Outline

1
Trading Service

• Outline
• What is trading service
• How to use trading service
• Printer Example

2
Motivation

• Locating objects in location transparent way
• Naming simple but may not be suitable when
• clients do not know server
• there are multiple servers to choose from
• Trading supports locating servers based on
  service functionality and quality
• Naming White pages
• Trading Yellow Pages

3
Trading Characteristics

• Trader operates as broker between client and
  server.
• Enables client to change perspective from who?
  to what?

Trader
1export
2 Query
3 invoke
Exporter
Importer

• Similar ideas in
• mortgage broker
• insurance broker
• stock brokerage

4
Trading Characteristics

• Common language between client and server
• Service types
• Qualities of service
• Server registers service with trader.
• Server defines assured quality of service
• Static QoS definition
• Dynamic QoS definition.

5
Trading Characteristics

• Clients ask trader for
• a service of a certain type
• at a certain level of quality
• Trader supports
• service matching
• service shopping

6
Example

• Hongkong Telecom video-on-demand server

Server
Video-on- demand provider
MGM
User
Warner
Trader
Independent
7
CORBA Trading Service

Server
Client Application
objectImpl
proxy
export() modify() withdraw()
Query()
CORBA Trading Service
lookup
register
link
proxy
Admin
8
The Trading Process

• Example Video-on-demand server

Client
Trader
MGMVoDS
WarnerVoDS
export( )
export( )
Query( )
modify( )
download( )
9
Trading Service Interface

• Service type repository
• Trader components
• Iterators
• Dynamic properties
• Lookup importers make queries
• Register exporters advertise new service offers
• Link admins federate traders
• Admin admins set policies
• Proxy legacy mechanisms for advertising services

10
Service Type Definition

• Service types define
• Functionality provided by a service and
• Qualities of Service (QoS) provision.
• Functionality defined by object type
• QoS defined based on properties, i.e.
• property name
• property type
• property value
• property mode
• mandatory/optional
• readonly/modifiable

11
Service Type Example

• typedef enum VGA,SVGA,XGA Resolution
• service video_on_demand
• interface VideoServer
• readonly mandatory property float fee
• readonly mandatory property Resolution res
• modifiable optional property float bandwidth

12
Service Type Hierarchy

• An object type might have several implementations
  with different QoS.
• Same object type might be used in different
  service types.
• Service type S is subtype of service S iff
• object type of S is identical or subtype of
  object type of S
• S has at least all properties defined for S
• Subtype relationship can be exploited by trader
  for service matching purposes

13
Constraint Definition

• Importer defines the desired qualities of service
  as part of the query
• Example
• feelt10 AND res gtSGA AND bandwidthgt256
• In a query, trader matches only those offers that
  meet the constraint

14
Trading Policies

• Depending on constraint and available services, a
  large set of offer might be returned by a query.
• Trading policies are used to restrict the size of
  the matched offers
• Specification of an upper limit
• Restriction on service replacements
• Restriction on modifiable properties (these might
  change between match making and service requests)

15
Federated Traders

• Scalability demands federation of traders
• A trader participating in a federation
• offers the services it knows about to other
  traders
• forwards queries it cannot satisfy to other
  traders
• Problems
• Non-termination of import
• Duplication of matched offers

16
Trading Graph
query.hop_count4
T1
def_follow_policyalways
max_hop_count5
max_hop_count1
T2
T3
query.hop_count0
Service Offer
T4
Trader Attribute
Link
17
CORBA Trading Service
Application Objects
CORBA facilities
Domain Interfaces
Object Request Broker
Object Trader
CORBAservices
18
CORBA Trading Interfaces
Admin
list_offers()

TraderComponents
Support Attributes
ImportAttributes
LinkAttributes
Register
Link
Lookup
export()
add_link()
query()
withdraw()
remove_link()
modify()
describe_link()
modify_link()
19
Defining Quality of Service

• typedef Istring PropertyName
• typedef sequenceltPropertyNamegt PropertyNameSeq
• typedef any PropertyValue
• struct Property
• PropertyName name
• PropertyValue value
• typedef sequenceltPropertygt PropertySeq
• enum HowManyProps none, some, all
• union SpecifiedProps switch (HowManyProps)
• case some PropertyNameSeq prop_names

20
Trader Interface for Exporters

• interface Register
• OfferId export(in Object reference,
• in ServiceTypeName type,
• in PropertySeq properties)
• raises(...)
• OfferId withdraw(in OfferId id)
• raises(...)
• void modify(in OfferId id,
• in PropertyNameSeq del_list,
• in PropertySeq modify_list)
• raises (...)

21
Trader Interface for Importers

• interface Lookup
• void query(in ServiceTypeName type,
• in Constraint const,
• in Preference pref,
• in PolicySeq policies,
• in SpecifiedProps desired_props,
• in unsigned long how_many,
• out OfferSeq offers,
• out OfferIterator offer_itr,
• out PolicyNameSeq Limits_applied)
• raises (...)

22
Service Type Repository

• Service type
• Name
• Interfact type
• Set of properties
• Name
• Typecode
• Read-only/modifiable
• Mandatory/optional

23
Service Type Repository

• Substitutability of service types
• The interface type of a derived service type may
  be a subtype of the interface type in the base
  service type
• The property set may be extended in a derived
  service type with new property name (and their
  associated type and mode specification)
• Inherited properties may be strengthened
• Optional ? mandatory
• Modifiable ? read-only

24
Service Type Repository

• creating service type

// operation signatures IncarnationNumber
add_type ( in CosTradingServiceTypeName
name, in Identifier if_name, in
PropStructSeq props, in ServiceTypeNameSeq
super_types ) raises ( CosTradingIllegalSer
viceType, ServiceTypeExists, InterfaceTypeMi
smatch, CosTradingIllegalPropertyName, Cos
TradingDuplicatePropertyName, ValueTypeRedefi
nition, CosTradingUnknownServiceType, Dupl
icateServiceTypeName )
25
Service Type Repository

• Removing service type

void remove_type ( in CosTradingServiceTypeName
name ) raises ( CosTradingIllegalServiceType
, CosTradingUnknownServiceType, HasSubTypes
)
26
Service Type Repository

• Obtaining service type info

ServiceTypeNameSeq list_types ( in
SpecifiedServiceTypes which_types ) TypeStruct
describe_type ( in CosTradingServiceTypeName
name ) raises ( CosTradingIllegalServiceType,
CosTradingUnknownServiceType ) TypeStruct
fully_describe_type ( in CosTradingServiceTyp
eName name ) raises ( CosTradingIllegalServic
eType, CosTradingUnknownServiceType )
27
Service Type Repository

• Masking/unmasking types

void mask_type ( in CosTradingServiceTypeName
name ) raises ( CosTradingIllegalServiceType
, CosTradingUnknownServiceType, AlreadyMaske
d ) void unmask_type ( in
CosTradingServiceTypeName name ) raises
( CosTradingIllegalServiceType, CosTrading
UnknownServiceType, NotMasked )
28
Service Type Repository

• properties

interface ServiceTypeRepository enum
PropertyMode PROP_NORMAL, PROP_READONLY, PROP_
MANDATORY, PROP_MANDATORY_READONLY struct
PropStruct CosTradingPropertyName
name CORBATypeCode value_type PropertyMode
mode
29
Printer Example

• Printer

interface Printer typedef string
filename exception PrinterOffLine
short code void print_file(in
filename fn) raises(PrinterOffLine)
short queue_length()
raises(PrinterOffLine) interface
TradingPrinter Printer,
CosTradingDynamicDynamicPropEval
30
Printer Example

• Printer property
• Building A Block, A Block A Block
• Floor 2, 3, 7
• Language PostScript, PostScript, PostScript
• Resolution 300, 600, 600
• Color black, black, 256color
• Queue_len -? PrinterObjectRef-gtqueue_length()
• Name 12ps, monster, rib

31
Printer Example

• Implementing the Printer Interface
• void print_file(string fn)
• Short queue_length()
• Any evalDP(string name, TypeCode returned_type,
  Any extra_info)

32
Printer Example

• Implementing the Printer Server
• ORB initialization
• Get trader reference
• Find service type repository
• Check for service type existence, if not exist,
  create it
• Create a prop struct ist with the property name
  for a printer service type
• Add the new service type
• Create service offer property seq to use for
  export
• Create printers (use command-line arguments) and
  making them available to ORB
• Update the template service offer to advertise
  them

33
Printer Example

• Implementing the Printer Client
• Arguments
• Name of the file to print
• Constraint expression for searching criteria
• Preference expression to order the printer
  service offers returned
• Implementation
• Check for arguments
• Find Lookup object
• Process the commend-line arguments
• Establish basic policies for a trader query
• Make the query
• The returned Printer object are tried in order
  until the file is successfully printed

34
Key Points

• Distributed objects can be located by naming and
  trading
• Naming binds externally known names to an object
  reference and supports name resolution to reveal
  the object reference
• Trading supports locating objects based on the
  functionality that they offer and the quality of
  service that they guarantee