naming csci 4780/6780. dht-based approaches lookup functionality distributed among multiple nodes...
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Naming
CSCI 4780/6780
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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
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Distributed Hash Table
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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
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Hierarchical Location Service
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Location Service for Replicated Entities
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Lookup in Hierarchical Location Service
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Insertion in Hierarchical Lookup Service
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Insertion (Contd.)
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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
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Name Spaces - Example
A general naming graph with a single root node.
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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
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Organizing Name Spaces• Tree-based approach
– Strict hierarchical approach
• Directed acyclic graph– Most commonly used
• General graphs– Rarely used
• Single root
• Multiple root
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Name Spaces in Unix File System
The general organization of the UNIX file system implementation on a logical disk of contiguous disk blocks.
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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?
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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