1 remote procedure call cisc 879 – spring 03 tam vu...
TRANSCRIPT
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Outline
Introduction RPC mechanisms Design issues Programming with RPC Case study: SUN RPC
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Introduction
Problem with sockets Socket interface is straightforward
Connect Read/write Disconnect
Forces read/write mechanism Not how we generally program We usually use procedure calls
To make distributed computing look more like centralized: I/O is not the way to go
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Introduction
1984: Birrell & Nelson Mechanisms to call procedures on other
machines Processes on machine A can call procedures on
machine B A is suspended Execution continues on B When B returns, control passed back to A
Goal: it appears to the programmer that a normal call is taking place
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Introduction
Remote Procedure Call (RPC) is a high-level model for client-sever communication.
RPC enables clients to communicate with servers by calling procedures in a similar way to the conventional use of procedure calls in high-level languages.
Examples: File service, Authentication service.
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The RPC model
Blocking state
client server
request
reply
Executing state
Calls procedure and wait for reply
Receives request and starts process executionSends reply and wait for next executionResume
s execution
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RPC Mechanisms
The client transfer call request (the procedure name) and the arguments to the server via client stub function
stub function marshals arguments and places them into a message
together with the remote procedure identifier. Sends message to server and waits for call return
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RPC Mechanisms
Server receives the call request and passes to an appropriate server stub function.
server stub function unmarshals the arguments, call the corresponding (local) service procedure.
On return, the server stub marshals the output arguments into a call return message and sends back to the client.
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RPC Mechanisms
Client stub receives call reply, unmarshals value, returns to client code
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RPC Mechanisms
client stubfunc.
Communicationmodule
Local return
Local call
Client computer Server computer
server stubfunc.
client
service procedure
Receivereply
Sendrequest
Unmarshalresults
Marshalarguments
Receiverequest
Sendreply
Select procedure
Unmarshalarguments
Marshalresults
Execute procedure
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Benefits
Familiar procedure call interface
Writing applications is simplified RPC hides all networks codes Programmers don’t have to worry about details (sockets,
port numbers, byte ordering)
RPC: presentation layer in OSI model
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Characteristics
The called procedure is in another process which may reside in another machine.
The processes do not share address space. Passing of parameters by reference and passing pointer
values make no sense. The called remote procedure executes within the
environment of the server process. The called procedure does not have access to the calling
procedure's environment.
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Design issues
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Parameter passing
By values easy, just copy data to network message.
By reference makes no sense without shared memory
Trick Copy items referenced to message buffer Ship them over Unmarshal data at server Pass local pointer to server stub function Send new value back
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Representing data
No such things as incompatibility on local systems
Remote machine may have: Different byte ordering Different sizes of integers and other types Different floating point representations Different character sets
Need standard encoding to enable communication between heterogeneous systems
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Representing data
Implicit typing Only values are transmitted, not data type or parameter
information E.g., Sun XDR (eXternal Data Representation)
Explicit typing Types are transmitted with values E.g., ISO ANS.1, XML
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Binding
How to locate host and server process? Solution 1: use a central DB
Server sends message to central DB indicating the services it can offer
Clients contact this authority whenever they need to locate a service
Solution 2: Client needs to know server name Server maintains a DB of available services
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Failures
Local procedure calls do not fail If they core dump, the entire process dies
RPC is more vulnerable to failure: Server could generate errors Problems in network Server crash Client crash while server is still running code for it
Transparency breaks here Applications should be prepared to deal with RPC failure
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Delivery guarantees
Retry request message: Client retransmits the request message until either a reply
or the server is assumed to have failed.
Duplicate filtering : server filters out duplicate message.
Retransmission of replies: Server keeps a history of reply messages to enable lost
replies retransmitted without re-executing the server operations.
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Call Semantics
Semantic of local procedure calls: exactly-once Exactly-once maybe difficult to achieve with RPC At-least-once
The client assumes that the RP is executed at least once (on return from the RP).
Can be implemented by retransmission of the request message on time-out.
Acceptable only if the server’s operations are idempotent. That is f(x) = f(f(x)).
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Call Semantics
At-most-once
When a RPC returns, the remote procedure (RP) is assume to have been called exactly once or not at all.
Implemented by the server's filtering of duplicate requests and caching of replies.
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Call Semantics
At-most-once
This ensure the RP is called exactly once if the server does not crash during execution of the RP.
When the server crashes during the RP's execution, the partial execution may lead to erroneous results.
In this case, we want the effect that the RP has not been executed at all.
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Call Semantics
RPC call semantics
Retry request message
Duplicate filtering
Response
At-least-once Yes No Re-execute procedure
At-most-once Yes Yes Retransmit reply
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More issues
Performance remote procedure call and return time can be significantly
slower than that for local procedure call (1 - 3 orders of magnitude).
Security Messages visible over the network Authenticate client Authenticate server
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Programming with RPC
Most languages (C, C++, Java,…) have no concept of remote procedure calls Language compilers will not generate client and server
stubs Common solution:
Use a separate compiler to generate stubs (pre-compiler)
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Programming with RPC
Interface Definition Language Allow programmers to specify remote procedure interfaces
(names, parameters, return values) Pre-compiler can use this to generate client and
server stubs Marshalling code Unmarshalling code Network transport protocols Conform to defined interface
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Programming with RPC
Client code has to be modified Initialize RPC-related options
Transport type Locate host/service
Handle failure of remote procedure call Server functions
Generally need little or no modification
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Case Studies: SUN RPC
Interface definition language: XDR a standard way of encoding data in a portable fashion
between different systems; Pre-compiler: rpcgen
A compiler that takes the definition of a remote procedure interface, and generates the client stubs and the server stubs;
Communication handling: TCP or UDP
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References
http://www.cisco.com/univercd/cc/td/doc/product/software/ioss390/ios390rp/rprpcgen.htm
http://pandonia.canberra.edu.au/OS/l14_1.html http://www.cs.cf.ac.uk/Dave/C/node33.html
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Thank you!