distributed systems principles and...
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واحد نجف آباد
Dr. Rastegari - Email: [email protected] - Tel: +98331-2291111-2488
Chapter 5
NAMING
DISTRIBUTED SYSTEMSPrinciples and Paradigms
Second Edition
ANDREW S. TANENBAUMMAARTEN VAN STEEN
Names, Identifiers, and Addresses
Names play a very important role in all computer systems. They are used to share resources, to uniquely identify entities, to refer to locations, and more.
Properties of a true identifier:• An identifier refers to at most one entity.• Each entity is referred to by at most one
identifier.• An identifier always refers to the same
entity2
Flat NamingIn many cases, identifiers are simply random bit strings, refer to as unstructured, or flat names.
It does not contain any information whatsoever on how to locate the access point of its associated entity. How flat names can be resolved, or, equivalently, how we can locate an entity when given only its identifier.Simple solutions for locating an entity, both applicable only to local-area networks
Broadcasting and MulticastingForwarding Pointers
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Forwarding Pointers (1)When an entity moves from A to B, it leaves behind in A, a reference to its new location at BFigure 5-1. The principle of forwarding pointers using (client stub, server stub) pairs.
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Forwarding Pointers (2)Figure 5-2. Redirecting a forwarding pointer by storing a shortcut in a client stub.
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Forwarding Pointers (3)Figure 5-2. Redirecting a forwarding pointer by storing a shortcut in a client stub.
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Home-Based Approaches
Figure 5-3. The principle of Mobile IP.7
Hierarchical Approaches (1)Figure 5-5. Hierarchical organization of a location service into domains, each having an associated directory node.
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Hierarchical Approaches …Figure 5-8. (a) An insert request is forwarded to the first node that knows about entity E.
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Hierarchical Approaches …
Figure 5-8. (b) A chain of forwarding pointers to the leaf node is created.
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Structured Naming
Naming systems generally support structured names that are composed from simple, human-readable names.
Not only file naming, but also host naming on the Internet follow this approach.
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Name Spaces
Figure 5-9. A general naming graph with a single root node.
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Name Space Distribution (1)
Figure 5-13. An example partitioning of the DNS name space, including Internet-accessible files, into three layers.
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Name Space Distribution (2)
Figure 5-14. A comparison between name servers for implementing nodes from a large-scale name space partitioned into a global layer, an administrational layer, and a managerial layer.
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Attribute-Based NamingFlat and structured names generally provide a unique and location-independent way of referring to entities.
In the most distributed systems an entity is described in terms of (attribute, value) pairs, generally referred to as attribute-based naming.An entity is assumed to have an associated collection of attributes. Each attribute says something about that entity. Naming system returns one or more entities that meet the user's description.
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Lightweight Directory Access Protocol
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DNS ImplementationFigure 5-21. Part of the description for the vu.nl domain which contains the cs.vu.nl domain.
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Hierarchical Implementations: LDAP
Figure 5-23. (b) Two directory entries having Host_Name as RDN.
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