AS6

Question 1

Purpose and Structure of Files

Answer 1

• Files enable data to persist in storage between executions of a program.
• One file contains many records, structured as multiple fields.

Question 2

Fixed Length Record

Answer 2

• Every record has to be the same size, so the programmer needs to define the size at the start.
• Consider data types being stored and the max amount of bytes they will require.
• Easier to program, can calculate how much storage will be required.

Question 3

Variable Records

Answer 3

• More difficult to calculate space as a different number of bytes in each record.
• Slower to process by a computer as start/end locations need to be calculated at read/write time.
• No blank spaces are left.
• Each field can extend to accommodate characters, avoids truncation.

Question 4

Estimating File Size

Answer 4

• Singular record: Adding together the size of the individual fields.
• Final size: Multiply by number of records + 10%. (for metadata)

Question 5

File Access

Answer 5

• Normally provided by the core language or imported libraries.
• Files are processed by being read line by line. The file handler will contain a pointer to tell the handler where the last read/write operation occurred up until it reached an End Of File designator.
• Normal practice to close files once finished with them.

Question 6

Serial Files

Answer 6

• New records being appended to back of file is easiest storing method. Important that file is opened in append mode.
• Normally stored in chronological order.
• Used when number of records is small/data must stored in the order it arrived.

Question 7

Sequential File Access

Answer 7

• Will order records based on primary key field.
• One field will determine if order is ascending or descending. Makes searching easier.
• A requirement for implementing an index.
• Necessary to find correct position in the file to insert it. Harder than appening to serial files.
• Not possible to move records to make room for a new one, a temp file copies all records before, the new record, and then records after. Replaces original file.

Question 8

Indexed Sequential File Access

Answer 8

• Contains index which makes accessing record groups faster.
• Locates records in logical chunks e.g. starting with same letter.
• Speeds up searches, slows down insertion/deletion. When a record is added, the start and finish points of the index will change the size of one record.
• Access can still be slow for large amounts of records.

Question 9

Direct (random) File Access

Answer 9

• Jumping to a record without needing to perform a search/use an index.
• Can be implemented using a hashing algorithm (primary key taken and used to produce file position).
• DA files are split into blocks which contain a fixed number of records. The file will take up the same amount of space no matter amount of stored records.

Question 10

Overflow

Answer 10

• A collision is when a hashing algorithm returns the same block position for two keys.
• Each block has max record count, so it can fill up.
• Either create an overflow file can allow for excess records or create a new file containing more blocks and re-hash every excess record in the new file.

Question 11

Overflow v. File Re-Organisation

Answer 11

• Overflow area is lower in cost than a new file, can slow down searches.
• When a search reaches the end of the block, overflow will also be checked. Because both areas work in the same way, the file will be less efficient.
• Creating a new file and re-hashing could be expensive. This is used when the load factor is too high.
• Selecting a larger block size reduces the need.

Question 12

Generations of Files

Answer 12

• File backups can be created in a structure to allow a series of steps to be taken so that the system will be restored in steps depending on when the loss occurred.
• Grandfather (monthly), father (weekly), son (daily).

Question 13

Transaction Logs

Answer 13

• Maintain the security of the system by giving accountability/ allowing file changes to be tracked.

Question 14

Master and Transaction Files

Answer 14

• To back up files with vast amounts of data, two files are used: A transaction file (short term interactions), and a master file (all data).
• Transaction files act as a temp file, holding data for a short period of time, to keep the system running quickly. At the end of a period, the data is copied to the master file.
• The master file stores all the data so they wont be used to carry out day-to-day operations (too large). Used for batch processing.

Question 15

Field

Answer 15

• To store data, it needs to be organised in fields.
• Each field represents a piece of data.

Question 16

Backup

Answer 16

• Copying files from their main area of use to a seperate area of use.
• If a file is data, the backup can be accessed to retrieve lost data.

Question 17

Archiving

Answer 17

• Files build up and take space. The more of space used up, the slower the computer.
• Files not used day-to-day could be archived.
• Moved to a seperate archive system, will speed up the overall performance of the system while still giving access to files if need arrives.

Question 18

Validation

Answer 18

• Ensures data entered is valid. Done through checks.

Question 19

Verification

Answer 19

• Ensures data entered is correct. Managable but often not wholly achievable.
• Proofreading: Compare original document to what has been keyed in.
• Double entry: Have data keyed in twice, allowing computer to identify any differences in the two versions.