Technical Explanation

We’ve had a few questions regarding the inner workings of Reserved Stock Pro (RSP) – This doc is a deep dive for advanced users.

How does Reserved Stock Pro work?

Our Reserved Stock Pro plugin uses a custom database table called rsp_reserved_stock to track stock reservations. We’ve decided to use a custom database table to provide the best performance and flexibility possible. Other plugins may store reservations via product meta.

What’s tracked in the database table?

There are a few columns with data for tracking reservations.

  • customer_id – The customer ID will be an auto generated ID for guests. If the customer is logged in, it it will be their WordPress user ID.
  • product_id – The ID of the product that has been reserved.
  • expiration – The timestamp when the reservation expires. This timestamp may be updated as the customer updates their cart.
  • order_id – If the customer proceeds through the checkout, WooCommerce will create a pending order. Reserved Stock Pro uses the order ID to track when WooCommerce reduces the product stock for the order. This happens at different times depending on the payment gateway.

How are stock reservations tracked?

The stock reservations are tied to the customer ID and expiration date. Therefore, only the customers who have reserved the product stock can purchase the product.

As a simplified explanation, the plugin compares the stock quantity and the reserved quantity to check what’s available for other customers. If there’s no available stock left, then other customers cannot add the product to cart.

Depending on your theme, the “Add to cart” buttons will be disabled. If the customer can still click the “Add to cart” button, the request goes through the Reserved Stock Pro validation and a customizable notice will be displayed letting the customer know that there is no available stock left.

What about concurrency?

Concurrency means multiple computations are happening at the same time. We are interested in what happens when WooCommerce has multiple customers trying to purchase the same product at the same time.

To mitigate concurrency issues the query used to check and reserve stock is performed in a single, atomic operation.

This operation locks the tables so that separate processes do not fight over the same stock. If there were two simultaneous requests for the same stock at the same time, one would succeed, and one would fail.

When do reservations expire?

Reserved Stock Pro checks that the current time is less than the customer’s expiration time that’s saved in the database. That allows for quick database queries to find if reservations have expired for the current customer, as well as to find the amount of stock that’s reserved for a specific product.

Reservations are cleaned up early from the database when different actions occur, such as:

  • Product has been removed from the cart.
  • Product was purchased.
  • Product quantity in cart has changed.
  • Customer loaded the site after the product expiration date.

Any other expired reservations in the database will automatically be deleted every 12 hours using a WP Cron Job. The scheduled cron job is called rsp_reserved_stock_twice_daily.

Validation to prevent overselling

Reserved Stock Pro reserves product stock only for simple & variable products. Those products must have stock management enabled, otherwise there is no stock to reserve in the first place.

Reserved Stock Pro includes validation at every step in the WooCommerce purchasing process to determine whether or not the customer can add the product to their cart and purchase.

Actions that trigger validation:

  • Attempting to add a product to cart.
  • Updating quantity of a product in cart.
  • Loading the cart from session (every time a page loads).
  • Attempting to checkout.

Reasons validation may fail:

  • All stock has been reserved by other customers.
  • All stock is sold out.
  • The customer already holds the last stock in their cart.
  • Current product reservation has expired.
  • Customer doesn’t have the product reserved.

Guest customers can log in

Guest customers are anyone who has added a product to their cart and haven’t logged in. Their cart products are reserved under the WooCommerce auto-generated guest ID. Reserved Stock Pro will handle transfering the reservations to an actual WordPress user ID if the customer logs in at anytime or creates a new account on the checkout,

Pending Payments

WooCommece will create orders with the status of “pending” depending on the payment gateway. One of those gateways is the PayPal Checkout gateway, which we’ll use for the below example.

When a customer is at the last step of the checkout process, they will click “Pay via PayPal” and 3 things will happen:

  • The customer will be redirected to finalize the payment on PayPal’s website.
  • WooCommerce creates a “Pending” order.
  • Reserved Stock Pro tracks the newly created order ID and extends the reservation time using the “Hold Stock Minutes” setting in WooCommerce.

Why extend the reservations?

  • The customer may cancel the payment to choose a different payment option.
  • The customer may browse the site in a different tab while finishing the payment.
  • The PayPal IPN (Instant Payment Notification) can be slow to mark orders as paid.

When is stock actual stock reduced?

Reserved Stock Pro keeps track of reservations in the custom database table and therefore doesn’t modify the stock inventory in the product’s meta data.

WooCommerce will reduce the product’s stock when the order status changes to “On Hold”, “Processing” or “Completed”. Reserved Stock Pro will delete the reservation for the customer as soon as the customer’s order reduces the actual product stock.

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