What to tests?

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• 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?