What to tests?

• Happy path

• Sad path

• Valid input

• Valid edge cases (0, -1, empty string, etc.)

• Broken input (string instead of number, invalid values, too long input, etc.)

• Extreme load

• System failure (power failure, network outage, lack of memory, lack of disk, ...)

• Third-party error or failure - How does your system work if the 3rd party API returns rubbish?