Throughput, latency, and availability in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is throughput in system design?

Throughput is the amount of work or number of tasks a system can complete in a given time, like how many requests a server can handle per second.

beginner

Define latency in the context of system performance.

Latency is the time delay between sending a request and receiving a response, similar to how long it takes for a letter to arrive after mailing it.

beginner

What does availability mean in system design?

Availability is the percentage of time a system is operational and accessible, like a store being open for customers most of the day.

intermediate

How are throughput and latency related but different?

Throughput measures how many tasks a system can do over time, while latency measures how fast each individual task is completed. A system can have high throughput but also high latency if tasks queue up.

beginner

Why is high availability important for user experience?

High availability ensures users can access the system whenever they need it, preventing frustration and loss of trust, much like a reliable service that is always open when customers want it.

What does throughput measure in a system?

A Percentage of uptime

B Time delay for a single task

C Number of tasks completed per unit time

D Amount of data stored

Throughput is about how many tasks or requests a system can handle in a given time.

Latency is best described as:

A Total tasks completed per second

B Time taken to respond to a single request

C System uptime percentage

D Number of users connected

Latency measures the delay between sending a request and receiving a response.

Which metric indicates how often a system is operational?

A Latency

B Throughput

C Bandwidth

D Availability

Availability shows the percentage of time the system is up and running.

If a system has high throughput but high latency, what does it mean?

A It processes many tasks but each takes a long time

B It processes few tasks quickly

C It is often down

D It has low capacity

High throughput with high latency means many tasks are done overall, but each task waits longer.

Why is availability critical for online services?

A It ensures users can access the service anytime

B It increases throughput

C It reduces latency

D It stores more data

Availability ensures the service is accessible whenever users need it.

Explain throughput, latency, and availability using a real-life example.

Think about how a busy store works and how fast customers get served.

How do throughput, latency, and availability impact user experience in a web application?

Consider what happens if a website is slow, busy, or down.

Practice

1. Which metric best describes how many requests a system can handle per second?

easy

A. Throughput

B. Latency

C. Availability

D. Scalability

Solution

1. Step 1: Understand throughput meaning

   Throughput measures the amount of work or requests a system can process in a given time.

2. Step 2: Compare with other metrics

   Latency measures response time, availability measures uptime, scalability is about growth capacity.

3. Final Answer:

   Throughput -> Option A

4. Quick Check:

   Throughput = Requests per second [OK]

Hint: Throughput = how much work done per time [OK]

Common Mistakes:

• Confusing latency with throughput
• Mixing availability with throughput
• Thinking scalability equals throughput

2. Which of the following correctly defines latency in system design?

easy

A. The total number of requests processed per second

B. The time taken to respond to a single request

C. The percentage of time the system is operational

D. The ability to add more servers to handle load

Solution

1. Step 1: Define latency

   Latency is the delay or time taken for a system to respond to one request.

2. Step 2: Eliminate other options

   The total number of requests processed per second describes throughput. The ability to add more servers to handle load describes scalability. The percentage of time the system is operational describes availability.

3. Final Answer:

   The time taken to respond to a single request -> Option B

4. Quick Check:

   Latency = Response time [OK]

Hint: Latency = response time per request [OK]

Common Mistakes:

• Confusing latency with throughput
• Thinking availability is latency
• Mixing scalability with latency

3. A system processes 1000 requests in 10 seconds. Each request takes 5 milliseconds to respond. What is the system's throughput and average latency?

medium

A. Throughput: 1000 requests/sec, Latency: 5 ms

B. Throughput: 100 requests/sec, Latency: 50 ms

C. Throughput: 10 requests/sec, Latency: 5 ms

D. Throughput: 100 requests/sec, Latency: 5 ms

Solution

1. Step 1: Calculate throughput

   Throughput = total requests / total time = 1000 / 10 = 100 requests per second.

2. Step 2: Understand latency

   Latency is given as 5 milliseconds per request.

3. Final Answer:

   Throughput: 100 requests/sec, Latency: 5 ms -> Option D

4. Quick Check:

   Throughput = 100, Latency = 5 ms [OK]

Hint: Throughput = total requests ÷ time; latency given directly [OK]

Common Mistakes:

• Dividing latency by number of requests
• Confusing throughput units
• Multiplying latency by total requests

4. A system claims 99.9% availability but is down for 10 minutes in a day. What is the actual availability and is the claim correct?

medium

A. Actual availability is 99.31%, claim is incorrect

B. Actual availability is 99.93%, claim is correct

C. Actual availability is 99.93%, claim is incorrect

D. Actual availability is 99.31%, claim is correct

Solution

1. Step 1: Calculate total minutes in a day

   24 hours x 60 minutes = 1440 minutes per day.

2. Step 2: Calculate availability

   Availability = (Total time - downtime) / Total time = (1440 - 10) / 1440 = 1430 / 1440 ≈ 0.9931 or 99.31%.

3. Step 3: Compare with claim

   Claim is 99.9%, actual is 99.31%, so claim is incorrect.

4. Final Answer:

   Actual availability is 99.31%, claim is incorrect -> Option A

5. Quick Check:

   Availability = 99.31%, Claim wrong [OK]

Hint: Availability = (uptime ÷ total time) x 100% [OK]

Common Mistakes:

• Using hours instead of minutes
• Incorrectly subtracting downtime
• Misreading percentage claim

5. You design a web service that must handle 10,000 requests per second with an average latency under 100 ms and 99.99% availability. Which design choice best balances these requirements?

hard

A. Use a single powerful server with caching to reduce latency

B. Use a database with strong consistency but no replication

C. Distribute load across multiple servers with load balancers and health checks

D. Optimize only latency by increasing CPU speed on one server

Solution

1. Step 1: Analyze throughput and availability needs

   10,000 requests/sec and 99.99% availability require scaling and fault tolerance.

2. Step 2: Evaluate design options

   Using a single powerful server with caching or optimizing CPU speed on one server risks overload and downtime. Using a database with strong consistency but no replication risks availability due to lack of fault tolerance. Distributing load across multiple servers with load balancers and health checks supports scaling and high availability.

3. Final Answer:

   Distribute load across multiple servers with load balancers and health checks -> Option C

4. Quick Check:

   Scaling + availability = load balancer + health checks [OK]

Hint: Scale horizontally with load balancers for throughput and availability [OK]

Common Mistakes:

• Relying on single server for high throughput
• Ignoring replication for availability
• Focusing only on latency without scaling