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

Transcription

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

2 2/23 Remote invocation Mechanisms for process communication on a Built on top of interprocess communication primitives Lower level Provided by the OS Applications This chapter Remote invocation, indirect communication Underlying interprocess communication primitives: sockets, message passing, multicast support, overlay networks UDP and TCP Middleware layers

3 3/23 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 aknowledge is redundant Connecytion establishing requires extra messages Flow control is redundant form the majority of incications Which pass only small arguments and results

4 4/23 Request Reply Protocols Communication primitives dooperation getrequest sendreply Client Server dooperation (wait) (continuation) Request message Reply message getrequest select object execute method sendreply

5 5/23 Request Reply Protocols dooperation Sends request to the remote server Arguments Server remote reference Operation id Arguments Returns reply send (TCP), sendto (UDP)

6 6/23 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 7/23 Request Reply Protocols MessageType requestid remotereference operationid arguments int (0=Request, 1= Reply) int RemoteRef int or Operation array of bytes Message idengtifier RequestId incresing sequence of integers (per sending process) RemoteReference Identification of the process (IP address, port, process id)

8 8/23 Request Reply Protocols Failure model (UDP) Omission failures Delivery order not guaranteed Timeouts dooperation can Repeats transmission of message

9 9/23 Request Reply Protocols Duplicate messages After time-outs Protocols must recognize them Lost reply messages Reception of duplicate request Ignore second repetitions Implement idepotent operation History required

10 10/23 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) TCP Explicit ack of the reply Server can clean history simplifies protocol Larger messages Acknowledgements not necessary

11 12/23 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 13/23 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 14/23 Provide/implement goof 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 15/23 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 Java interfaces Google protcolo buffers Webservices WSDL

15 Remote Procedure Call RPC call semantics 16/23 DoOperation can be implemented differently Retry request message Are retries made until reply is received? Duplicate filtering What appens when server receives duplicate message Retransmition of results Is history kept to prevent re-execution of operations?

16 Remote Procedure Call RPC call semantics 17/23 Call semantics Retransmit request message Fault tolerance measures Duplicate filtering Re-execute procedure or retransmit reply Maybe No Not Applicable Not Applicable No fault tolerance measures applied Omission failures (message lost), crash failures (server fails) At-least-once Yes No 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 Yes Yes 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 18/23 RPC tries to hide differences from local/remote procedure call Syntax is similar Data marshaling and retransmition 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 19/23 client process Request server process client program client stub procedure Communication module Generated automatically Stubs Marshal arguments /return values Dispatcher Redirects call Reply server stub procedure Communication module dispatcher General service procedure Communication module Hand coded Client program Service procedure

19 20/23 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 Object model 21/23 Object references remote Interfaces remote Remote interface m1 m2 m3 { Remote object Data Implementation of methods m4 m5 m6 Actions State change remote invocation Local invocation Local invocation Local invocation remote invocation Object creation Further invocations Exception Can be generated and returned Garbage collection

21 Remote Method Invocation Implementation 22/23 Proxy Dispatcher Skeleton Static Generation IDL+compiler (Similar to SUN RPC) Dinamic invocation Language support for refection method interception(clienet) Dynamic code loading

22 Remote Method Invocation Execution 23/23 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?