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

Naming Entities An entity can be anything An entity can be operated on To operate on an entity we need an access point An access point in a DS is called address An address is a special type of name A name for an entity independent from its address is called location independent A true identifier is a special type of name that uniquely identifies an entity having the 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

Name Spaces Names in a DS are organized in a name space Root node A naming space can be represented as a graph with leaf 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

Name Space UNIX – directory node file directory leaf node file The general organization of the UNIX file system implementation on a logical disk of contiguous disk blocks.

Linking and Mounting (1) The concept of a symbolic link explained in a naming graph within a single name space.

Linking and Mounting (2) in distributed file system (NFS) Name space A Name space B Mounting remote name spaces through a specific process protocol.

Name Space Distribution (1) An example partitioning of the DNS name space, including Internet-accessible files, into three layers.

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 Number of replicas None or few None Is client-side caching applied? Yes Sometimes A comparison between name servers for implementing nodes from a large-scale name space partitioned into a global layer, as an administrational layer, and a managerial layer.

Implementation of Name Resolution The principle of iterative name resolution.

Implementation of Name Resolution The principle of recursive name resolution.

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

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

The principle of forwarding pointers using (proxy, skeleton) pairs. 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.

Home-Based Approaches for mobile entities in large scale networks 1 0,3 2bis 2 4 The principle of Mobile IP (2 tiered case)

Hierarchical Approaches Hierarchical organization of a location service into domains, each having an associated directory node. Each entity in a domain is rapresented by a location record in the directory node. The root node has a location record for each entity.

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.

Hierarchical Approaches Looking up a location in a hierarchically organized location service.

Pointer Caches caching in a hierarchical location service supporting mobile entities is not effective pointer caching let the lookup start at the directory of the smallest domainin 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.

Scalability Issues hierarchical location services have bottlenecks at root directory partitioning high level nodes The scalability issues is related to uniformly placing subnodes of a partitioned root node across the network covered by a location service.

The Problem of Unreferenced Objects A graph representing objects containing references to each other Entities that are not directly or indirectly referenced by root nodes have to be removed.

Removing unreferenced entities Reference counting Reference listing Tracing-based garbage collection In a distributed system garbage collection requires network communication