SLA, SLO, and SLI definitions in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding SLA, SLO, and SLI

Which of the following best describes the relationship between SLA, SLO, and SLI?

A SLI is the agreement, SLA is the measurement, and SLO is the penalty.

B SLA is the metric, SLO is the agreement, and SLI is the penalty for not meeting the agreement.

C SLO is the service metric, SLA is the target, and SLI is the customer's feedback.

D SLI measures a service metric, SLO sets a target for that metric, and SLA is the formal agreement based on the SLO.

💡 Hint

Think about which term is a measurement, which is a goal, and which is a contract.

An SLI (Service Level Indicator) is a specific metric that measures service quality. An SLO (Service Level Objective) is the target value for that metric. An SLA (Service Level Agreement) is the formal contract that includes the SLO and consequences if the SLO is not met.

🧠 Conceptual

intermediate

Identifying an SLI example

Which of the following is an example of a Service Level Indicator (SLI)?

A The percentage of successful HTTP requests over a period.

B 99.9% uptime guaranteed in the contract.

C A penalty clause for downtime exceeding 0.1%.

D A customer agreement to pay for premium support.

💡 Hint

SLI is a measurable metric, not a contract or penalty.

SLI is a metric that measures service quality, such as the percentage of successful HTTP requests. The other options describe agreements or penalties, not metrics.

❓ Architecture

advanced

Designing Monitoring for SLI Collection

You need to design a system to collect SLIs for a web service's availability and latency. Which architectural component is essential to ensure accurate and scalable SLI collection?

A A centralized logging system that collects all request logs and computes SLIs offline daily.

B Distributed agents on each server that collect metrics in real-time and push to a metrics aggregator.

C A manual process where engineers review logs weekly to estimate SLIs.

D A client-side script that reports user satisfaction surveys.

💡 Hint

Consider real-time, scalable, and accurate metric collection.

Distributed agents collecting metrics in real-time and pushing to a central aggregator allow accurate, scalable, and timely SLI measurement. Centralized logging with offline processing is slower and less scalable. Manual review is not scalable or timely. Client-side surveys measure user experience but are not direct SLIs.

❓ scaling

advanced

Scaling SLA Enforcement for a Global Service

Your service has SLAs with different customers worldwide, each with unique SLOs. What is the best approach to scale SLA enforcement and reporting?

A Maintain a single global SLA enforcement system that applies the strictest SLO to all customers.

B Outsource SLA enforcement to a third-party vendor without integration to your monitoring.

C Implement a multi-tenant SLA enforcement system that tracks each customer's SLOs separately and generates customized reports.

D Ignore individual SLAs and focus only on average service metrics.

💡 Hint

Think about customization and scalability for multiple customers.

A multi-tenant SLA enforcement system that tracks each customer's SLOs separately allows accurate enforcement and reporting per contract. Applying the strictest SLO globally is unfair and inaccurate. Outsourcing without integration loses control. Ignoring individual SLAs risks contract violations.

❓ tradeoff

expert

Tradeoff Between SLI Granularity and System Overhead

You want to increase the granularity of SLIs by measuring latency per API endpoint every second. What is a key tradeoff to consider?

A Higher granularity improves accuracy but increases monitoring system overhead and storage costs.

B Higher granularity reduces accuracy but lowers system overhead.

C Higher granularity eliminates the need for SLAs.

D Higher granularity guarantees zero downtime.

💡 Hint

Think about the balance between detail and resource use.

Increasing granularity improves accuracy and insight but also increases the load on monitoring infrastructure and storage needs. It does not reduce accuracy or eliminate SLAs. It cannot guarantee zero downtime.

Practice

1. What does SLA stand for in system design and what does it represent?

easy

A. Service Level Objective; a specific target for system reliability

B. Service Level Indicator; a metric to measure system performance

C. System Load Average; the average system load over time

D. Service Level Agreement; the overall service promise between provider and customer

Solution

1. Step 1: Understand SLA meaning

   SLA stands for Service Level Agreement, which is a formal contract or promise about the service quality.

2. Step 2: Identify SLA role

   It defines the overall service promise between the provider and the customer, including availability, performance, and support.

3. Final Answer:

   Service Level Agreement; the overall service promise between provider and customer -> Option D

4. Quick Check:

   SLA = overall service promise [OK]

Hint: SLA is the big promise, not the metric or target [OK]

Common Mistakes:

• Confusing SLA with SLO or SLI
• Thinking SLA is a metric
• Mixing SLA with system load terms

2. Which of the following correctly defines an SLI?

easy

A. A contract between provider and customer

B. A specific goal to meet the service promise

C. A metric used to measure service performance

D. A penalty for failing service levels

Solution

1. Step 1: Define SLI

   SLI stands for Service Level Indicator, which is a measurable metric that shows how well the service is performing.

2. Step 2: Differentiate from SLO and SLA

   Unlike SLO (goal) or SLA (contract), SLI is the actual measurement like latency, error rate, or uptime percentage.

3. Final Answer:

   A metric used to measure service performance -> Option C

4. Quick Check:

   SLI = metric [OK]

Hint: SLI is the number you measure, not the goal or contract [OK]

Common Mistakes:

• Confusing SLI with SLO or SLA
• Thinking SLI is a goal or agreement
• Mixing SLI with penalties

3. Given the following definitions:
SLA: 99.9% uptime per month
SLO: 99.95% uptime per month
SLI: Measured uptime percentage
What happens if the measured uptime is 99.92% for the month?

medium

A. SLA is met but SLO is violated

B. Both SLA and SLO are met

C. Both SLA and SLO are violated

D. SLA is violated but SLO is met

Solution

1. Step 1: Compare measured uptime with SLA

   The SLA requires 99.9% uptime. The measured uptime is 99.92%, which is higher, so SLA is met.

2. Step 2: Compare measured uptime with SLO

   The SLO target is 99.95%. The measured uptime 99.92% is less than 99.95%, so SLO is violated.

3. Final Answer:

   SLA is met but SLO is violated -> Option A

4. Quick Check:

   Measured uptime 99.92% < SLO 99.95% but > SLA 99.9% [OK]

Hint: Check if measured metric meets SLO first, then SLA [OK]

Common Mistakes:

• Mixing up SLA and SLO thresholds
• Assuming meeting SLA means meeting SLO
• Ignoring the difference between SLA and SLO

4. You have these definitions:
SLA: 99.9% uptime
SLO: 99.95% uptime
SLI: uptime percentage
Measured uptime is 99.85%. Which is the error in this statement?
"Since uptime is 99.85%, both SLA and SLO are met."

medium

A. The measured uptime is below SLA, so SLA is violated

B. The measured uptime is above SLO, so SLO is met

C. SLI is not uptime percentage

D. SLA and SLO are the same, so both are met

Solution

1. Step 1: Check measured uptime against SLA

   SLA requires 99.9% uptime. Measured uptime is 99.85%, which is less, so SLA is violated.

2. Step 2: Check measured uptime against SLO

   SLO requires 99.95% uptime. Measured uptime 99.85% is less, so SLO is also violated.

3. Final Answer:

   The measured uptime is below SLA, so SLA is violated -> Option A

4. Quick Check:

   99.85% < 99.9% SLA means SLA violated [OK]

Hint: Always compare measured uptime to SLA threshold first [OK]

Common Mistakes:

• Assuming 99.85% is above 99.9%
• Confusing SLI with SLO or SLA
• Thinking SLA and SLO are identical

5. A company promises 99.9% uptime in its SLA. It sets an SLO of 99.95% uptime and uses error rate as the SLI. If the error rate increases causing uptime to drop to 99.85%, what should the company do to maintain trust and meet SLA?

hard

A. Change the SLA to match the current uptime

B. Improve system reliability to reduce error rate below SLO target

C. Ignore the SLI since SLA is the only important metric

D. Increase the error rate threshold in SLI to match uptime

Solution

1. Step 1: Understand the relationship between SLA, SLO, and SLI

   SLA is the promise (99.9%), SLO is the goal (99.95%), and SLI is the metric (error rate) used to measure uptime.

2. Step 2: Analyze the impact of increased error rate

   Higher error rate lowers uptime to 99.85%, which violates both SLO and SLA targets, risking customer trust.

3. Step 3: Determine corrective action

   The company should improve system reliability to reduce error rate and restore uptime above SLA and SLO levels.

4. Final Answer:

   Improve system reliability to reduce error rate below SLO target -> Option B

5. Quick Check:

   Fix error rate to meet SLO and SLA [OK]

Hint: Fix the metric causing SLA breach, don't change promises [OK]

Common Mistakes:

• Changing SLA instead of fixing system
• Ignoring SLI importance
• Adjusting thresholds to hide problems