1. 03 – Remote invoaction Request-reply RPC RMI Coulouris 5
Birrel_Nelson_84.pdf

2. Remote invocation, indirect communication
Mechanisms for process communication on a DS
Built on top of interprocess communication primitives
Lower level
Provided by the OS
Applications
This chapter
Remote invocation, indirect communication
Middleware layers
Underlying interprocess communication primitives: sockets, message passing, multicast support, overlay networks
UDP and TCP

3. Request Reply Protocols
Support client-server interaction
Usually synchronous
Client blocks until reply is received
Reliable (reply is an acknowledge)
May use UDP datagram instead of TCP streams
TCP knowledge is redundant
Connection establishing requires extra messages
Flow control is redundant for some uses
Which pass only small arguments and results

4. Request Reply Protocols
Communication primitives
doOperation
getRequest
sendReply
Client
Server
Request
doOperation
getRequest
message
select object
(wait)
execute method
Reply
sendReply
message
(continuation)

5. Request Reply Protocols
doOperation
Sends request to the remote server
Arguments
Server remote reference
Operation id
Returns
reply
send (TCP), sendto (UDP)

6. Request Reply Protocols
getRequest
Blocking function / waits for request
Returns request data
sendReply
Sends response to client
Arguments
Reply
Client id
send (TCP), sendto (UDP)

7. Request Reply Protocols
MessageType
int (0=Request, 1= Reply)
requestId
int
remoteReference
RemoteRef
operationID
int or Operation
arguments
array of bytes
Message identifier
RequestId
increasing sequence of integers (per sending process)
RemoteReference
Identification of the process (IP address, port, process id)

8. Request Reply Protocols
Failure model (UDP)
Omission failures
Delivery order not guaranteed
Timeouts
doOperation can
Repeats transmission of message

9. Request Reply Protocols
Duplicate messages
After time-outs
Protocols must recognize them
Reception of duplicate request
Ignore second repetitions
Implement idepotent operation
History
required

10. Request Reply Protocols
Exchange protocols
R (request)
Online request is sent
No need for response
Client may unblock imediatly
RR (request-reply)
Special ACK message not required
Implemented by the response
Subsequent Request is Acknowledgment of the reply
Exchange protocols
RRA (request-reply- ack)
Explicit ack of the reply
Server can clean history
TCP
simplifies protocol
Larger messages
Acknowledgements not necessary

11. Remote Procedure Call Request Reply protocols
Not transparent (w.r.t. distribution)
Local invocation ≠ remote invocation
Extends procedure call abstraction
RPCs are called as if in local address space
RPC system hides
Distribution
Encoding/decoding of parameters/reply
Passing of messages
Semantics of call

12. Remote Procedure Call Interfaces
RPCs should mimic programming language structures
Modules/ libraries / classes
Function/procedures/methods
Use the concept of interface
A set of well defined ointeraction points
Encapsulate implementation details
Service interfaces
Set of procedures offered by a server
Defining parameters and return values (and types)

13. Remote Procedure Call Interfaces
Provide/implement good programming practices
Implementation are hidden from the programmer
Server programming language /architecture is hidden
SW evolution is possible
Distribution imposes
Impossible access to remote variables
Call by reference not suitable
Input parameters: client → server transfer
Output parameters client ← server transfer
Addresses in one process are not valid on another
Addresses can not be passed

14. Remote Procedure Call Interface Definition Languages
Static languages require the a priori declaration of functions and types
Publication of a service requires its description
Interface Definition Languages (IDL) allow:
Specification of interfaces
Communication of interfaces
Production of .c/.h files
Examples
SUN XDR,
CORBA IDL
Webservices WSDL
Java interfaces

15. Remote Procedure Call RPC call semantics
DoOperation can be implemented differently
Retry request message
Are retries made until reply is received?
Duplicate filtering
What happens when server receives duplicate message
Retransmission of results
Is history kept to prevent re-execution of operations?

16. Remote Procedure Call RPC call semantics
Fault tolerance measures
Retransmit request message
Duplicate filtering
Re-execute procedure or retransmit reply
Maybe No
Not Applicable
No fault tolerance measures applied
Omission failures (message lost), crash failures (server fails)
At-least-once
Yes
Re-execute procedure
Invoker receives a result (code was executed at least once) or an exception (result was not received)
Retransmission of requests
Crash failures (server fails), arbitrary failures (requests are retransmitted, procedure may be executed more than once)
At-most-once
Retransmit reply
Invoker receives a result (code was executed once) or an exception (result was not received, but code not executed or executed once)

17. Remote Procedure Call Transparency
Minimum requirement
RPC tries to hide differences from local/remote procedure call
Syntax is similar
Data marshaling and retransmission is hidden
However RPC are more vulnerable to failure
Involve network, other computer, other process
Code should be able to recover
RPC latency is higher
Parameter passing is different

18. Remote Procedure Call Implementation
client process
server process
Request
Reply
client stub
procedure
server stub
procedure
client
program
service
procedure
Communication
module
Communication
module
dispatcher
Generated automatically
Stubs
Marshal arguments /return values
Dispatcher
Redirects call
General
Communication module
Hand coded
Client program
Service procedure

19. Remote Method Invocation
Extends RPC to the OO world
RPC and RMI
Support programming with interfaces
Built on top of Request-reply protocols
Offer similar semantics
Offer similar level of transparency
RMI
Use OO mechanisms
Object, classes inheritance, OO development methods/tools
Provide concept of object identity
All objects have references (local or remote)
That can be globally accessed

20. Remote Method Invocation Implementation
Proxy
Dispatcher
Skeleton
Static Generation
IDL+compiler (Similar to SUN RPC)
Dynamic invocation
Language support for reflection
method interception (client)
Dynamic code loading

21. Remote Method Invocation Execution
Server/client programs
Binder/ns
Process that provides object reference from textual name
Server threads
In order to allow concurrent method execution
Requires synchronization
Activation
When are objects killed?