naming csci 4780/6780. dht-based approaches lookup functionality distributed among multiple nodes...

Naming

CSCI 4780/6780

DHT-based Approaches• Lookup functionality distributed among multiple

nodes

• Chord – m-bit identifier space (IDs range from 0 to 2m-1)

• Lookup information on k will be maintained by succ(k)

• Simple implementation: Node q keeps track of succ(q+1)

• Finger table for scalable look up• q keeps track of succ(q+2i-1) for 0 < i < m

Distributed Hash Table

Hierarchical Approaches• Hierarchy of domains

• Leaf domain – lowest level of the hierarchy

• Location record for each entity

• Leaf domain node stores the actual location of the entity

• Upper level nodes store references to the children that contain the entity

• Multiple addresses if entity is replicated

Hierarchical Location Service

Location Service for Replicated Entities

Lookup in Hierarchical Location Service

Insertion in Hierarchical Lookup Service

Insertion (Contd.)

Structured Naming• 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 <edge label, node ID> 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:<label-1, label-2 …., label-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:<label-1, label-2 …. Label-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 in Unix – always refers to the user’s home directory