Distributed system in information technology

ths is distribued system

1. DISTRIBUTED SYSTEMS Distributed system: Name Services Module Code ITU 07318

2. Introduction Name service is a distinct service used by client processes to obtain attributes (IP address,Email address, Telephone numbers) when given their names. Hold addresses and other details of users, computers, network domains, services and remote objects. To communicate with an object we need to find the object, the address to the object, its access methods etc.

3. Introduction cont.. Name services provide clients with data about named objects in distributed systems; and the related concepts of directory and discovery services, which provide data about objects that satisfy a given description. We begin by examining the fundamental concepts of names and attributes

4. Names, Addresses and other Attributes Names In a distributed system, names are used to refer to a wide variety of resources such as computers, services, remote objects, and files as well as users. Names are used for identification as well as for describing attributes. Names facilitate communication and resource sharing e.g. processes, users etc.

5. Names, Addresses and other Attributes Addresses The object’s address is a value that identifies the location of the object rather than the object itself. But sometimes it is not adequate, when the object is relocated.

6. Names, Addresses and other Attributes Attributes An attribute is the value of a property associated with an object. I s a unique expression in a written format either by a code, by numbers or by the combination of both to distinguish variations from one another among a class of substances, items, or objects.

7. Identifiers For many purposes, names are preferable to identifiers. Because the binding of the named resource to a physical location is deferred and can be changed. Because they are more meaningful to users.

8. URIs,URLs,URNs Uniform Resource Identifier (URI): is a string of characters used to identify a name or a resource on the Internet. They come in two kinds: 1. Uniform Resource Locator (URL): Specifies where a known resource is available and the mechanism for retrieving it. Typed by the protocol field (http, ftp, nfs , etc.) Part of the name is service-specific Resources cannot be moved between domains

9. URIs,URLs,URNs 2. Uniform Resource Name (URN): names resource Format: urn: <namespace>: <name-within namespace> Examples a) urn:ISBN:021-61918-0 b) urn:inf.unibz.it:TR2007-5

10. Names, Addresses and other Attributes Any process that require to access a specific resource must posses a name or an identifier for it. Entities (Internet hosts, routers, file systems, services) are accessed using identifiers (numbers) IP Address File Descriptor Port number

11. Examples of Name Services File System maps file name to file DNS (Domain Name Service) maps domain names to IP addresses scalable, can handle change RMI registry binds remote objects to symbolic names X.500/LDAP directory service maps person’s name to email address, phone number

12. Name Services and DNS Name service W e have mentioned bindings before which are the associations between textual names and the atributes of the objects, for example computers, service, users and remote objects N ame service is a collections of sets of this kind of bindings

13. Name Services and DNS The major function of a name service is to look up attributes from a given name – name resolution, and other aspects related to the bindings. Name service is a simple technology used for single machine or in a small network. When put it in the interconnected networks area, the problem comes out that is a much larger name-mapping problem

14. Name Services and DNS With the invention of multi-domain name services and the development of the Global Name Service, it needs some requirement of handling an essentially arbitrary number of names and to serve an arbitrary number of administrative organizations; A good example is the Internet Domain Name System

15. Name Services and DNS Name Space A Name Space is the collection of all valid names recognized by a particular service. What kind of name is valid? A name can be looked up by the service, even though that name may prove not to correspond to any object – to be unbound. Name space requires a syntactic definition. For example, “...” is not acceptable as the DNS name of a computer.

16. Name Services and DNS Naming Domain A naming domain is a name space for which there exists a single overall administrative authority for assigning names within it. This authority is in overall control of which names may be bound within the domain.

17. Name Services and DNS In particular, domains in DNS are collections of domain names; syntactically, a domain’s name is the common suffix of the domain names within it, but it may not be distinguished from a computer name. What’s more, the administration of domains may be devolved to sub-domains.

18. Name Services and DNS Responsibility for a naming domain normally goes hand in hand with responsibility for managing and keeping up to date the corresponding part of the database stored in an authoritative name server and used by the name service. Naming data belonging to different naming domains are in general stored by distinct name servers managed by the corresponding authorities.

19. Name Resolution In general, Resolution is an iterative process whereby a name is repeatedly presented to naming context. To resolve a name, it is first presented to some initial naming context; resolution iterates as long as further contexts and derived names are output.

20. Name Resolution Name Servers and Navigation Any name server, such as DNS, that stores a very large database and is used by a large population will not store all of its naming information on a single server computer. Such a server will be a bottleneck and a critical point of failure.

21. Name Resolution Any heavily used name service should use replication to achieve high availability. Data belonging to a naming domain is usually stored by a local name server managed by the authority responsible for that domain But there are also name servers for the higher domains e.g. yahoo.com; ac.tz

22. Name Resolution The partitioning of data implies that the local name server cannot answer all enquiries without the help of other name servers. The process of locating naming data from among more than one name server in order to resolve a name is called Navigation

23. Name Resolution Iterative Navigation Multicast Navigation Recursive Navigation Non-Recursive Server-Controlled Navigation Recursive Server-Controlled Navigation

24. Iterative Navigation In this mode, a client presents the name to the local name server, which attempts to resolve it. If the local name server has the name, it returns the result immediately. If it does not, it will suggest another server that will be able to help.

25. Iterative Navigation Resolution proceeds at the new server, with further navigation as necessary until the name is located. Client 1 2 3 A client iteratively contacts name servers NS1–NS3 in order to resolve a name NS2 NS1 NS3 Name servers

26. Iterative Navigation Used in: DNS client presents entire name to servers, starting at the local server, NS1. If NS1 has the requested name, it is resolved, else NS1 suggests contacting NS2 (a Server for a domain that includes the requested name)

27. Multicast Navigation A client multicasts the name to be resolved and the required object type to the group of name servers. Only the server that holds the named attributes responds to the request. If the name proves to be unbound, then the request is greeted with silence.

28. Recursive Navigation Non-Recursive Server-Controlled Navigation Recursive Server-Controlled Navigation A name server NS1 communicates with other name servers on behalf of a client DNS offers recursive navigation as an option, but iterative is the standard technique. Recursive navigation must be used in domains that limit client access to their DNS information for security reasons.

29. Name Resolution - Recursive Navigation Non-recursive Server-controlled Navigation The client may choose any name server and this server communicates multicast or iteratively with its peers as we have mentioned above.

30. Name Resolution - Recursive Navigation 1 2 3 4 client NS2 NS1 NS3 Non-recursive server-controlled

31. Name Resolution - Recursive Navigation Recursive Server-controlled Navigation The client once more contacts a single server. If it does not store the name, the server contacts a peer storing a prefix of the name, which in turn attempts to resolve it. This procedure continues recursively until the name is resolved.

32. Name Resolution - Recursive Navigation Recursive server-controlled 1 2 3 5 4 client NS2 NS1 NS3

33. Caching In DNS and other name services, client name resolution software and servers maintain a cache of the results of previous name resolutions. When a client request a name lookup, the name resolution software consults its cache.

34. Caching If it holds a recent result from previous lookup for the name, it return it to the client; otherwise, it sets about finding it from a server. That server in turn may return data cached from other servers.

35. Caching Caching assists in maintaining availability of both name services and other services despite name server crashes. It facilitate response times by saving communication with name servers is clear. Caching can be used to eliminate high-level name servers-the root servers.

36. Thank you