Rpc (Distributed computing)

Remote Procedure Calls Paul Krzyzanowski [email_address] [email_address] Distributed Systems Except as otherwise noted, the content of this presentation is licensed under the Creative Commons Attribution 2.5 License.

Problems with sockets Sockets interface is straightforward [connect] read/write [disconnect] BUT … it forces read/write mechanism We usually use a procedure call To make distributed computing look more like centralized: I/O is not the way to go

RPC 1984: Birrell & Nelson Mechanism to call procedures on other machines Remote Procedure Call Goal: it should appear to the programmer that a normal call is taking place

How do regular procedure calls work in programming languages?

Regular procedure calls Machine instructions for call & return but the compiler really makes the procedure call abstraction work: Parameter passing Local variables Return data

Regular procedure calls You write: x = f(a, “test”, 5); The compiler parses this and generates code to: Push the value 5 on the stack Push the address of the string “test” on the stack Push the current value of a on the stack Generate a call to the function f In compiling f, the compiler generates code to: Push registers that will be clobbered on the stack to save the values Adjust the stack to make room for local and temporary variables Before a return, unadjust the stack, put the return data in a register, and issue a return instruction

Implementing RPC No architectural support for remote procedure calls Simulate it with tools we have (local procedure calls) Simulation makes RPC a language-level construct instead of an operating system construct

Implementing RPC The trick: Create stub functions to make it appear to the user that the call is local Stub function contains the function’s interface

Stub functions network routines server functions server stub (skeleton) network routines 1. Client calls stub (params on stack) client functions client stub client server

Stub functions server functions server stub (skeleton) network routines 2. Stub marshals params to net message client functions client stub network routines client server

Stub functions 3. Network message sent to server client functions client stub network routines server functions server stub (skeleton) network routines client server

Stub functions 4. Receive message: send to stub client functions client stub network routines server functions server stub (skeleton) network routines client server

Stub functions 5. Unmarshal parameters, call server func client functions client stub network routines server functions server stub (skeleton) network routines client server

Stub functions 6. Return from server function client functions client stub network routines server functions server stub (skeleton) network routines client server

Stub functions 7. Marshal return value and send message client functions client stub network routines server functions server stub (skeleton) network routines client server

Stub functions 8. Transfer message over network client functions client stub network routines server functions server stub (skeleton) network routines client server

Stub functions 9. Receive message: direct to stub client functions client stub network routines server functions server stub (skeleton) network routines client server

Stub functions 10. Unmarshal return, return to client code client functions client stub network routines server functions server stub (skeleton) network routines client server

Benefits Procedure call interface Writing applications is simplified RPC hides all network code into stub functions Application programmers don’t have to worry about details Sockets, port numbers, byte ordering RPC: presentation layer in OSI model

Parameter passing Pass by value Easy: just copy data to network message Pass by reference Makes no sense without shared memory

Pass by reference? Copy items referenced to message buffer Ship them over Unmarshal data at server Pass local pointer to server stub function Send new values back To support complex structures Copy structure into pointerless representation Transmit Reconstruct structure with local pointers on server

Representing data No such thing as incompatibility problems on local system Remote machine may have: Different byte ordering Different sizes of integers and other types Different floating point representations Different character sets Alignment requirements

Representing data IP (headers) forced all to use big endian byte ordering for 16 and 32 bit values Most significant byte in low memory Sparc, 680x0, MIPS, PowerPC G5 x86/Pentiums use little endian main() { unsigned int n; char *a = (char *)&n; n = 0x11223344; printf("%02x, %02x, %02x, %02x\n", a[0], a[1], a[2], a[3]); } Output on a Pentium: 44, 33, 22, 11 Output on a PowerPC: 11, 22, 33, 44

Representing data Need standard encoding to enable communication between heterogeneous systems e.g. Sun’s RPC uses XDR (eXternal Data Representation) ASN.1 (ISO Abstract Syntax Notation)

Representing data Implicit typing only values are transmitted, not data types or parameter info e.g., Sun XDR Explicit typing Type is transmitted with each value e.g., ISO’s ASN.1, XML

Where to bind? Need to locate host and correct server process

Where to bind? – Solution 1 Maintain centralized DB that can locate a host that provides a particular service (Birrell & Nelson’s 1984 proposal)

Where to bind? – Solution 2 A server on each host maintains a DB of locally provided services Solution 1 is problematic for Sun NFS – identical file servers serve different file systems

Transport protocol Which one? Some implementations may offer only one (e.g. TCP) Most support several Allow programmer (or end user) to choose

When things go wrong Local procedure calls do not fail If they core dump, entire process dies More opportunities for error with RPC: Transparency breaks here Applications should be prepared to deal with RPC failure

When things go wrong Semantics of remote procedure calls Local procedure call: exactly once A remote procedure call may be called: 0 times: server crashed or server process died before executing server code 1 time: everything worked well 1 or more: excess latency or lost reply from server and client retransmission

RPC semantics Most RPC systems will offer either: at least once semantics or at most once semantics Understand application: idempotent functions: may be run any number of times without harm non-idempotent functions: side-effects

More issues Performance RPC is slower … a lot slower Security messages visible over network Authenticate client Authenticate server

Programming with RPC Language support Most programming 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)

Interface Definition Language Allow programmer to specify remote procedure interfaces (names, parameters, return values) Pre-compiler can use this to generate client and server stubs: Marshaling code Unmarshaling code Network transport routines Conform to defined interface Similar to function prototypes

RPC compiler IDL client code (main) server functions RPC compiler client stub headers server skeleton data conv. data conv. compiler compiler server client Code you write Code RPC compiler generates

Writing the program Client code has to be modified Initialize RPC-related options Transport type Locate server/service Handle failure of remote procedure call Server functions Generally need little or no modification

RPC API What kind of services does an RPC system need? Name service operations Export/lookup binding information (ports, machines) Support dynamic ports Binding operations Establish client/server communications using appropriate protocol (establish endpoints) Endpoint operations Listen for requests, export endpoint to name server

RPC API What kind of services does an RPC system need? Security operations Authenticate client/server Internationalization operations Marshaling/data conversion operations Stub memory management Dealing with “reference” data, temporary buffers Program ID operations Allow applications to access IDs of RPC interfaces

Rpc (Distributed computing)

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