Distributed Systems

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Distributed Systems
Distributed Systems
Lecture 12 Name Services 9. July, 2002
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Schedule of Today
Distributed Systems
Objectives Definitions Problems in DS
Types of Names Name Service Architecture
Examples
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Name, Address, and Path
Motivation
Name describes the object under consideration
Name
Address identifies the objects location
Address
Path
Path describes how to get to the object
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Objectives (1)
Objectives

• Identification
• Names specify objects unambiguously
• Objects can be
• users, files, database objects, variables
• communication links ...
• Naming systems of different application areas
  are usually disjoint
• Data Sharing
• Shared access to the same object (by different
  users)
• Exchange of names among users

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Objectives (2)
Objectives

• Indirection
• Binding of names to objects can change
  dynamically
• A name can identify multiple (e.g. replicated)
  objects
• An object can have multiple names
• Naming is a prerequisite for transparency
• Location transparency
• Migration transparency
• Replication transparency

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Objectives Example
Objectives
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Definitions (1)
Definitions
Alphabet set of characters (e.g.
A, B, C

• Name/Identifier
• Sequence of characters of an alphabet
• Generally limited in length
• (e.g. AA, AB, ,CC)

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Definitions (2)
Definitions

• Binding
• Assignment of a name to an object
• Types of binding
• Static binding
• Dynamic binding

• Name Space
• Set of all possible identifiers
• (e.g. AA, AB,AC,
• BA, BB, BC,
• CA, CB,CC)

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Definitions (3)
Definitions

• Context
• Mapping information
• Names are interpreted relatively to a context
• Mapping Information
• Maps identifiers to identifiers of the next
  lower level

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Definitions (4)
Definitions

• Resolution
• The process of mapping an identifier to an
  object
• (possible via multiple stages)

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Example Resolution (Stage 1)
Definitions
Compiler as the mapping function and
context generator
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Example Resolution (Stage 2)
Definitions
Linker
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Example Resolution (Stage 3)
Definitions
Address resolver
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Problems in Distributed Systems
Problems

• Management of global context information
• Name assignment and resolution may depend on
• the availability of other nodes
• ? Loss of autonomy
• Size of systems many nodes, users, files etc.
• ? Large number of names
• Heterogeneity

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Context Management
Problems

• Goal
• Names are interpreted in a global context
• ? On all nodes, the same name identifies the
  same object
• ? Names can be passed on in messages
• Variants
• Centralized context management
• Distributed context management
• Context management with replication

In what system type addresses can be used within
inter-node messages?
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Centralized Context Management
Problems
Properties Easy to implement - Single point of
failure - Performance bottleneck
Non partitioned name space
Centralized Name Server
Clients will request names from the centralized
name server
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Distributed Context Management
Problems
Properties No single point of failure Load
sharing between several name servers - Failure
of a name server prevents name generation and
resolution for its corresponding partition -
Far more complex implementation
Partitioned name space
Distributed name servers without replication
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Distributed Context Management with Replication
Problems
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Size of Distributed Systems
Problems

• More names, more mappings
• Systems tend to grow, i.e. number of nodes grows
• ? Requires dynamical extension of the name space
• ? Requires scalability of involved algorithms
• Administrative tasks are distributed among many
• nodes, i.e. they require additional overhead

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Heterogeneity
Problems

• Local naming schemes are heterogeneous
• Global naming scheme has to be uniform
• Additional mapping from global to local
• naming scheme

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Types of Names
Types of Names
Examples /node1/files/john/imagine
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Components of a Name Service Architecture
Types of Names

• Name agent
• Supplies interface between
• name service and client
• Cooperates with name servers
• in order to generate, resolve,
• names
• Caches the results
• Name server
• Manages context information and
• implements the mapping function
• Can sometimes cooperate with
• other name servers
• Supplies an interface for name agent

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Name Service Operations
Name Service Architecture

• Manipulation of context information
• ADD Create a reference Name ? Object
• DELETE Delete a reference
• MODIFY Change a reference
• Queries
• READ Resolve a name into an object
• SEARCH Find a name or object based on some
  attributes
• LIST List all names
• Administration
• Assignment of access privileges
• Authentication
• Extension of the name space

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Name Server Architecture
Name Service Architecture

• Name Resolution
• mapping of names to objects
• Caching
• of remote context information
• for efficient access
• Replicated data management
• management of replicated context information
• Communication
• between agents and name servers
• between name servers for cooperation purposes
• Database
• management of local context information

Name service operations
Name resolution
Caching
Replicated data management
Communication
Database
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Resolution of Structured Names
Name Service Architecture

• Resolution mechanism is dependent on the type of
  name
• Location-dependent names
• Name contains the location of the object it
  identifies
• Authority-dependent names
• Name contains the location of the name server
  being
• responsible for the resolution of the name (the
  authority)
• Location-independent names
• Name contains no location information

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Location-Dependent Names (1)
Name Service Architecture

• Structure of the name space
• Every node has its own name server
• Name server only manages the
• names of local objects
• Name structure NodeID.ObjectID
• Names are generated
• by the local name server
• through concatenation of the
• NodeID and a local unique ObjectID

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Location-Dependent Names (2)
Name Service Architecture

• Name resolution (of N.O)
• Name agent send a query to node N.
• Name server of N maps O
• to a local object
• Properties
• Nodes are highly autonomous
• Name authority can be found in
• a simple and efficient way
• - Objects can not migrate

Partitioned name space
NS
NS
O11
O21
O12
Node 1
Node 2
Node 3
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Authority-Dependent Names (1)
Name Service Architecture

• Structure of the name space
• Every node has a name server
• Name server manages the
• names of all locally-created objects
• Objects may migrate
• Name server must be aware of the
• locations of all migrated objets
• Name structure NodeID.ObjectID
• Names are generated
• by the local name server
• through concatenation of the
• NodeID and a local unique ObjectID

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Authority-Dependent Names (2)
Name Service Architecture

• Name resolution (of N.O)
• Name agent send a query to node N.
• Name server of N maps O
• to a local object
• Properties
• Name generation is simple
• Name authority can be found in
• a simple and efficient way
• Objects can migrate
• - Additional overhead for the
• monitoring of migrated objects
• - Only one (fixed) authority per object

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Location-independent Names (1)
Name Service Architecture

• Structure of the name space
• Hierarchically structured name space
• Hierarchy can resemble
• organizational structure
• system topology
• geographical distribution
• etc.

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Location-independent Names (2)
Name Service Architecture

• Relative name of an object
• Identifies a name uniquely in the
• context of its parent object
• Example
• uka/ira/schmitt (parent de)
• Absolute name of an object
• Globally unique identifier for an object
• Concatenation of the relative names
• of all predecessors
• Example
• /de/uka/ira/liedtke

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Location-independent Names (3)
Name Service Architecture

• Partitioning of name space
• Hierarchical name space is split
• up in zones
• A zone has at least one name server
• Name server manages the name
• space covered by the zone
• Replication can be achieved by
• having multiple name servers
• per zone

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Location-independent Names (4)
Name Service Architecture

• Every name server knows
• at least one server of the root zone
• the root zone of every child zone
• and at least one name server of it
• Name server manages the name
• Name resolution
• Agent asks an arbitrary name server N
• If N knows the name
• ? local resolution
• If N does not know the name
• ? Resolution starts with a root server
• ? and works its way down the
• server hierarchy until the name
• has been completely resolved

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Location-independent Names (5)
Name Service Architecture

• Name resolution
• is done by the authority
• responsible for the object
• Properties
• Objects can migrate
• Authority for an object can be changed
• Multiple authorities possible
• (replication)
• - Resolution may be expensive
• ( - Efficiency may be improved by caching)
• - Name generation may require
• communication

F
A
F
F
D
E
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Caching
Name Service Architecture

• Cache contains mapping information
• On receipt of a request for resolution,
• agent checks if the required information
• is in the cache
• Yes Resolution is done locally
• No Agent sends a query to the
• naming system its result is
• stored in the cache
• A cache entry is invalidated when
• after a resolution it is found to be stale

Naming System
Agent
Cache
Client
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Replication (1)
Name Service Architecture

• Goal
• High availability and faster access
• Pessimistic methods
• One-copy serialisability
• Limited availability if
• the network is partitioned
• Optimistic methods
• Weak consistency
• But Available even if
• the network is partitioned

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Replication (2)
Name Service Architecture

• Weak consistency is sufficient
• If naming information is changed
• infrequently
• Even if stale information is used,
• in many cases no harm will be done
• If use of the information will reveal
• that it is stale

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Examples Standards
Examples

• DNS - Domain Name System
• NIS - Network Information Service
• X.500
• LDAP - Lightweight Directory Protocol

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Domain Name System
Examples

• DNS distributed database used by TCP/IP
  applications
• to map between hostnames and IP
  addresses
• to provide electronic mail routing information
• Primary task
• Mapping from a symbolic name to the 32 bit
  Internet(IP) address
• mcculler.uni-karlsruhe.de ? 129
• General functionality
• Mapping from hierarchical names to objects

Paul Mockapetris (1984, standard in the Internet
since 1987)
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Domain Name System
Examples

• Name space
• manages names for computer, email-server ...
• Name structuring
• hierarchical name space
• pathnames start at the leave and end at the
  root, e.g. i30www.ira.uka.de
• The top level domain has fixed names
• (can be changed only by an official
  Internet-office)
• below top level domain you can have as many
  levels as you need

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DNS Top-Level Domain
Examples
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DNS Name Space
Examples

• Hierarchical, location-independent
• names
• Name space is split up into domains
• which are ordered in a tree-like fashion
• Each domain receives a domain name
• that is unique among its sibling nodes
• Absolute name of a domain is
• obtained by concatenating the
• relative names on the path from
• this domain to the root of the tree
• Example i30www.ira.uka.de

de
au
tu-dresden
uka
sydney
informatik
ira
i20www
i30www
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Example of a DNS Name Server Hierarchy
Examples
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DNS Information Model
Examples

• Tables with entries (resource records)
  consisting of
• Domain name
• Class (e.g. IN for Internet, ISO for ISO)
• Type
• Time-to-live (how long cached info will be
  valid)
• Value (depending on the type)
• Application-dependent classes can be defined
• for each class there exists a set of object
  types.
• For each domain name, multiple entries may exist
  that differ in
• class and/or type and/or value

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DNS Systemarchitecture
Examples
user
Client
AP
OS
service access protocol
name resolver
TCP/IP
TCP/IP
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Example
Examples
user
ftp
name resolver
If the name could be resolved
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Replication and Caching in DNS
Examples

• Replication
• for every root server there are at least two
  replicas
• primary/backup principle
• backup servers update their state
• periodically via the primary server
• Caching
• each name server implements cache

Further reading F. Halsall Data
Communications, Computer Networks and Open
System, Addison-Wesley 1992 D. Comer
Computernetzwerke und Internets, Prentice Hall
1997
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X 500
Examples

• CCITT and ISO standard (1988)
• Names
• List of tuples ltattribute value)
• Attributes
• country c
• organisation o
• organiational unit on
• surname sn
• ...
• telephone number telephone
• Example
• /cde/ouni-karlsruhe/ourz/snjuling/telephone
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