Shopify Functions overview

Like a typical function you define an input, write the logic of your function, and then write out the output. Let’s dissect how each of these work:

• Input: The function input is a subset of the Cart API and is written as a GraphQL query. Functions can also query metafields so you can store necessary data and retrieve it.
• Function logic: The function logic can be written in any language that can compile a WebAssembly module which meets function requirements. Function templates and client libraries are available for JavaScript and Rust.
• Output: The function output is a payload that conforms to a GraphQL schema relevant to the specific domain. You can learn more about specific schemas in the Functions API reference.

The following diagram shows an example lifecycle of a Shopify Function:

• App developers create and deploy apps that contain functions.

• Merchants install the app on their Shopify store and configure the function. An API call is made with the function configuration.

• Customers interact with a Shopify store and Shopify executes the function.

For example, an app developer might create and deploy an app with a function that defines a new discount type. The merchant can then install the app on their Shopify store and create a new discount from a discount type provided by the app. Shopify executes the function to calculate the discount when a customer adds a product to their cart.

Shopify Functions are never invoked directly by URL or otherwise. Shopify invokes them as-needed within the customer journey.

Learn how to use Shopify Functions by following one of our use case tutorials:

Discounts

Use Shopify Functions to create a new discount type for users.

Payment customizations

Use Shopify Functions to hide a payment option offered to customers at checkout.

Delivery customizations

Use Shopify Functions to rename a delivery option offered to customers at checkout.

Cart and checkout validations

Use Shopify Functions to block progress on a checkout when the cart line quantities exceed a limit.

Order routing location rules

Use Shopify Functions to choose a different order location during checkout.

Customized bundles

Use Shopify Functions to group products together and sell them as a single unit.

Fulfillment constraints

Use Shopify Functions to customize fulfillment and delivery strategies.

Explore the developer tools and resources available for Shopify Functions.

Shopify Function APIs reference

Learn about the available APIs for Shopify Functions.

Language support

Learn about the language support and tooling that are available in Shopify Functions.

The following limitations and considerations apply to all functions:

• Apps can reference only their own functions in GraphQL Admin API mutations, such as discountAutomaticAppCreate and cartTransformCreate. Referencing a function from another app results in a Function not found error.
• Shopify doesn't allow nondeterminism in functions, which means that you can't use any randomizing or clock functionality in your functions.
• The compiled function can't exceed 256 kB in size.
• You can't debug your function by printing out STDOUT, which is reserved for function output. Use STDERR instead.

• Function logs to STDERR are truncated after 1 kB. Some Wasm toolchains might crash if STDERR fails to write full logs.

• The logic of the function must execute in a maximum of 11 million instructions, which can be tested locally using function-runner.

• At runtime, a function can't exceed the following memory limits:

  • Linear memory limit: 10 MB
  • Stack memory limit: 512 kB

• A function's input cannot exceed 64,000 bytes in size.

  Refer to input query limitations for more information.

• A function's output can't exceed 20 kB in size.

• An app can deploy a maximum of 50 functions.

• Network access isn't supported. For example, you can't send HTTP requests to your app's backend as part of the function's logic. However, you can pre-populate data by using metafields on products and customers, or passing data using cart attributes.

• Learn about how data is input to and output from Shopify Functions.
• Explore the references for each Function API.