Distributed System

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2. application layering

3. multi-tiered architecture

- Decentralized Architectures

Here there following examples

- Structured peer-to-peer

- Unstructured peer-to-peer

- Hybrid peer-to-peer

- Hybrid Architectures

-combining client-server solutions with decentralized architecture

Edge-server system is a class of distributed system organized according to a hybrid architecture

Collaboration distributed system is another example

General Approaches to Adaptive Software

Three basic approaches to adaptive software are:

- Separation of concern

- Computational reflection

- Component based design

Self Management in Distributed Systems

Distributed systems may be organized in high level feedback control systems allowing automatic adaptations to change

Example: Automatic Component Repair Management in Jade

Following steps are required in a repair procedure

- Terminate every binding between a component on a non-faulty node, and a component on the node that just failed

- Request the node manager to start and add a new node to the domain

- Configure the new node with exactly the same components as those on the crashed node

- Re-establish all the bindings that were previously terminated.

CHAPTER THREE

Processes

It refers to program in execution

Threads

Threads in distributed system are very useful to continue using CPU when a blocking I/O operation is performed.

This helps easier to build highly –efficient servers that run multiple threads in parallel.

Several may be blocking to wait until disk I/O or network communication completes.

Note: Client processes implement user interfaces

Client software aim at achieving distribution transparency

Multithreaded Server

Model

Characteristics

Threads

Parallelism, blocking system calls

Single Threaded Processes

No parallelism, blocking system calls

Finite state machine

Parallelism, non blocking system calls

Architectures of Virtual Machines

Interfaces at different levels

- There is an interface between the hardware and software containing machine instructions

It can be invoked by any program

- Interface between the hardware and software having machine instructions

Can be invoked only by privileged programs eg operating system

- There is an interface containing system calls that is offered by operating system

- There is an interface containing of library calls

In cases it forms an application programming interface

Server Design Issues

A server is implementing a specific service on behalf of a collection of clients

Each server is designed in the following way:

It waits for an incoming request from client and ensures a request is taken care of, then wait for next incoming request.

In iterative server, the server handles the request itself

Client contact the server

Whether and how server can be interrupted

Whether or not the server is stateless

Code Migration

This refers to moving a running process to a different machine

The main reason for migrating code is for improving performance

Performance can be improved if process is moved from highly loaded to lightly loaded machines

Also for flexibility

CHAPTER FOUR

Communication

The heart of all distributed system is the interprocess communication

In distributed system communication is based on low-level passing of information as it is offered by underlying network.

Modern distributed systems have thousands /millions of processes scattered across the network with unreliable communication such as internet.

Fundamentals of Communications in Distributed System

- Layer Protocols

Lack of shared memory has made all communications in distributed system to be based on sending and receiving (low-level) messages

For example if process A wants to communicate with process B, it has to build message on its own address space. Then it executes a system call that causes OS to send message over the network to B.

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