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Processes

Processes. Processes and threads. Process form s a building block in distributed systems Processes granularity is not sufficient for distributed systems Multiple threads make easier to build distributed applications.

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Processes

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  1. Processes

  2. Processes and threads • Process forms a building block in distributed systems • Processes granularity is not sufficient for distributed systems • Multiple threads make easier to build distributed applications

  3. Thread Usage in Nondistributed Systemslarge applications : many cooperating programs (processes) via InterProcess Communication mechanisms (Unix) Changes to MMU, TLB … Changes to MMU, TLB … • IPC needs extensive context switching Instead of processes : threads

  4. Thread Implementationuser level : switching in a few instructions, no change in memory maps etc. but a blocking system call blocks the entire processO.S. kernel level : no problem with blocking system call but every thread operation has to be carried out by the kernel, so switching contest similar to process Lightweight processes LWP • Manipulation of threads at user level • A blocking call doesn’t suspend the entire process • Applications don’t need to know LWPs • Different LWPs on different CPUs • Combining kernel-level lightweight processes and user-level threads.

  5. Multithreaded Servers • A multithreaded server (file server) organized in a dispatcher/worker model.

  6. Multithreaded Servers • Three ways to construct a server: • The multi- and single-threaded model are “sequential” and synchronous • The finite-state machine model is asynchronous and simulates the multi-threaded model

  7. Client-Side Software for Distribution TransparencyAccess, location, migration, replication, failure transparency:cooperation with client-side software • A possible approach to transparent replication of a remote object using a client-side solution.

  8. Servers: General Design Issues • Iterative server – the server itself handles the request • Concurrent server – the server passes the request to a separate thread or process • Stateless server – no information on clients state (Web) • Stateful server – maintains informations on clients state (file server) • Where clients contact a server? • Preassigned endpoint (port) • No preassigned endpoint a special daemon running on the server

  9. Servers: General Design Issues 3.7 • Client-to-server binding using a daemon as in DCE • Client-to-server binding using a superserver as in UNIX (inetd)

  10. Reasons for Migrating CodeImproving performances ( optimizing CPU load, minimizing communications…) and flexibility • The principle of dynamically configuring a client to communicate to a server. The client first fetches the necessary software, and then invokes the server (security!)

  11. Models for Code Migration Code segment, program starts from initial state Code and execution segment Migrated prog. starts from where it letf off • Alternatives for code migration.

  12. Migration and Local Resources Resource-to-machine binding Process-to-resource binding MV: Move the resource GR: Establish a global reference CP: Copy the resource value RB: Rebind process to local resource • Actions to be taken with respect to the references to local resources when migrating code to another machine.

  13. Migration in Heterogeneous Systems 3-15 • The principle of maintaining a migration stack to support migration of an execution segment in a heterogeneous environment

  14. Software Agents in Distributed Systemsautonomous units capable of performing tasks in collaboration with other agents • Collaborative agent is part of a multiagent system • Mobile agent can move between different machine • Interface agent assists user to interact with an application • Information agent manages informations from many different sources

  15. Agent Technology Agent Communication channel Creating, deleting, Looking up Naming service Other agents in platform • The general model of an agent platform (from Foundation for Intelligent Physical Agents (FIPA)).

  16. Agent Communication Languages ACL is an application level protocol providing an high level communication protocol between a collection of agents • Examples of different message types in the FIPA ACL [fipa98-acl], giving the purpose of a message, along with the description of the actual message content.

  17. Agent Communication Languages (example) • A simple example of a FIPA ACL message sent between two agents using Prolog to express genealogy information.

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