Distributed Systems
CS 15-440
Naming – 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
2
Recap: Three Classes of Naming
Flat naming
Structured naming
Attribute-based naming
3
Recap: Three Classes of Naming
Flat naming
Structured naming
Attribute-based naming
4
Recap: Name Spaces in Structured Naming
Structured names are organized into name spaces
Data stored in n1
n0
home
n2: “elke”
n3: “max”
n4: “steen”
n1
elke
n2
max
n3
keys
n5
“/keys”
steen
n4
Leaf node
twmrc
mbox
Directory node
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
6
Recap: Layers in Distributed Name Spaces
Distributed Name Spaces can be divided into three layers
Global
Layer
Administrational
Layer
Managerial
Layer
• Consists of high-level directory nodes
• Directory nodes are jointly managed by different administrations
• Contains mid-level directory nodes
• Directory nodes grouped together in such a way that each group is
managed by an administration
• Contains low-level directory nodes within a single administration
• The main issue is to efficiently map directory nodes to local name
servers
7
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
8
1. Iterative Name Resolution
In iterative name resolution, a client iteratively calls various
name servers to resolve a name
An example:
<a,b,c> = structured name in a sequence
#<a> = address of node with name “a”
Resolving the name “ftp.cs.vu.nl”
9
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
10
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:
<a,b,c> = structured name in a sequence
#<a> = address of node with name “a”
Resolving the name “ftp.cs.vu.nl”
11
Recursive Name Resolution (cont’d)
Approach
The 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)
12
Classes of Naming
Flat naming
Structured naming
Attribute-based naming
13
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 attributes
Users search for attributes, and a list of entities that match the
attributes are returned
14
Directory Service
Directory 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
15
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)
16
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.
17
Directory Information Tree in LDAP
All 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)")
18
Naming in Distributed Systems – Summary
Naming and name resolutions enable accessing
entities in a Distributed System
Three types of naming
Flat naming
e.g., Home-based approaches, Distributed Hash Table
Structured naming
Organizes names into name spaces
Name spaces can be distributed across multiple computers
Attribute-based naming
Entities are looked up using their attributes
19
Reference
http://en.wikipedia.org/wiki/Directory_service
20