initiated by RPC.

Through UDP RPC executes a routine on remote site. P ivokes a pre-defined procedure at A, the results are returned to P

– P sends a message to A. OS creates a new process Q that executes the task. When done Q sends the results back to P via the message system. This way P may execute concurrently with Q.

Either method can be used to access files at different sites

RPC might result in invocation of another RPC

Process migration in distributed system

• Migration: the process is not get executed at the site where it was initiated

• Reasons:– For load balancing– For speedup: if a process may be divided into subprocesses– Hardware preference: process is more suitable for execution on some

specialized processor– Software preference: process requires software that is available only at

particular site– Data access: the same as in computation migration - for big chunks of

data• If load balancing is needed then a system itself can move processes• when the process must satisfy hardware -software preference then

the user has to specify how process should migrate • Examples: Web server/database, Java Applets

Networks for distributed systems

Local area networks (LAN) :• Composed of processors distributed over small area (single

building)• Emerged in 1970

• More economical then large mainframe computer• Data sharing in a single enterprise

• Communication link: high quality cable• Communication speed: 1Mbit/sec - 1 Gbit/sec• Typical LAN consist of computers, shared peripheral devices

(printers, tapes) and one or more gateways that provide access to other networks

• Ethernet scheme is used to construct LAN• Wireless LAN networks can be built with only one wireless router• Compared to Ethernet systems WiFi networks run at slower speed

Networks for distributed systems

Wide area networks (WAN):

• Composed of autonomous processors distributed over a large area (such as US territory)

• Emerged in 1960s as academic research project

• First WAN - Arpanet - a cluster of four computers

• Communication link: phone lines, microwave links, satellite channels

Networks topologyConfigurations used for

distributed system network:– Partially connected network– Fully connected network– Tree-structured network– Star network– Ring network

• Criteria to compare the configurations:– Installation cost– Communication cost– Availability

Network communication structure

Communication network design issues:

• Naming and name resolution• Routing strategies• Packet strategies• Connection strategies• Contention

Naming and name resolution

• How could processes at different sites specify each other to communicate?– Process identifier (PID) is used for processes

within computer system, but since networked hosts share no memory different approach is needed

• Pair <host name, identifier> is used for process identification – Host name is alphanumeric

• Host name is human readable, but computers use host-id instead. There is a mechanism to resolve host name into host-id - Domain name system

• Hosts in Internet are logically addressed with with multipart names (IP addresses)

Naming and name resolution

• Each component in IP name has name server• A system that needs to use the DNS is configured with the

known addresses of the root servers • Example: bob.cs.brown.edHow to get IP address for IP name bob.cs.brown.edu ?• Kernel of system A issues a request to the root server for edu domain

asking for the address of the name server for brown.edu• The edu root server returns the address of the host on which brown.edu

name server resides• The kernel on system A then queries the name server at this address

and asks about cs.brown.edu • An address is returned ; and a request to that address returns host-id

128.148.31.100

1. Java provides API for mapping IP names to IP addresses2. Generally, OS is responsible for accepting a message from its

processes for <host name, identifier> and transferring it to appropriate host

3. Kernel of destination host transfers the message to the process named by identifier

Naming and name resolution

Routing strategies• When a process from site A wants to

communicate with process at site B how is a message sent?

• Use physical path from A->B• If multiple paths exists then use

Routing table – May include info about speed & cost of

communication paths

Routing strategies comparison

• Routing schemes– Fixed routing

• Path is specified in advance (shortest path) and doesn’t change– Virtual routing

• Path is fixed for the duration of one session. Examples: file transfer, remote login period etc

– Dynamic routing• Path is chosen when message is sent. The message is sent over the link that is

the least used at that particular time

• Tradeoffs– Order of messages

• Fixed & virtual scheme deliver messages in order– Environment

• Dynamic scheme performs better in complicated environment

Usually the combination of these schemes is used - for example: within a site each host has a static route to the gateway, but the gataway uses dynamic routing to reach any host on the network.

Communication Structure (Cont.)

Packet Strategies

• Variable length message divided into fixed-length message called packets/frames

• Packets transferred to destination using 2 kinds of services :• Reliable service• Unreliable service

Connection Strategies

• Circuit switching - A permanent physical link is established for the duration of the communication (i.e., telephone system)

• Message switching - A temporary link is established for the duration of one message transfer (i.e., post-office mailing system)

• Packet switching - Messages of variable length are divided into fixed-length packets which are sent to the destination• Each packet may take a different path through the

network• The packets must be reassembled into messages as

they arrive

Connection Strategies(Cont.)

• Circuit switching requires setup time, but incurs less overhead for shipping each message, and may waste network bandwidth

• Message and packet switching require less setup time, but incur more overhead per message

Contention

• Several sites may want to transmit information over a link simultaneously. Techniques to avoid repeated collision include

• CSMA/CD• Token Passing

Contention (Cont.)

• CSMA/CD - Carrier sense with multiple access (CSMA); collision detection (CD)

• A site determines whether another message is currently being transmitted over that link. If two or more sites begin transmitting at exactly the same time, then they will register a CD and will stop transmitting

• When the system is very busy, many collisions may occur, and thus performance may be degraded

• CSMA/CD is used successfully in the Ethernet system, the most common network system

Contention (Cont.)

• Token passing - A unique message type, known as a token, continuously circulates in the system (usually a ring structure)

• A site that wants to transmit information must wait until the token arrives

• When the site completes its round of message passing, it retransmits the token

• A token-passing scheme is used by some IBM and HP/Apollo systems

Communication Protocols

Communication Protocols

• Simplify the design problem by partitioning the problem into multiple layers.

• The 2 types of reference models

• ISO's (International Standard Organization) OSI (Open System Interconnection) Model

• TCP/IP Model

OSI Reference Model

• Defines 7 network layers

Physical Layer

• Handles the mechanical and electrical details of the physical transmission of a bit stream

• This layer is implemented in the hardware of the networking device

Data Link Layer

• This layer ensures that messages are delivered to the proper device on LAN using hardware address

• Handles the frames, or fixed-length parts of packets, including any error detection and recovery that occurred in the physical layer

Network Layer

• Responsible for tracking the location of devices in the network and determine the best way to move the data

• Routers work at this level

• Check if the packet belongs to that network else forward the packet to the destination address using routing table

Hop-to-hop delivery

Transport Layer

• Segments and reassembles the data into a data stream

• Provides the end-to-end transport services – using TCP/UDP

• Also called as Reliable Networking – acknowledgment, maintaining packet order (sequencing) and flow control

Session Layer

• Responsible for setting up, managing and tearing down the sessions between presentation layer entities

• Performs message synchronization

Presentation Layer

• Presents the data to the application layer

• Translator – provides coding and conversion functions (Before transmit the data it has to be in standard format. Computers are configured to receive this data and convert onto native format).

• By providing the translation service presentation layer ensures that data sent from the Application layer of one system is read by the Application layers of another one.

Presentation Layer (Cont.)

Application Layer

• Provides services to the end user

• Examples – File Transfer, E-Mail, Remote Login

OSI Reference Model

An OSI network message

Summary of layers

TCP/IP Protocol Layers

Robustness

• Failure Detection

• Reconfiguration

Robustness(Cont.)

• Distributed system may suffer from various types of failure

• Failure of a link

• Failure of a site

• Loss of message

Failure Detection

• Detecting hardware failure is difficult

• To detect a link failure, a handshaking protocol can be used

• The conclusion can be : • Other Site is down• The direct link between the two Site is down• The alternate link between the two Site is down• The message has been lost

Reconfiguration

• When Site A determines a failure has occurred, it must reconfigure the system:

• If the link from A to B has failed, this must be broadcast to every site in the system

• If a site has failed, every other site must also be notified indicating that the services offered by the failed site are no longer available

Recovery from failure

• Once the link is repaired both the systems must be notified (handshake)

• Recover about the site has been notified to all other sites (failed site must also receive information from other sites)

Design Issues• Transparency - the distributed system should appear

as a conventional, centralized system to the user

• Fault tolerance – the distributed system should continue to function in the face of failure

• Clusters – a collection of semi-autonomous machines that acts as a single system

• Scalability – as demands increase, the system should easily accept the addition of new resources to accommodate the increased demand

Silicon Graphics Cluster

An Example: Networking• The transmission of a network packet between hosts on an

Ethernet network

• Every host has a unique IP address and a corresponding Ethernet (MAC) address

• Communication requires both addresses

• Domain Name Service (DNS) can be used to acquire IP addresses

• Address Resolution Protocol (ARP) is used to map MAC addresses to IP addresses

• If the hosts are on the same network, ARP can be used• If the hosts are on different networks, the sending host will

send the packet to a router which routes the packet to the destination network

An Ethernet Packet

References

• Operating System Concepts 8th Edition by Abraham Silberschatz, Peter B. Galvin, Greg Gagne

• http://en.wikipedia.org/wiki/OSI_model• http://en.wikipedia.org/wiki/TCP/IP_model• http://en.wikipedia.org/wiki/Distributed_computing• citeseerx.ist.psu.edu/viewdoc• http://cs.gmu.edu/~setia/cs475-S03/slides/Client-server.pdf• http://www.rocksclusters.org• http://www.cs.jhu.edu/~yairamir/cs437/week7/sld005.htm

Thank you !