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Distributed Systems

Distributed Systems. Lecture #1: Remote Communication. Communication. Communication across address spaces Local Remote Same Rules apply Transparency Fault-isolation Consistency. Overview. Mechanism Message Passing Abstractions Message Passing Synchronous: Procedure Calls

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Distributed Systems

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  1. Distributed Systems Lecture #1: Remote Communication

  2. Communication • Communication across address spaces • Local • Remote • Same Rules apply • Transparency • Fault-isolation • Consistency

  3. Overview • Mechanism • Message Passing • Abstractions • Message Passing • Synchronous: Procedure Calls • Asynchronous: Events • Coupled: Shared Memory • De-coupled: Tuple-spaces

  4. Layered Communication Application Distributed System (RPC) Socket TCP/IP UDP/IP OS

  5. Local Communication • How would 2 processes communicate? • Message Passing: Sockets • Shared Memory: Files • Procedural call (across address space)? • Ever used local RPC? • Heard of OLE? COM?

  6. RPC Enables Modularization • Clients and Servers • Better software engineering • Well-defined interfaces between • Client • Service provider • Fault-isolation • Client failure does not affect the server • Centralized consistency management • Security • Scalable Performance: Server not constrained by client

  7. Local RPC • Transparency! • Makes sense? • Issues: • What transpires in a subroutine call?

  8. RPC Components Client Server Stub Skeleton Client Framework Server Framework Network

  9. (Local) RPC Issues • Transfer of control and data across address spaces • Binding • Data Marshalling • Thread Migration • Synchronization

  10. Remote RPC • Same Issues • BUT transparency makes sense? • To what degree? • Modularization • Fault-isolation • But masking? • Failure may be partial • Time? • Consistency • Scalable Performance?

  11. Case Study: Sun RPC • Used in NFS • What does an RPC system consist of? 1. Standards for wire format of RPC msgs and data types. XDR, XML 2. Library of routines to marshal / unmarshal data. 3. Stub generator, Client: marshal, invoke, wait, unmarshal reply. For server: unmarshal arguments, invoke server function, marshal reply. 4. Server framework: Dispatch each call message to correct server stub. 5. Client framework: Give each reply to correct waiting thread / callback. 6. Binding: how does client find the right server?

  12. Sun RPC • What does a Sun RPC request contain? • Wire format • Xid • Call orreply • RPC version • program # • program version • procedure # • auth bits • Marshalled arguments

  13. Marshalling: Sun RPC • Marshaling arguments • "Linearize" data • "Externalize" data: Formats defined by XDR standard Easy for e.g. int -- same representation, though portable byte order... Collections? include a length. Pointers? Pass by value or reference? • Return Values • How to return? • IDL: In, Out, INOut • RMI: Objects • Recursive packing

  14. RPC: Summary • Synchronous, Blocking • Mimics a procedural call • Involves: • Data transfer: Marshaling • Thread Migration

  15. NFS Transparency • Tension between performance and consistency • Syntax preserved • Not Semantics • File read/write semantics • Only check on open() • Consistency? • Events! • Expose Distribution: Java RMI

  16. Events • Subscribe/notify • Asynchronous • Non-blocking Subscribe Notify

  17. Events • Better performance than polling for larger tasks • Do it, let me know when you are done • Preferred mode of communication for interactive applications: none-blocking • But Harder to program • Synchronization • Spaghetti code

  18. Programming Events Main thread Subscribe Notify Events Handler

  19. Distributed Shared Memory Int x; x = 10; Int x; .. print x; • Consistency? • Deadlock? • DSM abstraction? • Make it explicit X = 10

  20. Tuple-spaces Tuple-space • Explicit DSM • Tuple as the atomic data object • Clearly defined semantics for insert/read/peak

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