Overview Of Process

ashish — Wed, 26 Aug 2009 09:59:05 +0000

A process is an activity which takes place over time and which has a precise aim regarding the result to be achieved. The concept of a process is hierarchical which means that a process may consist of a partially ordered set of sub-processes. A process is rather abstract. It describes the essentials of the purpose, structure, rationale, roles and timing, leaving plenty of implementation freedom. The power of a process is its abstraction, which enables its application in a wide range of applications, by tailoring its implementation to the specific application. A process can be tailored and elaborated in one or more procedures, which describe cookbook-like what need to be done when and by whom.
The process has been given many definitions for instance :
* A program in Execution.
* An asynchronous activity.
* The ‘animated sprit’ of a procedure in execution.
* The entity to which processors are assigned.
* The ‘dispatchable’ unit.
In Process model, all software on the computer is organized into a number of sequential processes. A process includes PC, registers, and variables. Conceptually, each process has its own virtual CPU. In reality, the CPU switches back and forth among processes.
A process goes through a series of discrete process states :
* New State: The process being created.
* Running State: A process is said to be running if it has the CPU, that is, process actually using the CPU at that particular instant.
* Blocked (or waiting) State: A process is said to be blocked if it is waiting for some event to happen such that as an I/O completion before it can proceed. Note that a process is unable to run until some external event happens.
* Ready State: A process is said to be ready if it use a CPU if one were available. A ready state process is runable but temporarily stopped running to let another process run.
* Terminated state: The process has finished execution.

A process in an operating system is represented by a data structure known as a process control block (PCB) or process descriptor. The PCB contains important information about the specific process including :
* The current state of the process i.e., whether it is ready, running, waiting, or whatever.
* Unique identification of the process in order to track “which is which” information. * A pointer to parent process.
* Similarly, a pointer to child process (if it exists).
* The priority of process (a part of CPU scheduling information).
* Pointers to locate memory of processes.
* A register save area.
* The processor it is running on.

Database System Concepts – Data Model, Schemas and Database state

ashish — Tue, 04 Aug 2009 03:05:29 +0000

A data model is a collection of concepts that can be used to describe the structure of a database. By structure of the database we mean the data types, relationships, and constraints that should hold on the data. Most data models also include a set of basic operations for specifying retrievals and updates on database.

Categories of Data Models:
- High level or Conceptual data models : These models provide concepts that are close to the way many users perceive data. They use concepts such as entities, attributes, and relationships. An entity represents a real-world object or concept such as an employee or a project. An attribute represents property of interest that describes an entity such as employee’s salary or name. A relationship represents an interaction among the entities.
- Representational data models : These models provides concept that may be understood by end users but that are not too far removed from the way data is organized within the computer. They are used most frequently in traditional commercial DBMSs and they include the widely used relational model as well as the network and hierarchical models. These models represent data by using record structures and hence are sometimes called record-based data models.
- Low level or Physical data models : These models provide concepts that describe the details of how the data is stored in the computer by representing information such as record formats, record orderings, and access paths. An access path is a structure that makes the search for particular database records efficient.

Schemas:
The description of a database in any data model is called the database schema which is specified during the database design and is not expected to change frequently. A displayed is called a schema diagram.

A schema diagram displays only some aspects of a schema, such as names of record types and data items, and some types of constraints.

Database State or Iinstance: The actual data in a database changes every time data is inserted, deleted, or modified. The data in the database at a particular moment in time is called a database state or a snapshot. It is also called the current set of occurrences or instances in the database.

Distinguish between Database State and Database Schema:
When a new database is defined, we specify its database schema only to the DBMS. At this point, the corresponding database state is empty state. The initial state of the database is got when the database is first populated or loaded with the initial data. From then on, every time an update operation is applied to the database, we get another database state.

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Overview Of Process

Database System Concepts – Data Model, Schemas and Database state