SLA, SLO, and SLI definitions in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does SLA stand for and what is its purpose?

SLA stands for Service Level Agreement. It is a formal contract between a service provider and a customer that defines the expected level of service, including specific performance and availability targets.

beginner

Define SLO in the context of service management.

SLO stands for Service Level Objective. It is a specific measurable goal within an SLA that defines the target level of service performance, such as 99.9% uptime.

beginner

What is an SLI and how is it used?

SLI stands for Service Level Indicator. It is a metric that measures the actual performance of a service, such as latency or error rate, used to determine if SLOs are being met.

intermediate

How do SLA, SLO, and SLI relate to each other?

An SLA is the overall agreement that includes one or more SLOs, which are specific targets. SLIs are the metrics used to measure if those SLOs are achieved, helping to monitor the SLA compliance.

intermediate

Why is it important to have clear SLIs in a service agreement?

Clear SLIs provide objective data to track service performance. They help both providers and customers understand if the service meets agreed targets, enabling trust and informed decisions.

What does SLO specifically define in a service agreement?

A The measurable target for service performance

B The overall contract between provider and customer

C The metric used to measure service quality

D The penalty for service failure

SLO defines the measurable target or goal for service performance within an SLA.

Which term refers to the actual metric used to measure service performance?

A SLO

B SLI

C SLA

D SLC

SLI is the metric that measures actual service performance.

What is the main purpose of an SLA?

A To define the technical architecture

B To write code for the service

C To monitor daily system logs

D To set formal service expectations between provider and customer

An SLA sets formal expectations and agreements on service levels.

If a service has an SLO of 99.9% uptime, what does this mean?

A The service is free for 99.9% of users

B The service will fail 99.9% of the time

C The service must be available 99.9% of the time

D The service has 99.9% error rate

An SLO of 99.9% uptime means the service should be available 99.9% of the time.

Which of the following is NOT part of SLA, SLO, and SLI definitions?

A Service Level Implementation

B Service Level Objective

C Service Level Indicator

D Service Level Agreement

Service Level Implementation is not a standard term; SLA, SLO, and SLI are the correct terms.

Explain the differences and relationships between SLA, SLO, and SLI in simple terms.

Think of SLA as the contract, SLO as the goals, and SLI as the measurements.

Why is it important for a service provider to monitor SLIs regularly?

Monitoring SLIs helps check if the service is doing what it promised.

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