AZ-900 Describe cloud concepts Practice Question
A company runs a seasonal e-commerce application. During holidays, traffic increases tenfold, but the company does not want to over-provision resources for the rest of the year. They want to add or remove server capacity automatically based on real-time demand and pay only for what they use. Which cloud computing characteristic does this scenario describe?
⚠ Common exam trap
Watch out — candidates often confuse elasticity with high availability, thinking that adding more servers automatically ensures uptime, but high availability focuses on fault tolerance and redundancy, not dynamic scaling based on demand.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Elasticity
Elasticity is the cloud computing characteristic that enables automatic scaling of resources up or down based on real-time demand. In this scenario, the e-commerce application needs to handle a tenfold traffic increase during holidays without over-provisioning for the rest of the year, which is exactly what elasticity provides by adding or removing server capacity dynamically and charging only for what is used.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
High availability
Why it's wrong here
High availability is about keeping a service operational with minimal downtime, typically achieved by redundant instances, load balancers, and fault domains, so that failures do not cause user-visible outages. The design goal is resilience to component loss, not responsiveness to changes in request volume. While a highly available system must also be elastic to handle peak e-commerce traffic, availability alone does not describe the dynamic scaling required for seasonal workloads.
When this WOULD be correct
A question asking: 'A company wants its e-commerce site to remain operational even if one of its servers fails. Which characteristic ensures the application is always accessible?' would make high availability correct.
- ✓
Elasticity
Why this is correct
Elasticity is the capacity to automatically provision and deprovision resources (e.g., virtual machines, containers, or serverless functions) in response to shifting workloads. For a seasonal e-commerce application, this means scaling out to handle holiday traffic surges and scaling in when demand retreats, ensuring performance without paying for idle capacity. It directly matches the scenario's variable usage pattern.
- ✗
Reliability
Why it's wrong here
Reliability describes a system's ability to perform its intended function without failure, including graceful recovery from crashes, network partitions, or data corruption. It involves automatic failover, retries, and durable storage, but addresses operational correctness and uptime guarantees rather than matching capacity to demand. A reliable application might run with fixed-sized resources and still be overwhelmed by Black Friday traffic spikes, which elasticity specifically solves.
When this WOULD be correct
A scenario where a cloud provider guarantees 99.99% uptime for a critical application, ensuring it remains operational despite hardware failures, would make reliability the correct answer.
- ✗
Fault tolerance
Why it's wrong here
Fault tolerance is a stricter property where a system continues functioning correctly even during component failures by leveraging redundant hardware, data replication, and isolation, so customers see no interruption or erroneous results. It fundamentally masks faults inside the system, but does not change the amount of compute, memory, or network capacity available. Seasonal e-commerce load varies over time, not just due to failures, so fault tolerance cannot substitute for automatically adding or removing resources as elasticity provides.
When this WOULD be correct
A company runs a critical application that must continue operating without interruption even if a server fails. The cloud provider automatically redirects traffic to healthy instances, ensuring no downtime. This describes fault tolerance.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The AZ-900 exam frequently reuses these exact scenarios with slightly different constraints.
✓ElasticityCorrect answer▾
Why this is correct
Elasticity is the capacity to automatically provision and deprovision resources (e.g., virtual machines, containers, or serverless functions) in response to shifting workloads. For a seasonal e-commerce application, this means scaling out to handle holiday traffic surges and scaling in when demand retreats, ensuring performance without paying for idle capacity. It directly matches the scenario's variable usage pattern.
✗High availabilityWrong answer — click to see why▾
Why this is wrong here
High availability focuses on ensuring the application remains accessible despite failures, not on dynamically scaling resources based on demand. The scenario specifically describes adding/removing capacity in response to traffic changes, which is elasticity.
★ When this WOULD be the correct answer
A question asking: 'A company wants its e-commerce site to remain operational even if one of its servers fails. Which characteristic ensures the application is always accessible?' would make high availability correct.
Why candidates choose this
Candidates may confuse high availability with the ability to handle increased traffic, but high availability is about uptime and redundancy, not dynamic scaling.
✗ReliabilityWrong answer — click to see why▾
Why this is wrong here
Reliability refers to the ability of a system to recover from failures and continue functioning, not to dynamically scale resources based on demand.
★ When this WOULD be the correct answer
A scenario where a cloud provider guarantees 99.99% uptime for a critical application, ensuring it remains operational despite hardware failures, would make reliability the correct answer.
Why candidates choose this
Candidates may confuse reliability with the ability to handle increased load, but reliability is about uptime and fault recovery, not automatic scaling.
✗Fault toleranceWrong answer — click to see why▾
Why this is wrong here
Fault tolerance ensures system operation during component failures, not dynamic scaling based on demand. The scenario describes adding/removing capacity automatically, which is elasticity, not fault tolerance.
★ When this WOULD be the correct answer
A company runs a critical application that must continue operating without interruption even if a server fails. The cloud provider automatically redirects traffic to healthy instances, ensuring no downtime. This describes fault tolerance.
Why candidates choose this
Candidates may confuse fault tolerance with elasticity because both involve automatic responses to changing conditions, but fault tolerance focuses on failure recovery, not scaling to meet demand.
Analysis generated from the official AZ-900blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Go deeper
Related to this question
Learn chapter
What is Cloud Computing?
Key term
Elasticity
Elasticity is the ability of a cloud system to automatically add or remove computing resources (like servers, storage, or bandwidth) in response to real-time changes in demand.
Key term
Cloud computing
Cloud computing is the on-demand delivery of IT resources over the internet, allowing users to access computing power, storage, and applications without owning physical hardware.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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