medusa-store/docs/services/CartService.md

Cart Service

In Medusa a cart is where a customer can add items that they intend to purchase later. The Cart service is responsible for making the operations necessary for handling items in the cart and completing a checkout.

Creating carts

When to create carts

You can create a cart when the user enters the site or when they first add something to the cart.

If you decide to wait with cart creation until the customer adds something to the cart you have to make sure that the customer sees correct prices/products/etc. for the region they are shopping in. You will also have to ensure that you make a POST /cart call before you call POST /cart/line-items.

This can be circumvented by creating the cart as soon as the user enters your site. You will then be able to keep the region data in the cart, which will be ready for any POST /cart/line-items. This has the trade-off of creating many unnecessary carts, but is easier to implement.

Shopping

Adding to the cart

The typical user journey is that a customer adds an item to her Cart this usually happens with POST /cart/line-items. The endpoint takes a variant_id and a quantity. The endpoint controller will then retrieve the variant and the product it is associated with, compile a valid LineItem and use the CartService function addLineItem to create the line item on the cart.

Updating line items

Updating line items happens with POST /cart/line-items/[line-id]. The endpoint takes quantity to update the line item's quantity. A quantity of 0 results in a removal of the line item. The endpoint controller uses updateLineItem to update the line item.

Removing line items

Deleting line items happens with DELETE /cart/line-items/[line-id]. Endpoint removes the line item with the given line id by calling removeLineItem.

Custom add to cart

It is possible to make custom endpoints to add to cart. For example if you are creating a gift card plugin you may want to create a custom endpoint that accepts values like amount which can be set by the customer. As long as the controller compiles a valid LineItem it can safely call addLineItem

Checking out

Initializing the checkout

When the customer is ready to check out they will reach your checkout page. At this point you want to display which payment and shipping options are offered. POST /cart/payment-sessions and POST /cart/fulfillment-sessions will initialize payment sessions and shipping methods offered by fulfillment providers. The payment sessions will typically have to be updated if any further changes to the cart total happens (shipping fee, promo codes, user exits checkout and adds more items to cart and returns to checkout). Medusa will make sure to update any payment sessions that have already been initialized.

In cases where shipping rates can vary based on order size, shipping address (with higher granularity than region) makes sense to call POST /cart/fulfillment-providers multiple times, e.g. if the customer updates their shipping address and new shipping rates have to be calculated. On each call the endpoint will make sure to only return relevant shipping methods.

Note that POST /cart/payment-providers should not be used to fetch available payment providers, only to initialize payment sessions. If you want to display the available payment providers to the customer you can find an array of these in region.payment_providers.

Getting customer data

To complete a purchase the customer has to fill in her details. The endpoints to do this are POST /cart/email, POST /cart/shipping-address and POST /cart/ billing-address. The endpoint controllers use the corresponding Cart service methods (updateEmail, updateShippingAddress, updateBillingAddress) to store the customer information in the cart.

Handling payments and completing orders

Authorization of payments happen with the payment provider. As such the typical payment flow will be:

1. the customer enters payment details
2. details are sent to the payment provider
3. the payment provider authorizes the payment
4. the payment method is store in the cart
5. the order is processed.

Steps 4. and 5. are handled with one API call in Medusa. Once the payment is authorized you call POST /order. The endpoint takes cart_id and payment_provider_id as data. The controller will call the Cart service function setPaymentMethod which will fetch the cart, search the payment_sessions array for the payment associated with payment_provider and check that the payment session is authorized. When the authorization is verified the payment method is set and the controller can safely call the Order service function create.

How do payment sessions work?

When the customer first enters the checkout page you should initialize payment sessions for each of the possible payment providers. This is done with a single call to POST /cart/payment-sessions. Calls to POST /cart/payment-sessions will either create payment sessions for each payment provider or if the payment sessions have already been initialized ensure that the sessions are up to date ( i.e. that the cart amount corresponds to the payment sessions' amounts). When the customer reaches the payment part of the checkout (or alternatively when she decides to use one of the payment providers as a checkout provider) the payment method will be saved once authorized.

How do fulfillment sessions work?

When the customer first enters the checkout page, fulfillment sessions should be initialized. The fulfillment session is responsible for fetching shipping options with a fulfillment provider. E.g. your store has an integration with your 3PL as a fulfillment provider. The 3PL has 4 shipping options: standard, express and fragile shipping as well as a parcel shop service where orders are delivered to a local store.

The store operator will have set up which shipping options are available in the customer's region. I.e. the store operator may have created a shipping option called Free Shipping, which uses the "Standard Shipping" method from the 3PL integration, and which is free when the order value is above 100 USD. The store operator may have also created an Express Shipping option which uses the "Express Shipping" method from the 3PL integration and which costs 20 USD. The store operator has also created a Fragile Shipping option which uses the "Fragile Shipping" method from the 3PL integration and which has variable pricing depending on the size of the shipment. The variable pricing is calculated by the integration depending on cart. Finally, the store operator has defined a parcel shop option, which uses the 3PL's parcel shop shipping method. The customer needs to provide the ID of the local store that she wants her order delivered to and the shipping method therefore takes some additional input to be a valid shipping method for an order.

When the customer enters the checkout page the POST /cart/shipping-options call will fetch each of the shipping options that the store operator has set up. Extending the above example, an array of shipping options would be stored in the cart in the format:

[
  {
    _id: [some-id],
    provider_id: "3pl_integration",
    name: "Free Shipping",
    price: 0,
    data: {
      // This will contain data specific to the shipping method, i.e. the 
      // id that the 3PL needs in order to process the order with this shipping
      // method
    }
  },
  {
    _id: [some-id],
    provider_id: "3pl_integration",
    name: "Express Shipping",
    price: 20,
    data: {
      // This will contain data specific to the shipping method, i.e. the 
      // id that the 3PL needs in order to process the order with this shipping
      // method
    }
  },
  {
    _id: [some-id],
    provider_id: "3pl_integration",
    name: "Fragile Shipping",
    price: 120, // Calculated from the cart
    data: {
      // This will contain data specific to the shipping method, i.e. the 
      // id that the 3PL needs in order to process the order with this shipping
      // method
    }
  },
  ...
]

If the customer changes her cart, all shipping options will be recalculated. For example, if the customer removes something from the cart so that they no longer qualify for free shipping, the free shipping method will be removed at the same time the fragile shipping method's price will be updated.