THE FREE ENGINEERING LAB

Good engineering
starts with evidence.

Make a request. Find the bug. Prove the fix.
Learn the testing habits that make software more dependable.

No payment required Work at your own pace About 40 minutes

API FOUNDATIONS

…/ 03 lessons

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LESSON 03 / 03 · 15 MIN

Build a release evidence pack

Combine positive, negative, and regression checks into a reviewable result.

Your team adds a discount code. The feature is small, but it touches existing price calculations and input validation. A release review should show what you checked, what failed, and what remains outside the scope of the test.

LEARN10 reduces the subtotal by ten percent, and the API treats coupon letters without regard to case. Two items cost 5,000 cents before the discount and 4,500 after it. An unknown coupon must return INVALID_COUPON. A good suite checks the feature, protects a normal order, and retains an important boundary test. Integer cents keep this exercise free from floating-point currency surprises.

Read the API contract POST /sandbox/quote

Teaching simulator: requests run entirely in your browser. They do not contact a server, create orders, or take payments.

quantity: Required integer, 1–10 inclusive. Missing, fractional, or below 1: 400 INVALID_QUANTITY. Above 10: 400 QUANTITY_LIMIT.
coupon: Optional. Omit or leave empty for no discount. LEARN10 gives 10% off and ignores letter case. Any other value: 400 INVALID_COUPON.
Success: 200 with currency, quantity, unitPrice, subtotal, discount, and total. Money uses USD cents. Unit price is 2500; total = subtotal − discount.
Validation order: Quantity type and minimum, then maximum, then coupon. Additional request fields are ignored.
Builds: Reference follows this contract. Candidate intentionally accepts 0, rejects 10, and checks coupon letters case sensitively.

YOUR CHALLENGE

Repair the lowercase-coupon total, run the five-case reference suite, and export the evidence for a review.

Read every test name and compare its expected result with the contract.
Repair the learn10 case: it should produce the same total as LEARN10.
Run all five cases on the reference build, keeping the normal order, invalid coupon, and maximum-quantity checks.
Download the run report. Its scope is this local simulator; it does not prove a production payment service is ready.

Your workbench

LOCAL SIMULATOR

Select a buildMatches the documented API contract.

01 Explore a requestTry an input. Inspect the response.

POST /sandbox/quote

quantity JSON value · empty means missingcoupon optional

RESPONSE

// Send a request to inspect
// its actual response here.

// Nothing leaves your browser.

Write your assertions

JSON data. Match the status and the body fields that matter.

JSON assertion suite for Build a release evidence pack

[
  {
    "name": "A normal order still works",
    "request": {
      "quantity": 2
    },
    "expect": {
      "status": 200,
      "body": {
        "total": 5000
      }
    }
  },
  {
    "name": "LEARN10 gives ten percent off",
    "request": {
      "quantity": 2,
      "coupon": "LEARN10"
    },
    "expect": {
      "status": 200,
      "body": {
        "total": 4500
      }
    }
  },
  {
    "name": "Coupon matching ignores letter case",
    "request": {
      "quantity": 2,
      "coupon": "learn10"
    },
    "expect": {
      "status": 200,
      "body": {
        "total": 5000
      }
    }
  },
  {
    "name": "Unknown coupons are rejected",
    "request": {
      "quantity": 2,
      "coupon": "UNKNOWN"
    },
    "expect": {
      "status": 400,
      "body": {
        "error": {
          "code": "INVALID_COUPON"
        }
      }
    }
  },
  {
    "name": "Maximum quantity remains supported",
    "request": {
      "quantity": 10
    },
    "expect": {
      "status": 200,
      "body": {
        "total": 25000
      }
    }
  }
]

1–12 cases · status is required · body fields are partial matches

03 / REVIEW THE EVIDENCE

What did your tests prove?

Run your suite to see the result of every assertion.

The checklist below measures coverage as well as correctness.

Required evidence

Protect a normal order: quantity 2, no coupon, total 5000

Check LEARN10 on quantity 2 gives total 4500

Check learn10 on quantity 2 also gives total 4500

Reject coupon UNKNOWN with status 400 and INVALID_COUPON

Keep the maximum order boundary: quantity 10, total 25000

Run a reference suite with every assertion passing

TAKE THIS WITH YOU

A release report is an argument supported by evidence. Name the build and checks, retain the failures, and state the limits. Passing these tests does not establish security, accessibility, or real payment correctness.

Complete the evidence checklist to finish this lesson.

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Good engineeringstarts with evidence.

Build a release evidence pack

Your workbench

Write your assertions

What did your tests prove?

Required evidence

Good engineeringstarts with evidence.

Build a release evidence pack

Your workbench

Write your assertions

What did your tests prove?

Required evidence

Good engineering
starts with evidence.

Good engineering
starts with evidence.