Naming A name in a distributed system is a string of bits or characters used to refer to an entity . To resolve na

1. NamingA name in a distributed system is a string of bits or characters used to refer to an entity.To resolve names a naming system is needed.

2. Naming Entities • An entity can be anything • An entity can be operated on • To operate on an entity we need anaccess point • An access point in a DS is calledaddress • An address is a special type of name • A name for an entity independent from its address is calledlocation independent • Atrue identifieris a special type of name that uniquely identifies an entity and has the following properties: • - 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 entity • Human friendly name is another type of name

3. Name SpacesNames in a DS are organized in a name space Rootnode Directory table • A naming space can be represented as a graph with leafnodes (containing entity information) and directory nodes. • Absolute and relative path names are related to a directory node • A global name denotes the same entity in the system • A local name depends on where the name is being used

4. Name ResolutionLinking and Mounting Name resolution can take place only if howand where to start is known (a.k.a. closure mechanism) An alias is another name for the same entity (multiple absolute reference or symbolic link) • The concept of a symbolic link explained in a naming graphwithin a single name space.

5. Linking and Mounting Name space B Name space A Protocol Server Mounting point Mounting point Mount point • Mountingremote name spaces through a specific process protocol ( i.e. in NFS) to merge different mane spaces

6. Linking and Mounting A different approach to merge name spaces (with scalability problems) Mapping table New root node • Organization of the DEC Global Name Service • Names in GNS always include the id of the node from where resolution should start

7. Name Space Distribution (1) Rarely changed Relatively stable • An example: partitioning of the DNS name space, including Internet-accessible files, into three layers. • A zone is a part of the name space implemented by a separate name server

8. Name Space Distribution (2) • 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. • Replication and caching can be difficult to implement maintaining consistency

9. Implementation of Name Resolution • Ex: resolution of root:<nl, vu, cs, ftp, pub, globe, index.txt> • The principle of iterative name resolution.

10. Implementation of Name Resolution • Ex: resolution of root:<nl ,vu, cs, ftp, pub, globe, index.txt> • The principle of recursive name resolution.

11. Implementation of Name Resolution • The comparison between recursive and iterative name resolution

12. Naming versus Locating Entities usually nodes in managerial layers vary very often… • Direct, single level mapping between names and addresses. • Two-level mapping using identifiers. • Simple example in LAN : ARP via broadcasting

13. Forwarding Pointers when an entity moves, it leaves a reference to its new location Exit item Entry items • The principle of forwarding pointers using (proxy, skeleton) pairs or SSP (stub and scion pair) • Problems when the chain is broken

14. Forwarding Pointers • Redirecting a forwarding pointer, by storing a shortcut in a proxy. • The current location is sent back to the caller

15. Home-Based Approachesfor mobile entities in large scale networksa home location keeps track of the current location of an entity 1 0,3 2bis 4 2 • The principle of Mobile IP (2 tiered case) • Each mobile host has a fixed IP add., communications directed to the host home agent • IP address is used ad an identifier

16. Hierarchical Approach Mechanism • Hierarchical organization of a location service into domains, each having an associated directory node. • Each entity in a domain is represented by a location record in the directory node. • The root node has a location record for each entity, storing a pointer to the directory node where the entity is.

17. Hierarchical Approaches • An entity may have multiple addresses. • If an entity has an address in different leaf domains D1 and D2, then the directory node of the smallest domain containing D1 and D2 will have 2 pointers.

18. Hierarchical Approaches pointer address client • Looking up a location in a hierarchically organized location service.

19. Hierarchical Approaches Update operation in a hierarchical location service (top-down) • An insert request is forwarded to the first node that knows about entity E (replica). • A chain of forwarding pointers to the leaf node is created.

20. Pointer CachesCaching in a hierarchical location service supporting mobile entities is not effectiveCaching is effective only if data rarely change, or entity moves within the domainPointer caching let the lookup start at the directory of the smallest domain in which a mobile entity moves regularly • Caching a reference to a directory node of the lowest-level domain in which an entity will reside most of the time.

21. Scalability Issueshierarchical location services have bottlenecks at root directory partitioning high level nodes • The scalability issues isrelated to uniformly placing sub-nodes of a partitioned root node across the network covered by a location service. Choice of partitions is critical

22. The Problem of Unreferenced ObjectsWhen an entity can no longer be accessed, it has to be removed • A graph representing objects containing references to each other • Entities that are notdirectly or indirectlyreferenced by root nodes have to be removed.

23. Removing unreferenced entities • Reference counting • reference to the object are counted, dynamically • Reference listing • a skeleton maintains the explicit listing of the proxies that point to it • Tracing-based garbage collection • all entities in a DS are traced, checking which entity can be reached • from the root set. • In a distributed systemgarbage collection requires network communication