CS603 Basics of underlying platforms January 9, 2002

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Basics of Underlying Platforms Why do we want distributed systems? Basic hardware concepts Software concepts System models

Distributed System Goals Connect users and resources –Why not place resource and user together? Multiple users share same resource We don’t know where the user will be (ATM) Mobile users Transparency –User shouldn’t be aware system is distributed –Is this always true?

Distributed System Goals (continued) Distributed systems should be open –Allow user to choose platform –Separate development of resource/client Distributed systems should scale –Number of users –Size of resource –Number of types of clients/servers

Hardware What machines are used (nodes)? –Homogenous –Client/server All clients identical Multiple types of clients –Multi-server Replicated servers Different resources

Hardware How are things connected? –Is a multiprocessor a distributed system? –Homogeneous networks (e.g., LAN) –Different types of networks (wireless, dial-up, LAN) How does this affect system? –Build to lowest common denominator? –Different views for different paths? Violates transparency!

Software concepts Distributed operating systems –Conceptually the ultimate in transparency User/programmer view is single system OS handles all mapping –Resource request is system call –Response comes automatically –Same code for single site or distributed –In practice, programmer can see distribution Enables concurrent processing Shared memory view OS provides constructs to support sharing

Software Concepts: Concurrency Primitives Semaphore –Value with operations: Increment and Decrement Decrement blocks if value = 0 –Alternative view: “Test and Set” –Key: Semaphore operations atomic Monitor –Object (package of values and procedures) –Atomicity: Only one process may execute object at any time Thought exercise: Implement Semaphore with Monitors

Software Concepts: Concurrency Primitives What went wrong? Blocked processes Solution: Additional primitives –Wait –Signal How does this affect “only one process at a time”? –Wait effectively exits monitor –Resume after wait

Software Concepts Problem: How to implement shared memory across a network? –Similar to paging, but “paged out” is on a different machine, not disk –Need global page table –Distributed OS handles the details What does this do to performance? Alternative: Expose distribution –Message passing –Remote procedure call

Software Concepts Network Operating System No single system view Primitives/commands handle distribution –rsh/rcp: Command level remote procedure call and remote file transfer –Primitives for cross-machine concurrency control (e.g., flock on NFS) Big problem: User must be location-aware!

Software Concepts: Middleware approach What does middleware do? –Doesn’t hide distribution –Hides location –Hides heterogeneity Examples –Unix (and especially Plan 9): Everything is a file –Remote Procedure Call –Distributed Objects –Distributed Documents

CS603 Basics of underlying platforms January 11, 2002

Distributed System Models Client-Server –Resource sits at a server –Application sits at a client –Server waits for client to request resource Three-tier Architecture –Client-(Client-Server) –Sometimes tighter coupling than client-server between lower levels Peer to Peer –Everyone is client and server What do these have in common?

Distributed System Models: What is next? Push models –Not just client driven –Otherwise looks like client-server (or multi- tier) Agent-based models –Is this anything new? Any other ideas?