Advanced Communication Among Distributed Objects

1
Advanced Communication Among Distributed Objects

• Outline
• Request synchronization
• Request multiplicity
• Request reliability

2
Request Synchronization

• Request synchronization
• Synchronous
• Wait for the results
• Oneway
• Do not expect any results or exceptions
• Request returns control to client as soon as the
  middleware accepts the request
• Server and Client execute concurrently
• Deferred synchronous
• Dynamic
• Explicit request object
• Invoke more than one concurrent operations with
  single thread
• Asynchronous
• Callback from the server to client
• Non-synchronous requests can be achieved by
  synchronous requests and multi-threading
• COBRA provide direct support for all 4 different
  requests

3
Oneway Request

• Using thread
• Spawn a child thread to make synchronous request
• Terminate the child thread once the request is
  finished
• Performance penalty

4
Oneway Request
Class PrintSquas static void main(string
args) Team team Date date // initialization
.. OnewayReq a new OnewayReq (team,
date) a.start() // continue Class
OnewayReq extends Thread Team team Date
date OnewayReq (Team t, Date d) team t date
d public void run() team.print(date) //
call remote method
5
Oneway Request

• Using thread
• Spawn a child thread to make synchronous request
• Terminate the child thread once the request is
  finished
• Performance penalty
• CORBA
• Static and dynamic invocation support oneway
  request
• IDL defines oneway operations
• Precondition
• No out, or inout parameter
• Viod return type
• No type-specific exceptions
• Static invocation, Server decides this
  synchronization scheme
• Dynamic invocation, client can choose oneway
  request (review the Request object, send
  operation.)

6
Oneway Request

• CORBA
• IDL defines oneway operations
• Static invocation, Server decides this
  synchronization scheme
• Dynamic invocation, client can choose oneway
  request

interface Team oneway void mail_timetable (in
Date date)
Server
Client
r create_request()
rRequest
send_oneway
Op()
delete()
7
Oneway Request

• Comparison
• Thread
• Less efficient (Create/delete thread)
• IDL (oneway)
• Less flexible
• Type safe
• DII
• Flexible using different flag with send
• Type unsafe
• Less efficient (Create/delete request object)

8
Deferred Request

• Using thread
• Spawn a child thread to make synchronous request
• Wait for termination of the child request thread
  
• Obtain the result
• Performance penalty

9
Deferred Request --- Using Thread

• class PrintSquad
• public void print (Team team, Date date)
• DefsyncReqPrintSquad a new
  DefSyncReqPrintSquad (team, date)
• // do something here.
• a.join(this) // wait for request thread to die
• System.out.println(a.getResult())
• class DefSyncReqPrintSquad extends Thread
• DefsyncReqPrintSquad (Team t, Date d)
• Public String getResult()
• Public void run()
• strings
• s team.asString(date) // call remote method
  and die

10
Deferred Request

• CORBA
• Only dynamic invocation interface support
  deferred synchronous request
• Cannot be issued using client stub
• get_response no-wait returns control back to
  client with indication if the result is ready

Server
Client
r create_request()
rRequest
send_defeered()
Op()
get_response()
delete()
11
Asynchronous Request

• Using thread
• Similar to deferred synchronous request
• Request thread invokes a call back operation
• Request thread join the main thread

12
Asynchronous Request --- Using Thread

• Interface Callback
• public void result (String s)
• class PrintSquad implements Callback
• public void print (Team team, Date date)
• AsyncReqPrintSquad a new AsyncReqPrintSquad
  (team, date, this)
• a.start()
• // continue to do some work here
• public void result (String s) // callback
• System.out.println(s)
• class AsyncReqPrintSquad extends Thread
• Team team
• Date date
• Callback call

13
Asynchronous Request

• CORBA

Server
Client
r create_request()
rRequest
send()
Op()
Callback ()
14
Asynchronous Request

• Using Message Queues

Request Queue
enter
remove
Client
Server
remove
enter
Reply Queue
15
Request Multiplicity

• Request multiplicity
• Unicast (from one client to one server)
• Group request
• same operations from multiple server objects
• Group requests are anonymous
• Group composition is managed by a channel
• Group membership is unknown to the request
  producer
• Group members are request consumers
• Multiple request
• Different operations from different server
  objects that are known to the client
• Requested operations are independent with each
  other with no precedence relation
• Client can collect results as they become
  available

16
Request Multiplicity

• Implementing group request
• Channel (Example 7.5)
• CORBA Event channel (Example 7.6)
• Implementing multiple request
• Thread
• Tuple space (Example 7.8)
• Out
• In (blocking call)
• Rd (blocking call)
• Performance and development overhead
• DII (Example 7.9)

17
Request Reliability

• Unicast Request Reliability
• Exactly-once
• Highest reliability
• Depends on middleware, fault-tolerant, hardware,
  os platforms
• Expensive
• Atomic
• Avoid side-effects when a failure occurs
• Transactions
• At-least-once
• Middleware guarantees the execution
• Multiple execution of a single request due to
  reply lose
• If request modify servers state, this semantics
  is not desirable
• At-most-once
• If not executed, errors are raised
• Maybe
• No guarantee of execution
• Client doesnt know the failure
• Oneway request in CORBA

18
Request Reliability

• Group and Multiple Request Reliability
• K-reliability
• Totally ordered
• Best effort
• Trade off between performance and Reliability