CS 162 Operating Systems and Systems Programming Professor Anthony D Joseph Spring 2003 2 When message is safely buffered on destination node 3 Right away if message is buffered on source node There are really two questions here Lecture 21 Remote Procedure Call RPC 1 When can the sender be sure that the receiver actually received the message 21 0 Main Point 2 When can the sender re use the memory containing the message Send receive One vs two way communication Remote Procedure Call Cross address space vs cross machine communication Receive buffer mbox Wait until mbox has message in it then copy message into buffer and return 21 1 Send Receive Note that send and receive are atomic the OS ensures this abstraction Never get portion of a message all or nothing Two receivers can t get same message How do you program a distributed application Need to synchronize multiple threads only now running on different machines no shared memory can t use test set 21 2 Message styles Key abstraction send receive messages Note this abstraction is similar to read and write of the file I O interface Using send receive for synchronization Producer int msg1 1000 Atomic operations send receive doesn t require shared memory for synchronizing cooperating threads while 1 prepare message make coke send msg1 mbox Questions to be answered Blocking vs Non blocking calls Buffered vs Non buffered I O How does addressing work Consumer int msg 1000 Mailbox temporary holding area for messages ports Send message mbox put message on network with name of mbox for where it is to go on far end while 1 receive msg2 mbox process message drink coke No need for producer consumer to keep track of space in mailbox handled by send receive When can Send return 1 When receiver gets message i e acknowledgement received CS 162 Spring 2004 Lecture 21 When packet arrives OS puts message into mbox wakes up one of the waiters 1 9 CS 162 Spring 2004 Lecture 21 2 9 What if file is too big for response e g maximum message size is 1KB Use remote file access protocol What about two way communication Request response For instance read a file stored on a remote machine or request a web page from a remote web server send open rutabaga mbox1 receive status mbox2 Also called client server Client requester server responder Server provides service file storage to the client if streq status OK Error status while TRUE send read 1024 mbox1 Request response receive buf mbox2 if streq status OK if streq status EOF break Client requesting the file char response 1000 Error status send read rutabaga mbox1 send close mbox1 receive status mbox2 receive response mbox2 Server char command 1000 answer 1000 Problem I O is not the most natural programming model for many situations What is receive command mbox1 decode command read file into answer send answer mbox2 21 3 Remote Procedure Call RPC Server has to decode command just as OS has to decode message to find mbox Call a procedure on a remote machine Client calls remoteFileSys Read rutabaga Translated into call on server fileSys Read rutabaga Implementation Request response message passing Stub provides glue on client server o Client stub is responsible for marshalling arguments and unmarshalling the return values CS 162 Spring 2004 Lecture 21 3 9 CS 162 Spring 2004 Lecture 21 4 9 o Server side stub is responsible for unmarshalling arguments and marshalling the return values 21 3 1 Comparison between RPC and procedure call o Marshalling involves depending on the system converting values to a canonical form serializing objects copying arguments passed by reference etc Client caller call return Server callee return call bundle args Client stub unbundle bundle ret vals Server stub unbundle arguments send Packet Handler receive Network transport 21 3 2 Implementation issues Network transport send receive Stub generator generates stubs automatically For this only need procedure signature types of arguments return value Generates code on client to pack message send it off on server to unpack message call procedure Input interface definitions in an interface definition language IDL Packet Handler Output stub code in the appropriate source language C C Java What if client server machines are different architectures Or they use different programming languages Client stub Build message Send message Wait for response Unpack reply Return result Examples of modern RPC systems CORBA Common Object Request Broker Architecture DCOM Distributed COM RMI java Remote Method Invocation How does client know which mbox to send to Binding Static fixed at compile time C Dynamic fixed at runtime Lisp RPC Server stub Create N threads to wait for work to do Loop Wait for command Decode and unpack request parameters Call procedure Build reply message with results Send reply CS 162 Spring 2004 Lecture 21 What s equivalent Parameters request message Result reply message Name of procedure passed in request message Return address mbox2 In most RPC systems dynamic binding via name service Name service provides dynamic translation of service mbox Why runtime binding Access control check who is permitted to access service Fail over if server fails use another 5 9 CS 162 Spring 2004 Lecture 21 6 9 What if there are multiple servers can they use same mbox OK if no state carried forward from one call to the next For example open seek read close each uses context of previous operation What if multiple clients All would receive results in same mbox Caching can help but that can make failure handling more complex In a distributed system you have to face up to these two problems Illustrate in lecture on network file systems On same machine for example microkernel OS the tradeoff is what do you gain for the loss of performance Fix by passing pointer to mbox in request message this is like pushing the return PC on the stack 21 5 Cross Domain Communication and Location Transparency How do address spaces communicate with one another Semaphores etc File system Shared memory Pipes 1 way communication Remote procedure call 2 way communication 21 4 Problems with RPC RPC provides location transparency except 21 4 1 Failures Different failure modes in distributed system than on single machine Several kinds of failures User level bug causes address space to crash Machine failure kernel bug causes all processes on same machine to fail Earthquake causes all machines to fail Before whole system would crash Now one machine can crash while others stay up If file
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