Distributed SystemsCS 15-440Naming – Part II

Lecture 6, Sep 26, 2011

Majd F. Sakr, Vinay Kolar, Mohammad Hammoud

Today…

Last session: Naming: Flat-Names, Structured-Names

Today’s session: Naming: Resolution of Structured Names, Attribute-based Naming Introduction to Synchronization and Clock Synchronization

In the next three lectures, we will cover: Synchronization

Announcement: Project 1 due on Oct 3rd

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Recap: Three Classes of Naming

Flat naming

Structured naming

Attribute-based naming

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Recap: Three Classes of Naming

Flat naming

Structured naming

Attribute-based naming

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Recap: Name Spaces in Structured Naming

Structured names are organized into name spaces

n0

n1

n4

n5

n2 n3

Leaf node

Directory node

home keys

steenmax

elke

n2: “elke”n3: “max”

n4: “steen”

Data stored in n1

“/keys”

twmrc mbox

Name Spaces may be distributed across multiple computers (e.g., NFS)5

Recap: Distributed Name Spaces

In large Distributed Systems, it is essential to distribute name spaces over multiple name servers

Distribute nodes of the naming graph

Distribute name space management

Distribute name resolution mechanisms

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Recap: Layers in Distributed Name Spaces

Distributed Name Spaces can be divided into three layers

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Distributed Name Resolution

Distributed Name Resolution is responsible for mapping names to addresses in a system where:

Name servers are distributed among participating nodes

Each name server has a local name resolver

We will study two distributed name resolution algorithms:1. Iterative Name Resolution

2.Recursive Name Resolution

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1. Iterative Name Resolution

In iterative name resolution, a client iteratively calls various name servers to resolve a name

An example:

Resolving the name “ftp.cs.vu.nl”

<a,b,c> = structured name in a sequence#<a> = address of node with name “a”

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Iterative Name Resolution Algorithm

1. Client hands over the complete name to root name server

2. Root name server resolves the name as far as it can, and returns the result to the client• The root name server returns the address of the next-level name

server (say, NLNS) if address is not completely resolved

3. Client passes the unresolved part of the name to the NLNS

4. NLNS resolves the name as far as it can, and returns the result to the client (and probably its next-level name server)

5. The process continues till the full name is resolved

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2. Recursive Name Resolution

In recursive name resolution, a client calls the root name server to resolve a name

The name server may recursively call other name servers for name resolution

An example:

Resolving the name “ftp.cs.vu.nl”

<a,b,c> = structured name in a sequence#<a> = address of node with name “a”

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Recursive Name Resolution (cont’d)

ApproachThe client provides the name to the root name server

If the root name server is not able to resolve the complete name, it passes the unresolved part of the name to the next associated name server

The process continues till the name is fully resolved

The root name server returns the result to the client

Drawback:Recursive name resolution incurs large overhead at name servers (especially, at the high-level name servers)

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Classes of Naming

Flat naming

Structured naming

Attribute-based naming

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Attribute-based Naming

In many cases, it is convenient to look up entities by means of their attributes

Similar to traditional directory services (e.g., yellow pages)

e.g., Search an organization for users in a department, Search for printers in a network

Attribute-based naming supports looking up for entities by means of their attributes

Entities are described using a set of (attribute, value) pairs

Name resolution happens through attributesUsers search for attributes, and a list of entities that match the attributes are returned

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Directory ServiceDirectory Services are systems that support naming and name resolution for attribute-based names

Many organizations use directory services to locate users, and resources such as files and printers in a network

Entity representation in a directory service:Each entity is described using an entry

An entry is a set of(attribute, value) pairs

All entries are stored in a database called directory

Directory service enables searching for entities by specifying attributes

The user specifies a query by describing a set of attributes

The directory service returns the entities that match the query

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Directory Service (cont’d)

Challenge in Implementing Directory Services: The lookup operations on directory services are extremely expensive

They require to match the requested attribute values against the actual attribute values; this needs inspecting all the entities

Solution: Implement the basic directory service as a database

Organize the database similar to traditional structured naming system for fast look up

We will study a well-known directory service Light-weight Directory Access Protocol (LDAP)

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Light-weight Directory Access Protocol (LDAP)

LDAP Directory Service consists of a number of records called “directory entries”

Each directory entry is made of (attribute, value) pair

LDAP Standard specifies five attributes for each record

Directory Information Base (DIB) is a collection of all directory entries

Each record in a DIB is unique

Each record is represented by a unique

name e.g., /C=NL/O=Vrije Universiteit/OU=Comp. Sc.

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Directory Information Tree in LDAPAll the records in the DIB can be organized into a hierarchical tree called Directory Information Tree (DIT)

LDAP provides advanced search mechanisms based on attributes by traversing the DIT

Example syntax for searching all Main servers in Vrije Universiteit: search("&(C = NL) (O = Vrije Universiteit) (OU = *) (CN = Main server)")

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Naming in Distributed Systems – Summary

Naming and name resolutions enable accessing entities in a Distributed System

Three types of namingFlat naming

e.g., Home-based approaches, Distributed Hash Table

Structured namingOrganizes names into name spaces

Name spaces can be distributed across multiple computers

Attribute-based namingEntities are looked up using their attributes

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Reference

http://en.wikipedia.org/wiki/Directory_service

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