Naming CSCI 6900/4900

Names & Naming System Names have unique importance –Resource sharing –Identifying entities –Location reference Name can be resolved to the entity it refers to Naming system resolves names Naming system in distributed systems can itself be distributed

Names, Entities & Addresses Name is a string of bits/characters used to refer to an entity Entity can resources/users/data/processes –Web pages, files, hosts, printers, network connections –Entities permit certain operations (reading/writing files) To operate we need to access the entity Access Point – A special kind of entity –Host of a server Name of access point is called its Address –Hosts IP address and port Address of access point is the address of the entity at that access point

More on Entities and Addresses Entities can have multiple access points –Multiple telephone numbers/ addresses –Replicated services Entities might change access points –Laptops get different IP addresses based on their location –New phone number/ when switching jobs Can we use the address as the name of an entity? –Difficulties in relocation/replication transparency Name that is independent of address – Location Independent Names

Identifiers An name with following properties –Identifier refers to at most one entity –Entity is referred to by at most one identifier –Identifier always refers to same entity (persistence) Facilitates unambiguous reference –Testing equality of identifiers suffices if entities being referred to are the same Addresses cannot be identifiers if they can be re- assigned –Example: Telephone numbers

Name Spaces Names are organized into name spaces Name space is represented as a directed graph with two types of nodes Leaf Node – Represents named entity –No outgoing edges –Stores information on the entity it is referring (address or state of the entity) Directory node – Can have multiple outgoing edges each with a name –Directory table – Contains for each outgoing edge

Name Spaces - Example A general naming graph with a single root node.

More on Name Spaces Path name: Sequence of edge labels leading from one node to another –N: Absolute path name –First node is the root Relative path name –First node can be arbitrary node Global name –Denotes same entity where ever it is used Local name –Interpretation is dependent upon where it is used

Organizing Name Spaces Tree-based approach –Strict hierarchical approach Directed acyclic graph –Most commonly used General graphs –Rarely used Single root Multiple root

Name Spaces in Unix File System The general organization of the UNIX file system implementation on a logical disk of contiguous disk blocks.

Name Resolution Process of looking-up an entity given its path name –Find the corresponding node Example: N: –Start at N –Proceed along the edges A crucial question: Where should the resolution begin? –Which node represents the root? –Unix example: Which i-node corresponds to the root?

Closure Mechanism Knowing how and where to start resolution mechanism – Closure Have a universally agreed upon root In Unix root directory is the first i-node –Actual byte offset is calculated by other values in superblock HOME variable is Unix – always refers to the user’s home directory

Linking and Mounting Aliases – Another name for same entity Two approaches –Multiple absolute path names Hard links in Unix –Represent entity by a leaf node, but the leaf node stores absolute path name Return the absolute path name instead of address Symbolic links in Unix