Third year sixth semester CS6601 Distributed System 2 mark with answer

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UNIT-1 PART-A 1.Define distributed Systems. A distributed system as one in which hardware or software Definition 1: componentslocated at networked computers communicate and coordinate their actions only by passing messages Defi Defi nition 2: .A collection of hardware and software systems that contain more .A than one processing or storage element but appearing as a single coherent system running under a loosely or tightly controlled regime is called Distributed Computing. The computers in the distributed system do not share a memory instead they pass messages asynchronously or synchronously between them. This is a type of segmented or parallel computing that runs on a heterogeneous system. 2.What are the examples of Distributed System? Web search, Massively multiplayer online games (MMOGs),Financial trading, Cloud computing, Grid computing, Ubiquitous or pervasive computing 3.List the advantages and disadvantages of Distributed System. Advantages 

Economics

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Speed

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Inherent distribution d istribution

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Reliability

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Incremental growth

Resource sharing Disadvantages 

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Heterogeneity

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Openness

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Failure handling

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Quality of service

Security 4.What are the challenges in Distributed System? 

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Heterogeneity

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Openness

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Failure handling

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Quality of service


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Security

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Transparency

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Scalability

Concurrency 5.Show where Distributed System is applied? Telecommunication networks: telephone networks and cellular and cellular networks, computer networks such as the Internet, the Internet, wireless sensor networks, routing algorithms; Network applications: 

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World wide web and peer-to-peer and peer-to-peer networks, massively multiplayer online games and virtual and virtual reality communities, distributed databases and distributed and distributed database management management systems, network file systems, distributed information processing systems such as banking systems and airline reservation systems; Real-time process control: aircraft control systems, industrial control systems; Parallel computation: scientific computing, including computing, including cluster cluster computing and grid and grid computing and various volunteer various volunteer computing projects computing projects (see the list the list of distributed computing projects), projects), distributed rendering in computer graphics. 6.Estimate how distributed system benefits resource sharing? Resource sharing benefits by allowing the users to share hardware resources such as printers, dataresources such as files, and resources with more specific functionality such as search engines.pages – not not the disks and processors on which they are implemented. Similarly, users think in terms of shared resources such as a search engine or a currency converter, without regard for the server or servers that provide these. 7.Define CSCW Computer-supported cooperative cooperative working (CSCW), a group of users who cooperate directly share resources such as documents ina small, closed group. 8.What is MMOG 

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Massively multiplayer online games is an online video game where very large numbers of users interact through the Internet with a persistent pe rsistent virtual world. Leading examples of such games include Sony’s EverQuest II and EVE Online from the Finnish company CCP Games. 9.Discuss the trends of Distributed System. Pervasive networking and modern internet Mobile and ubiquitous computing Distributed in Multimedia system Distributed computing as a utility. 10.Compare Centralized and Distributed System.    

Centralized systems

Distributed System

Several jobs are done on a particular Jobs are distributed among several CPU processors. The processors are interconnected by a computer network. They have shared memory and They have no global state (i.e No shared variables. shared memory and No shared variables) Global clock is present

No global clock

11.Generalize on Heterogeneity.

It means diversity of distributed system in terms of hardware, software,platform, etc. Modern distributed system will likely to be operating with different hardware devices, operating system,Network,programming language etc. 12.Explain the consequences faced by the designers in developing distributed systems. Refer challenges of distributed distributed system(Q.No:4) system(Q.No:4) 13.Define Ubiquitous computing. Ubiquitous computing is the harnessing of many small, cheap computationaldevices computationaldevices that are present in users’ physical environments, including the home, office and even natural settings. A user interacts with the computer,




which can exist in many different forms, including laptop computers, tablets and terminals in everyday objects such as a fridge or a a pair of glasses. glasses . 14.Show how distributed System helps cloud computing Cloud Computing is a specialized form of Distributed Computing.Cloud Computing takes Distributed Computing to a utility stage, ubiquitous and unmetered access to broadband Internet is the key to its success. In addition, better standardization, portability and interoperability of its distributed components will help move Cloud Computing to its full potential.Cloud Computing is all about delivering services or applications in on demand environment with targeted goals of achieving increased scalability and transparency, security, monitoring and management.In cloud computing systems, services are delivered with transparency not considering the physical implementation within the Cloud. 15.Illustrate the concept of concurrency. Both services and applications provide resources that can be shared by clients in adistributed system. There is therefore a possibility that several clients will attempt toaccess a shared resource at the same time. The process that manages a shared resource could take one client request at a time. But that approach limits throughput. Therefore services and applications generally allow multiple client requests to be processed concurrently. 16.What do you mean by transparency? Transparency is defined as the concealment from the user and the applicationprogrammer of the separation of components in a distributed system, so that the system is perceived as a whole rather than as a collection of independent components. Inter process communication is transparent at the application level, Whereas invisible to the user level to maintain a single system view. 17.Develop the scenario howmight the clocks in two computers that are linked by a local network be synchronized without reference to an external time source? Each computer in a distributed system has its own internal clock, which can be used by local processes to obtain the value of the current time. Therefore two processes running on different computers can each associate timestamps with their events.In a distributed a distributed system the problem takes on more complexity because a global time is not easily known. The most used clock synchronization solution on the Internet is the Network the Network Time Protocol (NTP) which is a layered client-server architecture based on UDP message passing. Lamport timestamps and vector and vector clocks are concepts of the logical clocks in distributed systems. 18.Define downloaded code




A CGI program runs at the server. Sometimes the designers of webservices require some service-related service-related code to run inside the browser, at the user’ s computer. In particular, code written in Javascriptis often downloaded with a web page containing a form, in order to provide better-quality interaction with the user than that supported by supported by HTML’s standard widgets. widgets. 19.Classify the security challenges faced by the distributed systems. : It is a category of security problem is that a user may Deni al of ser vice attacks wish to disrupt a service for some reason. This can be achieved by bombarding the service with such a large number of pointless requests that the serious users are unable to use it. This is called a denial of service attack. : Mobile code needs to be handled with care. Consider Secur ity of mobile mobi le code code someone who receives an executable program as an electronic mail attachment: the possible effects of running running the program are unpredictable; unpredictable; 20.State the features in fault tolerance Detecting failure  

Masking failure

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Tolerating failure

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Recovery from failure

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Redundancy

UNIT-II Two-Mark questions 1.What are the difficulties and threats in distributed system? Widely varying modes of use : The component parts of systems are subject to widevariations in workload – for example, some web pages are accessed several million times a day. Some parts of a system may be disconnected, or poorly connected some of the time – for for example, when mobile computers are included in a system. Some applications have special requirements for high communication bandwidth and and low latency – latency – for for example, multimedia applications. applications. Wide range of system environments: A distributed system must accommodate heterogeneous hardware, operating systems and networks. The networks may differ widely in performance – wireless wireless networks operate at a fraction of the speed of local networks. Systems of widely differing scales, ranging from tens of computers to millions of computers, must be supported.




Internal problems: Non-synchronized clocks, c locks, conflicting data updates and a nd many modes of hardware and software failure involving the individual system components. External threats: Attacks on data integrity and secrecy, denial of service s ervice attacks. 2. Classify the generations of distribute system 

Baseline physical model

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Early distributed systems

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Internet-scale distributed systems

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Contemporary distributed systems

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Distributed systems of systems

3. What are the entities that communicate in distributed system? The communication entities entities are Processes in distributed systems, Nodes in Wireless sensor networks, Object in object oriented systems, Components in

component based technology systems and web services. 4. List the types of Communication paradigms. Three types of communication paradigm: r elatively low-level low-level support for • Interprocess communication-refers to the relatively communication between processes in distributed systems, including message passing primitives primitives co mmon communication paradigm in • Remote invocation-represents the most common distributed systems, covering a range of techniques based on a two-way exchange between communicating communicating entities. Remote invocation invocation includes includes Request-reply protocol, Remote method invocation, Remote object Invocation. • Indirect communication-communication is indirect, through a third entity, allowing a strong degree of decoupling between senders and receivers. Senders do not need to know who they are sending to ( space uncoupling). Senders and receivers do not need to exist at the same sa me time (time uncoupling). uncoupling). Indirect communication includes Message queues, Group communication, Tuple space, Distributed shared memory. 5. Define remote invocation Remote invocation represents the most common communication paradigm in distributed systems, covering a range of techniques based on a two-way exchange between communicating entities in a distributed system(Request – system(Request – reply reply protocol) and resulting in the calling of a remote operation, procedure or method(RMI,RPC)




6. What are the request reply protocols? Request-reply protocols protocols represent a pattern on top of message passing and support the two-way exchange of messages as encountered in client-server computing. In particular, such protocols provide relatively low-level low-level support for requesting the execution of a remote operation, and also provide direct support for RPC and RMI 7. Design the roles and responsibilities of distributed systems. The main goal of a distributed computing system is to connect users and resources in a transparent, open, and scalable way. Ideally this arrangement is drastically more fault tolerant and more powerful than many combinations of stand-alone computer systems. Distributed systems replaced super computers by coordinating action in many ordinary systems. The larger amount of data stored in a single system , the longer it takes to access, and the heavier the load on the machine. This couples with the larger number of people trying to access the system at one time causes very poor access times. By splitting the data into logical sections and spreading it around several machines, there is a need of only small amount of indexing data to point people at the appropriate servers for their requirements. 8. What are the middleware layers? The term middleware applies to a software layer that provides a programming abstraction as well as masking the heterogeneity of the t he underlying networks, hardware, operating systems and programming languages. Eg:CORBA, Eg:CORBA, Java Remote Method Invocation 9. Differentiate marshalling and unmarshalling. Marshalling is is the process of taking a collection of data items and assembling them into a form suitable for transmission in a message. Unmarshalling is is the process of disassembling them on arrival to produce an equivalent collection of data items at the destination. Thus marshaling consists of the translation of structured data items andprimitive values into an external data representation. Similarly, unmarshalling consists of the generation of primitive values from their external data representation and the rebuilding of the data structures. 10 .Illustrate the Characteristics and application of Interprocess communication. Message passing between a pair of processes can be supported by two messagecommunication operations, send and receive, receive, defined in terms of destinations and messages. To communicate, one process sends a message (a sequence of bytes) to a destination and another process at the destination receives the message. This activity involves the communication of data from the sending process to the receiving process and may involve the synchronization of the two processes. Application: UDP datagram communication, TCP stream communication




11. Where remote object reference is applied? When a client invokes a method in a remote object, an invocation message is sentto the server process that hosts the remote object. This message needs to specify which particular object is to have its method invoked. A remote object reference is passed in the the invocation message to specify which object object is to be invoked 12. Evaluate on overlay networks. An overlay network is a virtual network consisting of nodes and virtual links, which sitson top of an underlying network (such as an IP network) and offers something that is not otherwise provided: • A service that is tailored towards the needs of a class of application or a particular higher-level service – service – for for example, multimedia content distribution; • More efficient operation in a given networked environment – for for example routing in an ad hoc network; feature – for for example, multicast or secure s ecure communication. communication. • An additional feature – 13. Formulate why there is no explicit data typing in CORBA CDR?

The type of a data item is not given with the data representation in the message ineither the CORBA CDR or the Sun XDR standard. This is because it is assumed that the sender and recipient have common knowledge of the order a nd types of the data items in a message. In particular, for RMI or RPC, each method invocation passes arguments of particular particular types, and the result is a value value of a particular particular type. 14. Classify the main arguments for adopting a super node approach in Skype. Super nodes are selected on demand based a range of criteria includingbandwidth available, reachability (the machine must have a global IP address and not be hidden behind a NAT-enabled router, for example) and availability (based on the length of time that Skype has been running continuously on that node). The main goal of super nodes is to perform the efficient search of the global index of users, which is distributed across the super nodes. The search is orchestrated by the client’s chosen super node and involves an expanding search of other super nodes until the specified user is found. 15. Describe Remote procedure call. ca n use to request a Remote Procedure Call (RPC) is a protocol that one program can service from a program located in another computer in a network without without having to understand network details. (A procedure call is also a lso sometimes known as a function call or a subroutin s ubroutinee call.)RPC uses the client/server client/server model. 16. Classify the issues for the design of RPC. The design issues of RPC are




• The style of programming promoted by RPC – programming programming with interfaces; • The call semantics associated with RPC; • The key issue of transparency and how it relates to remoteprocedure calls. 17. Distinguish RMI and RPC. The main difference between the two is the approach or paradigm used. RMI uses an object oriented paradigm where the user needs to know the object and the method of the object he needs to invoke. In compari co mparison, son, RPC isn’t object oriented and doesn’t deal with wi th objects. Rather, it calls specific subroutines that are already established. RPC handles the complexities involved with passing the call from the local to the remote computer. RMI does the very same thing; handling the complexities complexities of passing along the the invocation from the local local to the remote computer. computer. But instead of passing a procedural procedural call, RMI passes a reference reference to the object and the the method that is being called. Summary: 1.RMI is object oriented while RPC isn’t 2.RPC is C bases while RMI is Java only 3.RMI invokes methods while RPC invokes functions 4.RPC is antiquated while RMI is the future 18. Discuss the design issues of RMI. The design issues of RMI are • Programming with interfaces; • The call semantics associated with RMI; • The key issue of transparency. The object model The distributed object model Garbage collection Remote object reference 

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19. Explain on shared memory approach. Indirect communication paradigms offer an abstractionof shared memory. Two approaches of shared memory are: Distributed Shared Memory -Distributed shared memory (DSM) is an abstraction used for sharing data between computers that do not share physical memory. Processes access DSM by reads and updates to what appears to be ordinary memory within their address space. Tuple space-In this approach, processes communicate indirectly by placing tuples in a tuple space, from which other processes can read or remove them. 

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Tuples do not have an address but are accessed by pattern matching on content (content-addressable memory) 20. Show how will you make use of message queues Message queues is an important category of indirect communication systems. Message queues provide a point-to-point service using the concept of a message queue as an indirection, thus achieving the desired properties of space and time uncoupling. They are point-to-point in that the sender places the message into a queue, and it is then removed by a single process. Message queues are also referred to as Message-Oriented Middleware 21. What is EJB? EJB is an extension of RMI. Enterprise JavaBeans (EJB) is a comprehensive technology that provides the infrastructure for building enterprise-level server-side distributed Java components. The EJB technology provides a distributed component architecture that integrates several enterprise-level requirements such as distribution, transactions, security, messaging, persistence, and connectivity to mainframes and Enterprise Resource Planning (ERP) systems. When compared with other distributed component technologies such as Java RMI and CORBA, the EJB architecture hides most the underlying system-level semantics that are typical of distributed component applications, such as instance management, object pooling, multiple multiple threading, and connection connection pooling. pooling. 22. Give the use of proxy in RMI . The role of a proxy in RMI is to make make remote method method invocation invocation transparent toclients by behaving like a local object to the invoker; but instead of executing an invocation, it forwards it in a message to a remote object. It hides the details of the remote object reference, the marshalling of arguments, unmarshalling of results and sending and receiving of messages from the client.

UNIT-III Two marks questions 1.Define Peer to peer system. Peer-to-peer systems represent a paradigm for the construction of distributed systemsand applications applications in which w hich data and computational resources are contributed co ntributed by many hosts on the the Internet, all of which participate participate in the provision of a uniform service. Peer-to-peer applications have been used to provide file sharing, web caching, information distribution and other services, exploiting the resources of tens of thousands of machines across the Internet. I nternet. 2.Classify the characteristics of Peer to peer system.




Peer-to-peer systems share these characteristics: Their design ensures that each user contributes resources to the system. Although they may differ in the resources that they contribute, all the nodes in apeer-to-peer system have the same functional capabilities and responsibilities. Their correct operation does not depend on the existence of any centrallyadministered centrallyadministered systems. s ystems. They can be designed to offer a limited degree deg ree of anonymity to the providers andusers of resources. A key issue for their efficient operation is the choice of an algorithm for theplacement of data across many hosts and subsequent access to it in a manner thatbalances the workload and ensures availability without adding undue overheads. 3. Discuss how IP and overlay routing for peer-to-peer applications differ from each other?  

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IP Routing 1.IPv4 is limited addressablenodes.

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2.Loads on routers are determined by network topology and associated traffic patterns. IP routing tables are updatedasynchronously Network connectivity failure. nfoldreplication is costly. Each IP address maps to exactly onetarget node. Addressing is only secure when allnodes are trusted. Anonymity for theowners of addresses is not achievable.

Overlay Routing Peer-to-peer systems can address more objects. The GUID namespace is very 128 large and flat (>2 ), allowing it to be much more fully occupied. Object locations can be randomized and hence traffic patterns are divorced from the network topology. Routing tables can be updated synchronously or asynchronously Routes and object references can be replicated n-fold, ensuring tolerance of n failures of nodes or connections. Messages can be routed to the nearest replica of a target object. Security can be achieved even in environments with limited trust A limited degree of anonymity can be provided




4. Classify the functional and nonfunctional requirements of peer to peer middle ware systems. Functional requirements : The function of peer-to-peer middleware is to simplify theconstruction of  services that are implemented across many hosts in a widely distributednetwork. Enable clients to locate and communicate with anyindividual resource made  available available to a service The ability to addnew resources and to remove them at will and to add hosts  to the service and removethem. should offer a simpleprogramming si mpleprogramming interface to application programmers  Non-Functional requirements: Global scalability  Load balancing  Optimization for local interactions between neighbouring peers  Accommodating to highly dynamic host availability  Security of data in an a n environment with heterogeneous trust  Anonymity, Anonymity, deniability de niability and resistance to censorship  5.Illustrate what is the use of routing overlay? Routing overlay takes responsibility for locating nodes and objects. The routing overlay ensures that any node can access any object by routing eachrequest through a sequence of nodes, exploiting knowledge at each of them to locate the destination object. The main task of a routing overlay is the following: Routing of requests to objects  Insertion of objects  Deletion of objects  Node addition addition and removal  6.Define pastry. Pastry is themessage routing infrastructure. All the nodes and objects that can be accessed through Pastry are assigned 128-bit GUIDs. For nodes, these are computed by applying a secure hash function.An object’s GUID is computed from all or part of the state of the object using a function that delivers a value that is, with very high probability, unique. Distributed Hash Table(DHT) is used to generate the GUID from the object’s value 7.Differentiate Structured and unstructured peer-to-peer systems. Structured P-to-P system Unstructured P-to-P system 1.Guaranteed to to locate objects objects and can Self-organizing and naturally resilient to



offer time and complexity bounds on this operation; relatively low message overhead. 2.Need to maintain often complexoverlay structures, which can be difficult and costly to achieve, especially in highly dynamic environments.


node failure.

Probabilistic and hence cannot offer absolute guarantees on locating objects; prone to excessive messaging overhead which can affect scalability

8.Classify the modules available in file system.

9.Express in diagram the file attributes record structure.




10.What are the requirements of distributed file system (or) Give the design issues of Distributed file system?        

Transparency Concurrent file updates File replication Hardware and operating system heterogeneity Fault tolerance Consistency Security Efficiency

11.Explain the working of Andrew File system. The design of the Andrew File System (henceforth abbreviated AFS) reflects an intention to support information sharing on a large scale by minimizing clientserver communication. This is achieved by transferring whole files between server and and client computers and caching caching them at clients clients until the server receives a more up-to-date version. 12.What do you mean by cache consistency and how it is achieved in AFS? Cache consistency means ensuring that the cached copies of files are up -to-date when files may be updated by several clients.In AFS it achieved by Callback promise pro mise mechanism in which a token issued by the Vice server that is the custodian of the file, guaranteeingthat it will notify the Venus process when any other client modifies the file. Callback promises are stored with the cached files on the workstation disks and have two states: valid or cancelled. When a server performs a request to update a file it notifies all of the Venus processes to which it has issued callback promises by sending a callback to each a callback is a remote procedure call from a server to a Venus process. When the Venus process receives a callback, it sets the callback promise token for the relevant relevant file to cancelled. 13. What are venus and vice processes ? AFS is implemented as two software components that exist as UNIX processes called Viceand Viceand Venus. Vice Venus. Vice is the name given to the server software that runs as a user-level UNIX process in each server computer, and Venus is a user-level process that runs in each client computer andcorresponds andcorresponds to the client module in abstract model. Venus manages the cache,removing the least recently used files when a new file is acquired from a server to make the required space if the partition is full. The Vice servers accept requests only in terms of fid s. s. Venus




translates the pathnames supplied by clients into fid s using a step-by-step lookup to obtain the information from the file directories held in the Vice servers. 14.Develop How does AFS deal with the risk that callback messages may be lost? Callbacks must be renewed before an open if a time T (typically on the order of a few minutes) has elapsed since thefile was cached without communication from the server. This is to deal with possible communication failures, which can result in the loss of callback messages. 15.Define URI, URL and URN. Uniform Resource Identifiers (URIs) is used to identify resources on the Web, and other Internet resources such as electronic mailboxes Uniform Resource Locator (URL) is often used for URIs that provide location information andspecify the method for accessing the resource. URLs are efficient identifiers for accessing resources Uniform Resource Names (URNs) are URIs that are used as pure resource names rather than locators. The URI distinguishes that message from any other email message. But it does not provide the message’s address in any store, so a lo okup operation is needed to find it. 16.List out the goals of Global Naming Service(GNS)? To handle an essentially arbitrary number of names and to serve an arbitrary number of administrative organizations A long lifetime High availability Fault isolation Tolerance of mistrust 17.How will you make use of name space and DNS? A name space is the collection of all valid names recognized by a particular service. The DNS name space has a hierarchic structure: a domain name consists of one ormore strings str ings called name components or labels, labels , separated by the delimiter ‘.’. There isno delimiter at the beginning or end of a domain nam e, although the root of the DNS name space is sometimes referred to as ‘.’ for administrative purposes. 18.Formulate how caching helps a name service’s availability? 

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In DNS and other name services, client name resolution software and serversmaintain a cache of the results of previous name resolutions. Caching is key to a name service’s service’ s performance and assists in maintaining the availability of both the name service and other services in spite of name server crashes. Its role in enhancing response times by saving communication with name servers is clear. Caching can be used to eliminate high-level name servers – the root server, in particular – particular – from from the navigation path, allowing resolution to proceed despite some server failures.Caching by client name resolvers is widely applied in name services and is particularly successful because naming data are changed relatively rarely. 19.Define LDAP. Lightweight Directory Access Protocol (LDAP), in which a DUA accesses X.500 directory services directly over TCP/IP instead of the upper layers of the ISOprotocol stack. LDAP also simplifies the interface to X.500 in other ways: for example, it provides a relatively simple API and it replaces ASN.1 encoding with textual encoding. LDAP has been widely adopted, particularly for intranet directoryservices. It provides secure access to directory data through authentication. 20.Demonstrate the use of name cache? 

To increase the performance of name service

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To reduce overall system overhead

Unit-4 Part-A 1.Define clock screw and clock drift. The difference between the readings of any twoclocks is called their skew.A clock’s driftrate is the change in the offset (difference in reading) between the clock and a nominal perfect reference clock per unit of time measured by the reference clock. 2.Describe How will you synchronize physical clock? Physical clock can be synchronizedby any one of the two methods. It is necessary to synchronize theprocesses’ clocks, Ci, with an authoritative, external source of time. This is externalsynchronization. And if the clocks Ciare synchronized with one another to a known degree of accuracy, then the interval between two events occurring at different computers at their local clocks clocks are measured, even even though they are not necessarily synchronized to an external source of time. ti me. This is internal synchronization. 3.What is Network time protocol?




The Network Time Protocol (NTP0 defines an architecture for atime service and a protocol to distribute time information over the Internet. The aim of NTP is as follows: To provide a service enabling clients across the Internet to be synchronized accurately to UTC To provide a reliable service that can survive lengthy losses of connectivity To enable clients to resynchronize sufficiently frequently to offset the rates of drift found in most computers To provide protection against interference with the time service, whether malicious or accidental 4.Explain why is computer clock synchronization necessary Clock synchronization is important to maintain theconsistency of distributed data (the use of timestamps to serialize transactions), to check the authenticity of a request sent to a server and a nd to eliminate the processing of duplicate updates. 5.Define distributed mutual exclusion with its requirements. In a distributed environment if a collection ofprocesses share a resource or collection of resources, then distributed mutual exclusion is required to prevent interference and ensure consistency when accessing the resources. 6. Define global state. It is harder to collect the state of the collection of processes, There emerged an agree on a time at which each process would record its state – the the result would be an actual global state of the system. 7.Define cut and its types. A cut of the system’s execution is a subsetof its global history that is a union of prefixes of process histories: histories: 

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The set of events is called the frontier of the cut: There are two types of cuts : Consistent cut and inconsistent cut. A cut is said to be inconsistent if if it shows an effect without cause, that is receipt of a message M is included where as sending of M is not included in the cut. A cut is said to be consistent if it includes both the sending and receipt of M and also sending of M without including receipt of M also said to be consistent cut because receipt of M is not assured. 8.Show what is the use of transaction?




A transaction defines a sequence of server operations that is guarenteed by the server to atomic in the presence of multiple clients server crashes. The goal of transactions is to ensure that all of the objects are manged by a server remain in a consistent state when they are accessed by multiple transactions and in the presence of server crahses.Transactions deal with crash failures of process and omission failures in communication. 9.Formulate the ACID properties. ACID represent the properties of transactions: nothing. Atomicity : a transaction must be all or nothing. Consistency : a transaction takes the system from one consistent state to another consistent state. Isolation Durability 10.Illustrate what is concurrency control? Give its use. Each server manages a set of objects and is responsible for ensuring that they remainconsistent when accessed by concurrent transactions. Therefore, each server is responsible for applying concurrency control to its own objects. The members of a collection of servers of distributed transactions are jointly responsible for ensuring that they are performed in a serially s erially equivalent manner. 11.Show how will you make use of nested transaction? What are its rules? Nested transaction allow transactions to be composed of other transactions. The outermost transaction in a set of nested transaction is called the top-level transaction. Transactions other than the top-level transaction are called subtransaction. Rules of Nested Transaction: A transaction may commit or abort only after its child transactions have completed. When a sub transaction completes, it makes an independent decision either to commit provisionally or to abort. When a parent aborts, all of its sub transactions are aborted. When a subtransaction aborts, the parent can decide whether to abort or not. 12.Define deadlock and starvation state. A deadlock wouldinvolve two or more of the processes becoming stuck indefinitely while attempting to enter or exit the critical section. starvation: starvation : the indefinite postponement of entry for a process that has requested it.The absence of starvation is a fairness condition. Another fairness issue is the order in which processes enter the critical critical section. 13.What is multi version time stamp ordering? 

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In multi-version timestamp ordering a list of old committed versions as well as tentative versions is kept for each object. This list represents the history of the values of the object. object. The benefit of using multiple versions versions is that read operations that arrive too late need not be rejected. 14.Formulate the need for atomic commit protocol. The atomicity property of transactionsrequires that when a distributed transaction comes to an end, either all of its operationsare carried out or none of them. In the case of a distributed transaction, the client has requested operations at more than one server. A transaction comes to an end when the client requests that it be committed or aborted. A simple way to complete the transaction in an atomic manner is for the coordinator to communicate the commit or abort request to all of the participants in the transaction and to keep on repeating the request until all of them have acknowledged that they have carried it out. 15.Define the two phase commit protocol. The two-phase commit protocol is designed to allow any participant to abort itspart of a transaction. Due to the requirement for atomicity, if one part of a transaction is aborted, then the whole transaction must be aborted. In the first phase of the protocol, each participant votes for the transaction to be committed or aborted.In the second phase of the protocol, every participant in the transaction carries out the joint decision. If any one participant votes to abort, then the decision must be to abort the transaction. If all the participants vote to commit, then the decision is to commit the transaction. 16.What are the task of recovery manager. The tasks of a recovery manager are: • to save objects in permanent storage (in a recovery file) for committed transactions; • to restore the server’s objects obje cts after a crash; • to reorganize the recovery file to improve the performance perfor mance of recovery; • to reclaim storage space (in the recovery recover y file). 17.Analyze and list the need for transaction status and intentions list entries in a recovery file?




18.DefineLinearizability and sequential consistency. There are various correctness criteria forreplicated objects. The most strictly correct systems are linearizable, linearizable, and this property is called linearizability.A linearizability.A replicated shared object service is said to be linearizable if for any executionthere executionthere is some interleaving of the series of operations issued by all the clients that satisfiesthe following two criteria: The interleaved sequence of operations operations meets the specification of a (single)  correct copy of the objects. The order of operations in the interleaving is consistent with the real times at  which the operations occurred in the actual execution. 19. Define sequential consistency. A weaker correctness condition is sequential consistency, consistency , which captures an essential requirement concerning the order in which requests are processed without appealing to real time. A replicated shared object service is said to be sequentially consistent if for any execution there is some interleaving of the series of operations issued by all the clientsthat satisfies the following two criteria: operations meets the specification of a (single) • The interleaved sequence of operations correct copy of the objects. interleaving is consistent with the t he program order in • The order of operations in the interleaving which each individual client executed them. 20.Summarize on coda file system. Constant Data Availability(CODA) is distributed file system. The aim was to provide users with the benefits of a shared file repository but b ut to allow them to rely re ly entirely on local resources when the repository is partially or totally inaccessible. In addition to these aims, Coda retains the original goals of AFS with regard to scalability and the emulation of UNIX file semantics. 21. What are the requirements of Mutual Exclusion?




ME1: (safety) At most one process may execute execute in the critical section (CS) at a time. ME2: (liveness) Requests to enter and exit the critical section eventually succeed. ME3: (ordering) If one request to enter the CS happened-before another, then entry to the CS is granted in that order.

UNIT-V Two Mark questions 1. Define Process Migration and Process management. Process migration deals with the movement of a process from its current location to the processor to which it has been assigned. Process management involves the activities aimed at defining a process establishing responsibilities, evaluating process performance, identifying

and opportunities or improvement 2. Classify the desirable features of good process migration mechanism. 

Transparency



Minimal Interference



Minimal Residual Dependencies



Efficiency



Robustness

Communication between coprocesses of a job 3. What are the advantages of process migration? 



Reducing average response time of processes



Speeding up individual jobs



Gaining higher throughput



Utilizing resources effectively



Reducing network traffic



Improving system reliability



Improving system security




4. Define thread. A thread is the entity within a process that can be scheduled for execution. All threads of a process share its virtual address space and system resources.It is a basic unit of CPU utilization used for improving a system

performance through parallelism. 5. List the advantages of thread. 





Overheads involved in creating a new process is more than creating a new thread. Context switching switching between threads is cheaper cheaper than processes due to their same address space. Threads allow parallelism to be combined with sequential execution and blocking system calls.

Resource sharing can be achieved more efficiently and naturally between threads threads due to same address space. 6. What are the sub activities involved in process migration? 

Freezing the process on its source node and restarting it on its destination node. 

Transferring the process’s address space s pace from its source its source node to its destination node. 



Forwarding massages meant for the migrant process.

Handling communication between cooperating processes that have been separated as a result of process migration. migration. 7. Design how process migration be done in heterogeneous system? All the concerned data must be translated from the source CPU format to the destination CPU format before it can be executed on the destination node. 





A heterogeneous system having n CPU types must have n(n-1) pieces of translation software.

Handles problem of different data representations such as characters, integers and floating-point numbers. 8. List the models for organizing threads. 



Dispatcher-workers model



Team model



Pipeline model




9. Analyze how signal handling is done? 

Create a separate exception handler thread in each process.

Assign each thread its own private global variables for signaling conditions. 10.Explain 10. Explain how thread scheduling is classified? 



Priority assignment facility



Flexibility to vary quantum size dynamically



Handoff scheduling

Affinity scheduling 11.Distinguish 11. Distinguish static versus dynamic load balancing algorithm. 

Static load balancing Use only information about the average behavior of the system, syste m, ignoring the current state of the system.

Dynamic load balancing React to the system state that changes dynamically.

Simpler because no need to maintain and process system state information.

More complex algorithms.

Do not react to the current system state.

React to the current system state.

than

static

12.Rank 12. Rank the issues in designing the load balancing algorithm. 

Load estimation policy- to estimate workload of a node



Process transfer policy- decides to execute locally/remotely



 

State information exchange policy-Exchange state s tate information among nodes Location policy-determines the transfer location Priority assignment policy-determines the priority to execute processes

Migration limiting policy-total number number of times t imes a process can migrate 13.Assess 13. Assess on the issues in designing the load sharing approaches. 






Load sharing algorithms do not attempt to balance the average a verage workload on all the nodes of the system, rather they only attempt to ensure that no node is idle when a node is heavily loaded.

The priority assignment policies and migration limiting policies 14.Give 14. Give the techniques and methodologies for scheduling process of a distributed system. 



No a prior knowledge knowledge about the processes processes



Dynamic in nature



Quick decision making capability



Balanced system performance and scheduling overhead



Stability



Scalability



Fault tolerance

Fairness of service 15.What 15. What is Thread pool? Thread pool consists of a number of threads in a pool where they await work. This is slightly faster to service a request with an existing thread than create a new thread. This allows the number of threads in the application to be bound to to the size of the pool. pool. 16.Discuss 16. Discuss the goals achieved by task assignment approach. 



Minimization of IPC costs



Quick turnaround time for the complete process



A high degree of parallelism

Efficient usage of system resources 17.Point 17. Point out the priority assignment rules. 





Selfish -Local processes are given higher higher priority than remote processes. Altruistic- Remote processes are given higher priority than local processes.

Intermediate- The priority of processes depends on the number of local processes and the number of remote processes at the concerned node. 18.List 18. List out location policies? 



Threshold method




Shortestmethod



Biddingmethod



Pairingmethod 19.Discuss 19. Discuss the migration limiting policies. 



A decision about the total no. of times a process pr ocess should be allowed to migrate.

Two migration-limiting policies: o Uncontrolled o Controlled 20.Generalize 20. Generalize state information exchange policies. 



Periodic broadcast



Broadcast when state changes



On- demand exchange



Exchange by polling

Third year sixth semester CS6601 Distributed System 2 mark with answer

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