Processes Threads OS Threads in distributed systems Object

Processes Threads & OS Threads in distributed systems Object servers Code migration Software agents CS-495 Distributed Systems Fabián E. Bustamante, Winter 2004

Threads Idea – we build virtual processors in software – Processor, processes and threads Context switch – Process and thread context – Observations • Threads shared the same address space • Process switching involves getting the OS in the loop - >$ • Creating and destroying threads is cheaper than doing so with processes Threads and the OS – User or kernel threads? – Two level threading and scheduler activations CS-495 Distributed Systems

Threads in Distributed Systems Multithreaded clients – to hide network latency – Web client – RPCs Multithreaded servers – improve performance & structure – – – Starting a thread is cheaper than starting a process A single-threaded server is a problem for scalability Move on to next request instead of waiting Using blocking calls simplifies overall structure Simplified flow control (? ) CS-495 Distributed Systems

Alternatives to Threaded Servers Alternatives Model Characteristics Threads Parallelism, blocking system calls Single-threaded process No parallelism, blocking system calls Finite-state machine Parallelism, nonblocking system calls Problems with threads? – – Hard to program: synchronization & deadlocks Hard to debug: data and timing dependencies Hard to get good performance Should you abandon threads? No – but avoid them if you can CS-495 Distributed Systems

Servers – Design Issues General organization – Basic model & superservers (inetd) Out-of-band communication – Use a separate port – Use a special message State – Stateless – no memory/no problem – Stateful – faster + reliability headache Object servers – Objects – data/state + method – Transient and persistent objects, thread per object or per invocation … – Activation policies and object adapters CS-495 Distributed Systems

Code Migration Why? – Performance – Flexibility Models – Process segments: • Code: set of instructions • Resource: external resources needed (files, printers, …) • Execution: current execution state –. . . CS-495 Distributed Systems

Models for Code Migration CS-495 Distributed Systems

Migration and Local Resources Problem: what if resources are not available at target? Type of resources: – Fixed: it cannot be migrated – Fastened: it can but it will cost you a lot – Unattached: easy to move Resource-to machine binding Process-toresource binding By identifier By value By type Unattached Fastened Fixed MV (or GR) CP ( or MV, GR) RB (or GR, CP) GR (or MV) GR (or CP) RB (or GR, CP) GR GR RB (or GR) MV: just move it GR: Establish global systemwide refer CP: Copy the value of the resource RB: Re-bind to a locally available resource CS-495 Distributed Systems

Migration in Heterogeneous Systems Main problem: – Target machine may not be suitable to run it – The def. of process/thread/processor context is highly dependent on HW/OS/Runtime system One solution: use abstract machine Current solutions: – Interpreted languages running on virtual machine – State the migration points CS-495 Distributed Systems

What’s an Agent? Def. : Autonomous process that can react/initiate changes in its environment, possible in collaboration – – Collaborative agents Mobile agents Interface agents Information agents Property Common to all agents? Description Autonomous Yes Can act on its own Reactive Yes Responds timely to changes in its environment Proactive Yes Initiates actions that affects its environment Communicative Yes Can exchange information with users and other agents Continuous No Has a relatively long lifespan Mobile No Can migrate from one site to another Adaptive No Capable of learning CS-495 Distributed Systems

Agent Technology Management: Keeps track of where the agents on this platform are Directory: Mapping of agents names and attributes to agent IDs ACC: Agent Communication Channel – used to communicate with other platforms CS-495 Distributed Systems

Agent Communication Languages ACL is application-level protocol making distinction bet/ purpose and content of msg. Message purpose Description INFORM Inform that a given proposition is true QUERY-IF Query whether a given proposition is true QUERY-REF Query for a give object CFP Ask for a proposal PROPOSE Provide a proposal ACCEPT-PROPOSAL Tell that a given proposal is accepted Field Value REJECT-PROPOSAL Tell that a given proposal is rejected Purpose INFORM REQUEST Request that an action be performed Sender weather@http: //www. weather. com: 7239 SUBSCRIBE Subscribe to an information source Receiver mafalda@http: //www. cs. nwu. edu: 5623 Language Prolog Ontology weather Content weather(today, 60201, sunny) CS-495 Distributed Systems