Showing posts with label SAD Notes. Show all posts
Showing posts with label SAD Notes. Show all posts

Sunday, February 17, 2013

Maintenance


Maintenance:
Once a system has been installed and fully operational then the task of system maintenance begins. The maintenance programming term involves keeping the software operational that is undertaking preventive measures to keep computer program running, monitoring the systems operations, fixing problems if any and modifying programs in response to user requirements.

Types of maintenance:
                               I.            Corrective
                             II.            Adaptive
                          III.            Perfective

                               I.            Corrective maintenance:
It is performed to correct any errors in newly implemented system.
The errors that can occur in the system –
                                                     i.)            Logical error (Program logic error)
                                                   ii.)            System error (hardware problem like disk crash, memory overflow etc.)
                                                 iii.)            Operational error- It occurs due to inappropriate computer schedules.
Ex:
·        Improper backup of computer files.
·        Failure to change printer ribbon.
                                                iv.)            User error - It occurs due to deficiency of user training.

The maintenance programmers repair such errors by redesigning the modules in the implemented software system.



                             II.            Adaptive maintenance:
It performs to cope up with the changing environment of the market and organisation. It is performed with a view to keeping the software up to date. This is because, as internal and external business condition changes the data processing requirement also changes. It begins with a request for service from users. Then the request is approved by committee. Then maintenance programmer start work for new requirements.

                          III.            Perfective maintenance:
It is performed to further improve the efficiency of working system. Modifying data structure, baking records, eliminating temporary work files etc are some methods of improving overall system performance.
Some ways to improve efficiency are –
                                                     i.)            Improving loops and calls to external procedures.
                                                   ii.)            Improving the evolution of algebraic expressions.
                                                 iii.)            Modifying expensive parts of the system.

System security


System Security:
System security is a process to protect the system from damage, error and unauthorized access. The level of protection depends upon the sensitivity of data, the reliability of the user and the complexity of the system. A well designed system includes control procedures to provide physical protection, maintenance, data integrity and restrict system access.
The system security includes for related issues –
                               I.            System integrity:
It refers to proper functioning of hardware and programs, safety against external threats such as eavesdropping.
The data integrity makes sure that data do not differ from its original form and have not been accidently or intentionally disclosed, altered or destroyed.
                             II.            System security:
It refers to the protection of data from loss, disclosure, modification and destruction.
                          III.            Privacy:
It defines the rights of the user or organisation to determine what information, we want to show or accept from other and how organisation can be protected against unfair or excessive dissemination of information.
                         IV.            Confidentiality:
It is a special status given to the sensitive information in the database for privacy.

Threats to the system security:
a.     Errors and omissions
b.    Fire
c.     Natural  disaster
d.    External attack
e.     Disgruntled and dishonest employees.

File structure and organisation

File structure–
A file is a collection of records which are related to each other. The size of file is limited by the size of memory and storage medium.
Two characteristics determine how the file is organised:
                                                        I.            File Activity:
It specifies that percent of actual records proceeds in single run. If a small percent of record is accessed at any given time, the file should be organized on disk for the direct access in contrast.
If a fare percentage of records affected regularly than storing the file on tape would be more efficient & less costly.

                                                      II.            File Volatility:
It addresses the properties of record changes. File records with many changes are highly volatile means the disk design will be more efficient than tape.

File organisation –
A file is organised to ensure that records are available for processing. It should be designed with the activity and volatility information and the nature of storage media, Other consideration are cost of file media, enquiry, requirements of users and file’s privacy, integrity, security and confidentiality.
There are four methods for organising files-

1.      Sequential organisation
2.      Indexed Sequential organisation
3.      Inverted list organisation
4.      Direct access organisation
5.      Chaining

1.       Sequential organization:
Sequential organization means storing and sorting in physical, contiguous blocks within files on tape or disk. Records are also in sequence within each block. To access a record previous records within the block are scanned. In a sequential organization, records can be added only at the end of the file. It is not possible to insert a record in the middle of the file without rewriting the file.
In a sequential file update, transaction records are in the same sequence as in the master file. Records from both the files are matched, one record at a time, resulting in an updated master file. In a personal computer with two disk drives, the master file is loaded on a diskette into drive A, while the transaction file is loaded on another diskette into drive B.  Updating the master file transfers data from drive B to A controlled by the software in memory.
Advantages:
i.                     Simple to design
ii.                   Easy to program
iii.                  Variable length and blocked records available
iv.                 Best use of storage space

Disadvantages
i.                     Records cannot be added at the middle of the file.

2.       Indexed sequential organization:

Like sequential organization, keyed sequential organization stores data in physicallycontiguous blocks. The difference is in the use of indexes to locate records. There are  three areas in disk storage: prime area, overflow area and index area.

The prime area contains file records stored by key or id numbers. All records are initially stored in the prime area.

The overflow area contains records added to the file that cannot be placed in logical sequence in the prime area.

The index area is more like a data dictionary. It contains keys of records and their locations on the disk. A pointer associated with each key is an address that tells the system where to find a record.


Advantages:
i.                     Indexed sequential organization reduces the magnitude of the sequential search and provides quick access for sequential and direct processing.
ii.                   Records can be inserted in the middle of the file.

Disadvantages:
i.                     It takes longer to search the index for data access or retrieval.
ii.                   Unique keys are required.
iii.                  Periodic reorganization is required.





3.       Inverted list organization:
Like the indexed- sequential storage method the inverted list organization maintains an index. The two methods differ, however, in the index level and record storage. The indexed sequential method has a multiple index for a given key, where as the inverted list method has a single index for each key type. In an inverted list, records are not necessarily stored in a particular sequence. They are placed in the data storage area, but indexes are updated for the record key and location. The inverted keys are best for applications that request specific data on multiple keys. They are ideal for static files because additions and deletions cause expensive pointer updating.
Advantages
i.                     Used in applications requesting specific data on multiple keys.

Example:
Data for the flight reservation system. 

The flight number, description and the departure time are as given as keys. In the data location area, no particular sequence is followed. If a passenger needs information about the Houston flight, the agent requests the record with Houston flight. The DBMS carries a sequential search to find the required record. The output will then be That the flight number is 170 departing at 10.10 A.M and flight number 169 departing at 8.15 A.M.

if the passenger searches for information about a Houston flight that departs at 8.15,then the DBMS searches the table and retrievesR3 and R6. Then it checks the flight departure time and retrieves R6 standing for flight number 169.


4.       Direct access organization:
In direct access file organization, records are placed randomly throughout the file. Records need not be in sequence because they are updated directly and rewritten back in the same location. New records are added at the end of the file or inserted in specific locations based on software commands.
Records are accessed by addresses that specify their disk locations. An address is required for locating a record, for linking records, or for establishing relationships. Addresses are of two types:
i.         Absolute
ii.       Relative.
A absolute address represents the physical location of the record. It is usually stated in the format of sector/track/record number. One problem with absolute address is that they become invalid when the file that contains the records is relocated on the disk.
A relative address gives a record location relative to the beginning of the file. There must be fixed length records for reference. Another way of locating a record is by the number of bytes it is from the beginning of the file. When the file is moved, pointers need not be updated because the relative location remains the same.
Advantages:
i.                     Records can be inserted or updated in the middle of the file.
ii.                   Better control over record allocation.

Disadvantages:
i.              Calculating address required for processing.
ii.             Impossible to process variable length records.

5.       Chaining:
File organization requires that relationships be established among data items. It must show how characters form fields, fields form files and files relate to each other. Establishing relationship is done through chaining. It uses pointers

Example: The file below contains auto parts that are an indexed sequential file sequenced by part no. A record can be retrieved by part no. To retrieve the next record, the whole file has to be searched. This can be avoided by the use of pointers.


System quality control and assurance


System quality control & assurance:
When a system is designed, it must fulfil the goal of the organization so some control must be developed to ensure a quality system.

            Quality assurance defines the objectives of the system that is what is expected from the system. Quality assurance activity in the system development involves four types of sub activities –

                               I.            Verification-
It is a process of confirmation as what is supplied as per system specification to the system. Verification controls are carried out under given environment and test results are obtained for acceptance of the new system.

                             II.            Validation-
Validation is a process of using system in actual environment in order to find errors and checks whether the validations applied to the system are working properly or not. If the results obtained from the system are proper, the system is valid otherwise it is not valid.

                          III.            System testing-
It is a review of specification, design and coding. It may be based on feedback obtained from the users.

                          IV.            Certification-
It is a confirmation of the correctness of the program. It provides authenticity of the system design that whether the system is according to its predefined standards and objectives or not.

Testing


Testing:
It is a process to check the software or system whether it gives the required output or not, on given set of data.
It other terms we can say it is a process to fail particular system because in testing we prepare all possible set of data and apply to the system to check it gives expected output so system must give expected output or must be properly shutdown in abnormal cases.

Testing can be categorised into –
1)    Black Box Testing
2)    White Box Testing

1)   Black Box Testing
It emphasizes on input data and expected output of the system. It does not includes the process from which the inputted data converts into required output such as internal control conditions, loop statements etc.
2)   White Box Testing
In white box testing we emphasizes on each & every step from which data moves and converts into required output such as control statement, repetitions, constraints, comparisons etc.

Structured english


Structured English:
It is used to provide step by step specification for any algorithm. It can be used at any desirable level of description & procedures. It is modified form of English that is used to specify the contents of the process box in data flow diagram (DFD). It uses English language instead programming language.
            It uses there basic types of statements to describe a process –
                                i.            Sequence structure –
                                                It is a single step or action included in a process. It does not depend on the existence of any condition.
For Ex:
To buy a computer science book we follow the steps –
a.     Pick out a desirable book.
b.     Take it to the sells counter.
c.      Pay cash for the book.
d.     Collect cash receipt.
e.     Collect the book and leave the store.
It is used when two or more actions can be taken depending on the value of a specific condition. We can describe it by using following example –
If
            Book found on the store
Then
            Take the book to sales counter
            Pay cash
            Take receipt & book
            Leave store
Else
            Do not take book
            Leave store.

                              ii.            Iterative structure –
                                                In various operating condition it is common to find that certain activities are repeatedly executes while certain conditions exist. It permits analysts to describe these conditions.
Until the category of student is SC
            Do give the discount to students on fee 50%
            Take next student
End Until

Data dictionary


­­Data Dictionary
It is store house of data, giving information about data. During system implementation it serves as common base for programmers who are working on the system, It compares their data description. It is also an important step in building database.
            “Data dictionary contains description & definition consulting the data structure, data elements, their interrelationship & other characteristics of a system.”
Objectives of Data Dictionary
                                i.            A standard definition of all terms in a system i.e. each data item is uniquely identified and defined.
                              ii.            Easy cross referencing between subsystem’s program and modules.
                            iii.            Simple program maintenance.
                           iv.            It contains information about the data of the system and there is an entry in the data dictionary for every element of DFD. Thus DFD and Data Dictionary are compliment of each other.

Data Items:
There are three classes of data items –
a.     Data Element –
                                    It is the smallest unit of data which cannot be meaningfully decomposed further.
For Ex:
            Employee code, unit of measurement
b.     Data Structure –
                                    A group of data elements forms a data structure.
For Ex:
            Data Structure of employee consist of a group of data elements such employee code, name, age, experience, phone no., address etc.
c.      Data Flow & Data stores –
                                    Data flows are data structures in motion whereas Data stores are data structure at rest. Data stores may be files, database etc.





Format of Data Dictionary:
A data dictionary is organised into five sections –
                                                        i.            Data Elements
                                                      ii.            Data Flows
                                                    iii.            Data Store
                                                   iv.            Process
                                                     v.            External Entities


Data Dictionary lists all data elements, flows, stores, process of the system and it gives the detail about each item in following format –
Data types, data name, data description, data characteristics, data control information, composition, physical location of data, etc.




Data dictionary for data element Employee code in dictionary –


Data Element  -           Employee code
Description      -           Unique code assigned for each employee
Type                -           char
Length             -           4
Range              -           0-9999
Data Stores     -           Employee table, Payroll table

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