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The Jini Technology Lookup Service
How Does It Work
Services and clients find a lookup service using the discovery protocol
Services advertise themselves by registering with the lookup service
Clients query the lookup service to find the services they need
Peer-to peer communication between client and service after that
Discovery/Join and Lookup Protocols
Figure 1 Discovery
Discovery/Join and Lookup Protocols
Figure 2 Join
Discovery/Join and Lookup Protocols
Figure 3 Lookup
Discovery/Join and Lookup Protocols
Figure 4 Service Invocation
Discovery and Join
How does a client or service find lookup services
How does a service join a lookup serviceHow does a client know which ones to do
lookups in
Initial Discovery
Performed by both clients and servicesSend multicast request packet
Groups you are looking for Host and TCP port to use contact you
Waiting for lookup services to connect back
Download lookup service’s proxy object Register with or do lookup in
Lookup Service
Network service For finding other network services “Workgroup”-scale, not the entire Internet
Not a naming or directory serviceFocus is on types, not names
Yellow pages, not white pages
Both programmatic and end-user access
Lookup Service 101
Each service registers its proxy objectClient does lookup by interface type
Instance of semantics API uses java.lang.Class instances, not strings
Client gets proxy object back Needed classes are automatically downloaded
Proxy communicates with remote service
Service Type Matching
Can match on any type Interface or concrete class Including super interface or superclass Can get subinterface or subclass back
Java.lang.Object match everythingCan match using multiple types
Service Type Matching
“I want a combination TV/VCR” org.Ce.TV AND org.Ce.VCR
Yellow pages Automatic placement in all categories
Base class and all interfaces All superclasses and superinterfaces
Attributes
Service can also register with attributesAttributes allow further discrimination
between services of the same typeLookup support matching on
Just service types Just attributes Combinations
Example Generic Attributes
NameLocationManufacturerModel
Example Printer Attributes (1)
Monochrome or colorResolutionSpeedPaper size
Attribute Characteristics
Some are defined by service itself e.g., manufacturer, model, resolution
Some are defined by administrator e.g., Name, location
Attributes can have multiple values
Example Printer Attributes (2)
Printer supports these combinations Monochrome, 600 dpi, 10 ppm Monochrome, 300 dpi, 15 ppm Color, 300 dpi, 5 ppm
For each combination, printer supports Letter Legal Envelope
Attribute Representation
Each attribute is a public field of a class Attribute name = field name Attribute type = field type
Each class defines an attribute set Co-dependent attributes in the same class Orthogonal attributes in different classes
Each service has set of attribute sets Instances of multiple classes Multiple instances of the same class
Example Printer Attribute (3)
PrintEngine{color: false, dpi: 600, ppm: 10}
PrintEngine{color: false, dpi: 300, ppm: 15}
PrintEngine{color: false, dpi: 300, ppm: 5}
PaperSize{size: PaperType.Letter}
PaperSize{size: PaperType.Legal}
PaperSize{size: PaperType.Envelope}
Location{floor: “3”, room: “1234”, building: “A1”}
Name{name: “rainbow”}
Attribute Set Matching
Match on attribute set class Including superclasses
Match on attribute values (fields) Exact value match only
java.rmi.MarshalledObject.equals
Template is just an attribute set instance Null field = match any value (wildcard)
Matching Examples
template PrintEngine{color: false, dpi: null,
ppm: null}
matches PrintEngine{color: false, dpi: 600, ppm: 10}
and PrintEngine{color: false, dpi: 300, ppm: 15}
Matching Examples
template PrintAttrs{}
matches PrintEngine{color: true, dpi: 300,
ppm: 5}
and PageSize{size: Legal}
Service Registration
Service registers by providing: Service id
Every service has a universally unique id Lookup service will assign one if requested
Proxy object Zero or more attribute sets
Service Registration
public class ServiceItem implements
java.io.Serializable {
public ServiceID serviceID;
public Object service;
public Entry [ ] attributeSets;
…
}
Service Template
Service template contains: Optional service id
Null is wildcard = match any
Zero or more service types Zero or more attribute set templates
Null fields are wildcards = match any
Service Template
public class ServiceTemplate implements
java.io.Serializable {
public ServiceID serviceID;
public Class [ ] serviceTypes;
public Entry [ ] attributeSetTemplates;
…
}
Template Matching
Service registration matches template if: Service id matches, and Proxy is instance of each service type, and For each attribute set template:
Service has a matching attribute set
Example Queries
Any service named “rainbow”Any Printer named “rainbow” in A1Any color envelope PrinterAll services on floor 3
Complex Queries
“Between 300 and 600 dpi”“On floor 1, 3, or 7”“Letter or Legal”“Printer or Copier”
Complex Queries
Technique 1 Change ranges and ors to wildcards Do single template-based lookup Then do finer-grained match locally
Technique 2 Query for actual values in ranges and ors Do multiple lookups
Attribute Modification
Can only be done by service itself Allows service to control policy
Internal state changes
Administrative interfaces
Service is responsible for persistent storage
Add, modify, delete, replace allAttributes should change infrequently
Primary search keys Query service directly for very dynamic state
Lookup Groups
Each lookup service is configured with s set of group names Is in (is a member of) each group
Each service is configured with a set of group names Registers with all lookup services in any of those
groupsEach client is configured with a set of group
names Uses lookup service in any of those groups
Summary
Service perspective Register with desired attributes Keep renewing registration lease Add, modify, delete attributes as desired
Client perspective Lookup any 1 of
Get just proxy object
Lookup to N of Get proxies attributes, and service IDS
