Transparency In Distributed Systems Hiremath,Naveen

OUTLINE ➲ What is a Distributed System? ➲ Why D.S.? ➲ Design Issues of D.S. ➲ Transparency in a D.S. ➲ Transparency Goals ➲ Degree of Transparency ➲ Case Study

What is a Distributed System ? ➲ A distributed system is a collection of autonomous computers linked by a computer network that appear to the users of the system as a single computer

An Example Local Area Network Workstations Servers WAN Gateway

What is a Distributed System ? (Alternate take) “A distributed system is a system in which I can’t do my work because some computer that I’ve never even heard of has failed.” Leslie Lamport, Microsoft Research (ex DEC)

Why Distributed System ? ➲ Allows collaboration,information exchange irrespective of geographical location(banking,reservation systems) ➲ Doing tasks faster by doing them in parallel ➲ Avoiding a single point of failure

Continued... ➲ Incremental Growth ➲ Facilitates human to human communication ➲ Economics[J. Wein]

Disadvantages of Distributed Systems ➲ Difficulties of developing distributed software ➲ Networking Problems ➲ Security problems[B. Karp, S. Hailes]

Design Issues of Distributed Systems ➲ Heterogeneity ➲ Openness ➲ Security ➲ Reliability and fault tolerance ➲ Scalability ➲ Concurrency ➲ Achieving Transparency[A. S. Tanenbaum]

Transparency In a Distributed System ➲ Definition: Concealment from the user and the application programmer of the separation of components in a distributed system, so that the system is perceived as a whole than rather as a collection of independent components.

Transparency Goals ➲ How to achieve the single system image? ➲ How to “fool” everyone into thinking that the collection of machines is a “simple” computer ?

Goals Contd... ➲ Access Transparency Local and remote resources are accessed using identical operations ➲ Location Transparency Users cannot tell where hardware and software resources(files,CPU's) are located; the name of the resource shouldn't encode the location of the resource

Goals Contd... ➲ Migration(Mobility) Transparency Resources should be free to move from one location to another without having their names changed ➲ Failure Transparency Applications should be able to complete their task despite failures occurring in certain parts of the system

Goals Contd... ➲ Replication Transparency The system is free to make additional copies of files and other resources(for purpose of performance and/or reliability), without the users noticing. Example: several copies of a file; at a certain request that copy is accessed which is closest to the client

Goals Contd... ➲ Concurrency Transparency The users will not notice the existence of other users in the system(even if they access the same resources) ➲ Performance Transparency Load variation should not lead to performance degradation. This could be achieved by automatic reconfiguration as response to changes of the load

Goals Contd... ➲ Scaling(Size) Transparency Can expand in scale(incremental growth) without change to system's structure or application algorithms ➲ Persistence Transparency Hide whether a (software)resource is in memory or on disk[R. Chow]

Goals Contd... ➲ Parallelism Transparency This permits parallel activities without users knowing how,where, and when these activities are carried out by the systems ➲ Revision Transparency This refers to the vertical growth of systems as opposed to the horizontal growth as in scalable transparency. Revision of softwares not visible to users [B. Karp,2006]

Degree of Transparency ➲ Distribution transparency is generally preferable, but not always a good idea: ● It is undesirable to hide the location of the printer from its users

CONTD... ● Need for trade-off between a high degree of transparency and the performance of a system ● It is impossible to hide the fact that Mother Nature will not allow it to send a message from one process in Atlanta to the other in Beijing in less than approximately 35 ms

Case Study:Internet Naming Service ➲ How name things and find things in a distributed system? ➲ How to build one? Lets evaluate this with respect to Transparency

Role of Names & Name Services ➲ Resources are accessed using identifier or reference ● An identifier can be stored in variables and retrieved from tables quickly ● Identifier includes or can be transformed to an address for an object e.g. NFS file handle,CORBA object reference

CASE STUDY CONTD... ● A name is a human-readable value(usually a string) which can be resolved to an identifier or an address ● Internet domain name, file path name ● e.g. /etc/passwdwww.gsu.edu

CASE STUDY CONTD... ➲ Names are preferable to identifiers because ● The binding of the named resource to a physical location is deferred and can be changed ● They are more meaningful to users

Resource access using a URL gosolar/index.html Web Server Network address Socket 2:37:6a:1:d0:4a file Web Server ARP lookup DNS lookup Resource ID(IP number,port number,pathname)

CONTD... ➲ DNS Lookup can be ● Iterative NS1 NS2 NS3 Client Name Servers A client iteratively contacts name servers NS1-NS3 in order to resolve a name

CONTD... ● Recursive server-controlled Client NS 2 N S3 NS

References [1]A. S. Tanenbaum, “Distributed Operating Systems”,Prentice Hall, pp [2]R. Chow,T. Johnson, “Distributed Operating Systems & Algorithms”, Addison Weley, pp [3]J. Wein, “Parallel & Distributed Systems” [4]B. Karp, “RPC & Transparency”,UCL Computer Science,2006 [5]Y. Lu,”Distributed Operating Systems”,UNL [6]J. Holliday,”Distributed Computing”,SCU [7]B. Karp, S. Hailes,”Distributed Systems & Security:An Introduction,UCL Computer Science,2006

References CONTD... [8]P. Eles, “Distributed Systems”, IDA, Linkopings University [9]Advanced Distributed Systems,School of Computing, Napier University [10]P. Boulet,”Distributed Systems,Fundamental Concepts”,TIIR,2006