DCB Example: Product price change with grace period

example.js

/*
This example demonstrates how a product price change can be rolled out with grace period for customers to order it for the previous price
*/


// little helper function to generate dates X minutes ago
const minutesAgo = (minutes) => new Date((new Date).getTime() - minutes * (1000 * 60));

const events = [
  {
    type: "PRODUCT_DEFINED",
    data: { id: "p1", price: 123 },
    tags: ["product:p1"],
    recordedAt: minutesAgo(15),
  },
  {
    type: "PRODUCT_PRICE_CHANGED",
    data: { id: "p1", newPrice: 130 },
    tags: ["product:p1"],
    recordedAt: minutesAgo(10),
  },
  {
    type: "PRODUCT_PRICE_CHANGED",
    data: { id: "p1", newPrice: 145 },
    tags: ["product:p1"],
    recordedAt: minutesAgo(5),
  },
];


const productPriceDecisionModel = (productId) => {
  const projection = {
    $init: () => ({currentPrice: null, previousPrice: null, lastChanged: null}),
    PRODUCT_DEFINED: (state, event) => ({...state, currentPrice: event.data.price, lastChanged: event.recordedAt}),
    PRODUCT_PRICE_CHANGED: (state, event) => ({...state, currentPrice: event.data.newPrice, previousPrice: state.currentPrice, lastChanged: event.recordedAt}),
  };
  return events
    .filter(event => event.tags.includes(`product:${productId}`))
    .reduce((state, event) => projection[event.type]?.(state, event) ?? state, projection.$init?.());
};

// The actual example:

const purchaseProduct = (productId, displayedPrice) => {
  const productPrice = productPriceDecisionModel(productId);
  if (displayedPrice !== productPrice.currentPrice && displayedPrice !== productPrice.previousPrice) {
    throw new Error(`invalid price ${displayedPrice}`)
  }
  if (displayedPrice === productPrice.previousPrice) {
    // number of minutes before a changed product price is enforced
    const priceGracePeriodInMinutes = 10;
    if ((new Date - productPrice.lastChanged) / (1000 * 60) > priceGracePeriodInMinutes) {
      throw new Error(`price ${displayedPrice} refers to a price that was changed more than ${priceGracePeriodInMinutes} minutes ago`)
    }
  }
  // success -> process purchase...
}

for (const displayedPrice of [123, 130, 145]) {
  try {
    purchaseProduct('p1', displayedPrice);
    console.log(`purchase with displayedPrice ${displayedPrice} succeeded`)
  } catch (e) {
    console.error(`purchase with displayedPrice ${displayedPrice} failed: ${e.message}`)
  }
}

The output of this example would be:

'purchase with displayedPrice 123 failed: invalid price 123'
'purchase with displayedPrice 130 succeeded'
'purchase with displayedPrice 145 succeeded'

And, if priceGracePeriodInMinutes in line 51 was changed to some value below 5 (e.g. 3):

'purchase with displayedPrice 123 failed: invalid price 123'
'purchase with displayedPrice 130 failed: price 130 refers to a price that was changed more than 3 minutes ago'
'purchase with displayedPrice 145 succeeded'

Of course, this example can just as well be done with "classical" Event Sourcing, but we can easily adjust the example to work with multiple products (i.e. shopping cart) at once now