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Comment by bijowo1676

1 hour ago

not the best design to have 1000 rows for each shop*SKU combination. If a candidate proposed this solution during Shopify's System Design interview, i doubt he would be vetted for Senior+ position.

Instead of having 1000 rows per shop*SKU, why not just have one row per shopping cart*SKU?

That way a single row would represent a single cart, and will hold info of multiple items of the same SKU.

No need a cludge with 1000 rows limit and replenishment process. Instead of dealing with N rows, you always deal with a single row.

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bijowo1676

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soontimes  1 hour ago

> Instead of having 1000 rows per shopSKU, why not just have one row per shopping cartSKU?

At what point that row is inserted?

  • bijowo1676  1 hour ago

    per my reading of the article, the protection is only needed for a few seconds, while payment is being processed by the payment system.

    so the row is inserted when Payment is initiated, and row is deleted when Payment succeeds

      What is oversell protection?
  Reserve: When payment starts, we mark items as reserved (a short hold, e.g. several minutes).
  Claim: When payment succeeds, we permanently deduct quantity from the inventory ledger (source of truth).

    but that system could be easily improved to reserve item when user Adds item to a cart, to prevent scenario when user adds item to a cart, goes through checkout, and after initiating payment gets "soldout error":

      1. Let user add item to a cart by default (happy path)
  2. Initiate async check in the background for SKU and quantity
  2a. The check sums up rows for all SKUs and compares to Inventory table (very cheap check since its done to only active shopping carts)
  3. After few seconds the check comes back, and we let user know that item is soldout, before/the moment user goes to Checkout.

    • soontimes  42 minutes ago

      Ok, but before inserting you must ensure that inventory is not depleted, which means you need to know the count and you need to lock the row. So you still have contention on that item. Them having a 1k buffer allows not to take a lock on a single row every time, and only do it when buffer is empty

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