Distributed Systems - Name Services - Thoai Nam

Nội dung text: Distributed Systems - Name Services - Thoai Nam

1. Teaching material based on Distributed Systems: Concepts and Design, Edition 3, Addison-Wesley 2001. Distributed Systems Course Name Services Copyright © George Coulouris, Jean Dollimore, Tim Kindberg 2001 CDK3 - Chapter 9: email: authors@cdk2.net This material is made 9.1 Introduction available for private study and for direct use by 9.2 Name services and the DNS individual teachers. It may not be included in any 9.3 Discovery services product or employed in any service without the written 9.6 Summary permission of the authors. Viewing: These slides must be viewed in slide show mode.
2. The role of names and 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 remote object reference – A name is human-readable value (usually a string) that can be resolved to an identifier or address  Internet domain name, file pathname, process number  E.g ./etc/passwd,  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  Resource names are resolved by name services – to give identifiers and other useful attributes 3 *
3. Composed naming domains used to access a resource from a URL Figure 9.1 URL DNS lookup Resource ID (IP number, port number, pathname) 138.37.88.61 8888 WebExamples/earth.html ARP lookup (Ethernet) Network address 2:60:8c:2:b0:5a file Socket Web server 5 *
4. Iterative navigation Figure 9.2 NS2 2 Name Client 1 NS1 servers 3 NS3 A client iteratively contacts name servers NS1–NS3 in order to resolve a name Reason for NFS iterative name resolution Used in: This is because the file service may encounter a symbolic link (i.e. an DNS: Client presents entire name to servers, starting at a local server, NS1. alias) when resolving a name. A symbolic link must be interpreted in If NS1 has the requested name, it is resolved, else NS1 suggests thecontacting client’s file NS2 system (a server name for aspace domain because that includes it may thepoint requested to a file name). in a directory stored at another server. The client computer must determine NFS: Client segments pathnames (into 'simple names') and presents them whichone atserver a time this to ais, server because together only withthe clientthe filehandle knows ofits the mount directory points. that (p.362.)contains the simple name. 7 *
5. DNS - The Internet Domain Name System  A distributed naming database  Name structure reflects administrative structure of the Internet  Rapidly resolves domain names to IP addresses – exploits caching heavily – typical query time ~100 milliseconds BasicScales DNS to algorithm millions for of namecomputers resolution (domain name -> IP number) • Look– partitioned for the name database in the local cache • Try– cachinga superior DNS server, which responds with: – another recommended DNS server  Resilient– the IP address to failure (which of may a server not be entirely up to date) – replication 9 *
6. DNS in typical operation a.root-servers.net Without caching (root) uk ns1.nic.uk purdue.edu (uk) yahoo.com ns.purdue.edu (purdue.edu) co.uk ac.uk ns0.ja.net (ac.uk) * .purdue.edu ic.ac.uk qmw.ac.uk IP: alpha.qmw.ac.uk alpha.qmw.ac.uk dns0.dcs.qmw.ac.uk dns0-doc.ic.ac.uk 2 client.ic.ac.uk (qmw.ac.uk) (dcs.qmw.ac.uk) (ic.ac.uk) IP:jeans-pc.dcs.qmw.ac.ukIP:ns0.ja.net dcs.qmw.ac.uk *.dcs.qmw.ac.uk *.ic.ac.uk *.qmw.ac.uk 14 jeans-pc.dcs.qmw.ac.uk ? 3 IP:dns0.dcs.qmw.ac.uk11 *
7. DNS resource records Figure 9.5 Record type Meaning Main contents A A computer address IP number NS An authoritative name server Domain name for server CNAME The canonical name for an alias Domain name for alias SOA Marks the start of data for a zone Parameters governing the zone WKS A well-known service description List of service names and protocols PTR Domain name pointer (reverse Domain name lookups) HINFO Host information Machine architecture and operating system MX Mail exchange List of pairs TXT Text string Arbitrary text 13 *
8. Directory and discovery services  Directory service:- 'yellow pages' for the resources in a network – Retrieves the set of names that satisfy a given description – e.g. X.500, LDAP, MS Active Directory Services  (DNS holds some descriptive data, but: • the data is very incomplete • DNS isn't organised to search it)  Discovery service:- a directory service that also: – is automatically updated as the network configuration changes – meets the needs of clients in spontaneous networks (Section 2.2.3) – discovers services required by a client (who may be mobile) within the current scope, for example, to find the most suitable printing service for image files after arriving at a hotel. – Examples of discovery services: Jini discovery service, the 'service location protocol', the 'simple service discovery protocol' (part of UPnP), the 'secure discovery service'. 15 *
9. Topics not covered  GNS case study (Section 9.4) – an early research project (1985) that developed solutions for the problems of:  large name spaces  restructuring the name space  X.500 and LDAP (Section 9.5) – a hierarchically-structured standard directory service designed for world-wide use – accommodates resource descriptions in a standard form and their retrieval for any resource (online or offline) – never fully deployed, but the standard forms the basis for LDAP, the Lightweight Directory Access Protocol, which is widely used  Trading services (see Section 17.3) – Directories of services with retrieval by attribute searching – Brokers negotiate the contract for the use of a service, including negotiation of attribute such as quality and quantity of service 18 *