System Design

Question 1

What is latency?

Answer 1

How LONG it takes a packet to travel from one computer on the internet to another.

Average ‘ping’ speed

Question 2

What units is latency measured in?

Answer 2

Milliseconds (ms)

Question 3

At what latency might you start to notice a lag? (What’s good latency?)

Answer 3

Anything under 100ms should be good but it depends on the application.

For gaming, it’s suggested less than 50ms and ideally less than 30ms.

Average speeds faster than 50ms are best for most purposes.

Question 4

What is network throughput?

Answer 4

How MUCH data can be transferred from source to destination in a given time.

Question 5

What is database throughput?

Answer 5

How MANY reads/writes can be handled in a given time frame (often seconds).

Question 6

How many reads and writes / second can a typical RDBMS handle?

Answer 6

Reads/second: 10K

Writes/second: 5K

Question 7

How many reads and writes / second can a typical Distributed Cache handle?

Answer 7

Reads/second: 100K

Writes/second: 100K

Question 8

How many reads and writes / second can a typical Message Queue handle?

Answer 8

Reads/second: 100K

Writes/second: 100K

Question 9

How many reads and writes / second can a typical NoSQL database handle?

Answer 9

Reads/second: 20K - 50K

Writes/second: 10K - 25K

Question 10

What is the average max effective capacity of a RDBMS instance?

Answer 10

3TB

Question 11

What is the average max effective capacity of a Distributed Cache?

Answer 11

16GB - 128GB

Question 12

How many bits does the extended-ASCII character set use and how is it different from the basic ASCII set?

Answer 12

The extended-ASCII character set uses 8 bits (256 possible characters).
The basic ASCII character set uses 7 bits (128 possible characters).

The extended-ASCII character adds foreign language characters.

Question 13

How many bytes to store one extended-ASCII character in a text file (e.g. ‘A’)?

Answer 13

8 bits = 1 byte

Question 14

How many bytes to store ‘Monday’?

Answer 14

6 bytes

Question 15

How many bytes to store a plain text email?
Assumptions:
- the average length of a word is 5 characters
- the average number of words in an email is 400

Answer 15

Character = 1 byte

Total Characters = 1 * 5 * 400 = 2000 bytes = 2KB

Question 16

How many bytes to store a 64 * 64 pixel GIF?
Assumptions:
- colours can be represented with 3 bytes (one byte each for its red, green and blue values)

Answer 16

Around 12KB

64 * 64 * 3 = 12,288 bytes

Question 17

Estimating Data
-----------------------
How many bytes to store a 3 minute MP3 audio file?
Notes:
- 1 kbit = 125 bytes
- common MP3 quality is 256 kbit/s

Answer 17

Around 6 MB

256 * (60 * 3) * 125 = 5,760,000 bytes = 5.76 MB

Question 18

How many bytes to store a one hour film compressed as MPEG4?

Answer 18

4 GB

Question 19

What are UUIDs?

Answer 19

Universally Unique IDentifiers, that can be generated without the use of a centralized authority.

UUID collisions are extremely unlikely to happen, especially not in a single application space.

All UUIDs are 128 bits in length, but are commonly represented as 32 hexadecimal characters separated by four hyphens:

123e4567-e89b-12d3-a456-426614174000