Naming. Naming entities

Transcription

1 Naming Naming entities Locating mobile entities Removing unreferenced entities 1 Name: Just a string Naming entities used to denote entity in a system Identifier: Uniquely refers to an entity Each entity has only one identifier Identifier always refers to the same entity Address: Means of gaining access to entity 2

2 Name Spaces Nodes: leaf vs. directory Path: absolute vs. relative Graph: purely hierarchical vs. directed acyclic 3 Name resolution Looking up a name in a name space Closure mechanism: Selection of context from where to start Ex: /home/steen/mbox.twmrc

3 Name aliases Multiple names refer to same node in name space Term: link Hard link Symbolic link 5 Symbolic link 6

4 Solution 1: Merging name spaces Introduce naming scheme 7 Solution 2: Merging name spaces Introduce nodes that contain the name of a node in a foreign name space Mount point: Node in name space that refers to node in foreign name space Mounting point: Node in foreign name space 8

5 Merging name spaces 9 Merging name spaces Solution 3: Use only full pathnames home 10

6 Name Space Distribution (1) 11 Name Space Distribution (2) Item Global Administrational Managerial Geographical scale of network Worldwide Organization Department Total number of nodes Few Many Vast numbers Responsiveness to lookups Seconds Milliseconds Immediate Update propagation Lazy Immediate Immediate Number of replicas Many None or few None Is client-side caching applied? Yes Yes Sometimes 12

7 Iterative Name Resolution 13 Recursive Name Resolution 14

8 Implementation of Name Resolution Server for node Should resolve Looks up Passes to child Receives and caches Returns to requester cs <ftp> #<ftp> #<ftp> vu <cs,ftp> #<cs> <ftp> #<ftp> #<cs> #<cs, ftp> ni <vu,cs,ftp> #<vu> <cs,ftp> #<cs> #<cs,ftp> #<vu> #<vu,cs> #<vu,cs,ftp> root <ni,vu,cs,ftp> #<nl> <vu,cs,ftp> #<vu> #<vu,cs> #<vu,cs,ftp> #<nl> #<nl,vu> #<nl,vu,cs> #<nl,vu,cs,ftp> Recursive name resolution of <nl, vu, cs, ftp>. Name servers cache intermediate results for subsequent lookups. 15 Comparison of Name Resolution The comparison between recursive and iterative name resolution with respect to communication costs. 16

9 DNS Name Space Type of record SOA A MX SRV NS CNAME PTR HINFO TXT Associated entity Zone Host Domain Domain Zone Node Host Host Any kind Description Holds information on the represented zone Contains an IP address of the host this node represents Refers to a mail server to handle mail addressed to this node Refers to a server handling a specific service Refers to a name server that implements the represented zone Symbolic link with the primary name of the represented node Contains the canonical name of a host Holds information on the host this node represents Contains any entity-specific information considered useful 17 An excerpt from the DNS database for the zone cs.vu.nl. DNS Implementation (1) 18

10 DNS Implementation (2) Name cs.vu.nl solo.cs.vu.nl Record type NS A Record value solo.cs.vu.nl Part of the description for the vu.nl domain which contains the cs.vu.nl domain. 19 The X.500 Name Space (1) Attribute Country Locality Organization OrganizationalUnit CommonName Mail_Servers FTP_Server WWW_Server Abbr. C L L OU CN Value NL Amsterdam Vrije Universiteit Math. & Comp. Sc. Main server , , A simple example of a X.500 directory entry using X.500 naming conventions. 20

11 The X.500 Name Space (2) Part of the directory information tree. 21 Locating Mobile Entities Naming vs. locating objects Simple solutions Home-based approaches Hierarchical approaches 22

12 Naming versus Locating Entities a) Direct, single level mapping between names and addresses. b) T-level mapping using identities. 23 Broadcasting Simple solutions broadcast request to entity to return its current address Limited to LAN Forwarding pointers Each time entity moves, leave pointer to new address behind 24

13 Forwarding Pointers (1) Problem: chain length, and broken chains 25 Forwarding Pointers (2) Redirecting a forwarding pointer, by storing a shortcut in a proxy. 26

14 Home-Based Approaches 27 Hierarchical Approaches (1) Directory node contains record for every entity in its domain with: Address of entity, or Pointer(s) to directory node of subdomain(s) 28

15 Hierarchical Approaches (2) An example of storing information of an entity having two addresses in different leaf domains. 29 Hierarchical Approaches (3) Looking up a location in a hierarchically organized location service. 30

16 Hierarchical Approaches (4) a) An insert request is forwarded to the first node that knows about entity E. b) A chain of forwarding pointers to the leaf node is created. 31 Pointer Caches (1) Caching a reference to a directory node of the lowest-level domain in which an entity will reside most of the time. 32

17 Pointer Caches (2) A cache entry that needs to be invalidated because it returns a nonlocal address, while such an address is available. 33 Scalability Issues The scalability issues related to uniformly placing subnodes of a partitioned root node across the network covered by a location service. 34

18 Removing unreferenced entities Distributed garbage collection Reference counting Reference listing Scalability issues 35 The Problem of Unreferenced Objects 36

19 Reference Counting (1) The problem of maintaining a proper reference count in the presence of unreliable communication. 37 Reference Counting (2) a) Copying a reference to another process and incrementing the counter too late b) A solution. 38

20 Weighted Referencing Counting (1) a) The initial assignment of weights in weighted reference counting b) Weight assignment when creating a new reference. 39 Weighted Referencing Counting (2) c) Weight assignment when copying a reference. 40

21 Weighted Referencing Counting (3) Creating an indirection when the partial weight of a reference has reached Generation Referencing Counting Creating and copying a remote reference 42

22 Reference Listing Object maintains list of objects that refer to it Add/remove from list is idempotent (i.e. can be repeated) Does not require reliable communication Problem: Scalability Solution: lease with potential refresh 43 Identifying Unreachable Objects Reference counting does not avoid unreachable objects Solution: Tracing-based garbage collection Ex: make and sweep Phase I: Mark all objects that can be reached from root Phase II: sweep all unmarked objects 44

23 Tracing in Groups: initial marking 45 Tracing in Groups: local propagation 46

24 Tracing in Groups: final marking 47