← QCE Digital Solutions Unit 4

Availability

Authorised users can reach data and systems when they need them. Attacks like denial of service and hardware failure reduce it. It is one part of the CIA triad.

Part of: QCE Digital Solutions Unit 4 — Complex Digital Data Exchange Problems and Solution Requirements — Module 19

Understanding Availability in QCE Digital Solutions Unit 4

What it does: When requests exceed what the server can handle, authorised users are blocked, so availability is reduced. Redundancy keeps the system running when hardware fails. Encryption protects confidentiality but does not help users reach data during an outage.

Common mistakes: Availability is the share of servers that are still working. It should divide working by servers, not failed by servers. A single = assigns a value, so it is a syntax error here. Checking whether a system is up needs the comparison operator ==.

Class: _____________________ · Name: _____________________ · Date: _________

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This module's key terms are also in a printable crossword (Modules 127–333) and word search (Modules 127–401).

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Learning

Duo

Which statement best describes availability?

Only the data owner can ever read the dataAuthorised users can reach data and systems when they need them

Matches

Availability means only the data owner can ever open the data

↓

This describes confidentiality or restricted access, not availability. Availability means authorised users can reach data when they need it.

Applying

Meanings

Term

Availability

Meaning

Authorised users can reach data and systems when they need them. Attacks like denial of service and hardware failure reduce it. It is one part of the CIA triad.

Practice

totalHours ← 720

Testing

Gaps

if uptime / totalTime 0.99 then

Check Answer

Spots

3working = servers - failed

4availability = failed / servers

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Quick Questions

1. Which statement best describes availability?

A. Only the data owner can ever read the data

B. Authorised users can reach data and systems when they need them ✓

Availability is about authorised users getting access when needed. Restricting who can read data is confidentiality.

2. A denial of service attack floods a school portal with traffic so students cannot log in. Which part of the CIA triad is reduced?

A. Confidentiality

B. Availability ✓

The data was not read or changed. Users just could not reach the system, so availability was reduced.

3. A server's hard drive fails and a business cannot access its customer records for two days. What has been reduced?

A. Availability, caused by hardware failure ✓

B. Confidentiality, caused by a social engineering attack

Hardware failure is a non-malicious cause that stops authorised users reaching data, so it reduces availability.

4. Which action best improves availability if a single server fails?

A. Keep a redundant backup server ready to take over ✓

B. Encrypt the files on the single server

Redundancy keeps the system running when hardware fails. Encryption protects confidentiality but does not help users reach data during an outage.

5. Which of these is an availability problem?

A. An authorised user is locked out because the system is offline ✓

B. An unauthorised person reads a customer's address

Being unable to reach data when needed is an availability issue. Unauthorised reading breaks confidentiality.

Match the Pairs

uptime = 99.9 → A measure of availability: the percentage of time a system is working and reachable by authorised users

if disk_status == "failed": → A check for hardware failure, which reduces availability when data or systems can no longer be reached

A denial of service attack reduces availability by flooding a server with requests so real users cannot get in → requests_per_second > server_limit

Hardware failure can be reduced by keeping a spare copy of the data on a second server that takes over → if main_server == "down": use(backup_server)

Availability means only the data owner can ever open the data → This describes confidentiality or restricted access, not availability. Availability means authorised users can reach data when they need it.

A denial of service attack reduces availability by changing the values stored in a database → Changing stored values harms integrity, not availability. A denial of service attack reduces availability by overwhelming a system so users cannot reach it.

Key Term

Availability

Authorised users can reach data and systems when they need them. Attacks like denial of service and hardware failure reduce it. It is one part of the CIA triad.

Fill in the Blanks

1. if uptime / totalTime < 0.99 then alert(admin)

Hint: Type the comparison operator that checks whether availability has fallen below the target.

2. if not serverOnline then switchTo(backupServer)

Hint: Type the Boolean operator that reverses a value, so the backup runs when hardware failure takes the main server down.

3. while not connected connected ← connect(backupServer)

Hint: Type the loop keyword that keeps repeating while a condition is true, so users stay connected.

4. if requestsPerMinute > limit then block(ipAddress)

Hint: Type the comparison operator that checks whether traffic exceeds the limit, which could signal a denial of service attack.

5. define availability(uptime, totalTime) return uptime / totalTime * 100

Hint: Type the keyword that sends the calculated result back from the function.

Spot the Error

Question 1

A servers = 3

B failed = 1

C working = servers - failed

D availability = failed / servers

E print(availability)

Line D — Availability is the share of servers that are still working. It should divide working by servers, not failed by servers.

Question 2

A def is_available(status):

B return status = 'up'

C print(is_available('up'))

Line B — A single = assigns a value, so it is a syntax error here. Checking whether a system is up needs the comparison operator ==.

Question 3

A requests = 5000

B limit = -100

C if requests > limit:

D print('Possible DoS')

Line B — A negative limit means every request is over the limit, so normal traffic would be flagged as a denial of service attack. The limit should be a positive number such as 100.

Question 4

A def get_server(primary_up)

B if primary_up:

C return 'primary'

D return 'backup'

Line A — A function header must end with a colon. It should be def get_server(primary_up):

Question 5

A disks = 2

B disks_failed = 1

C if disks_failed >= disks:

D print('Data available')

Line C — With mirrored disks, data stays available while at least one disk survives. That means failed disks must be fewer than total disks (<), not greater than or equal.

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