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Build a fulfillment constraints function

You can use fulfillment constraints to customize fulfillment and delivery strategies throughout the fulfillment process. In this tutorial, you'll use Shopify Functions to create a function which ensures that certain products that are purchased together are always fulfilled from a specific store location.

Anchor to ScenarioScenario

You have three products which can be sold individually. The products are fulfilled from your warehouse location, which has been set as a shipping origin within your shipping profile. You want to group and ship the three products in a special package only when they're purchased together. The grouping of the products should only be assembled at your physical store location in Ottawa.

The function you'll build in this tutorial adjusts the fulfillment location for the three products only when they are purchased together.

Anchor to What you'll learnWhat you'll learn

In this tutorial, you'll learn how to do the following tasks:

  • Generate starter code for Shopify Functions.
  • Use GraphQL to define the input of your function.
  • Deploy functions to the Shopify platform.
  • Setup your app to register your fulfillment constraint rule with your Shopify store.
  • Review logs for your function.

Anchor to RequirementsRequirements

  • You're using API version 2025-07 or higher for your function.

Anchor to Rust-specific requirementsRust-specific requirements

The following requirements are specific to Rust-based development with Shopify Functions.

  • You've installed Rust.

    On Windows, Rust requires the Microsoft C++ Build Tools. Make sure to select the Desktop development with C++ workload when installing the tools.

  • You've installed the wasm32-wasip1 target:

    Terminal

    rustup target add wasm32-wasip1

Anchor to Step 1: Create the fulfillment constraint rule functionStep 1: Create the fulfillment constraint rule function

To create your fulfillment constraint rule function, you can use Shopify CLI to generate a starter function, specify the inputs for your function using an input query, and implement your function logic using Rust.

  1. Navigate to your app directory:

    Terminal

    cd <directory>

  2. Run the following command to create a new fulfillment constraint rule extension:

    Terminal

    shopify app generate extension --template fulfillment_constraints --name my-fulfillment-constraint-function

  1. Choose the language that you want to use. For this tutorial, you should select either Rust or JavaScript.

    Shopify defaults to Rust as the most performant and recommended language choice to stay within the platform limits. For more information, refer to language considerations.

    Terminal

    ? What would you like to work in?
    > (1) Rust
    (2) JavaScript
    (3) TypeScript
    (4) Wasm
Tip

Shopify Functions support any language that compiles to WebAssembly (Wasm), such as Rust, AssemblyScript, or TinyGo. You specify the Wasm template option when you're using a language other than Rust and can conform to the Wasm API. Learn more about the Wasm API.

  1. Navigate to extensions/my-fulfillment-constraint-function:

    Terminal

    cd extensions/my-fulfillment-constraint-function

  2. Replace the contents of src/run.graphql file with the following code:

    run.graphql

    src/run.graphql

    query Input {
    cart {
    deliverableLines {
    id
    merchandise {
    __typename
    ...on ProductVariant {
    id
    product {
    hasAnyTag(tags: ["Promotional candle"])
    }
    }
    }
    }
    }
    locations(names: ["Ottawa Store"]) {
    id
    name
    }
    }
    query RunInput {
    cart {
    deliverableLines {
    id
    merchandise {
    __typename
    ...on ProductVariant {
    id
    product {
    hasAnyTag(tags: ["Promotional candle"])
    }
    }
    }
    }
    }
    locations(names: ["Ottawa Store"]) {
    id
    name
    }
    }
    Tip

    If a store has many locations, then running this query can result in a big input to parse. To optimize performance, consider using the identifier and names input options to fetch only relevant locations.

  3. If you're using JavaScript, then run the following command to regenerate types based on your input query:

    Terminal

    shopify app function typegen

  4. Replace the src/run.rs or src/run.js file with the following code:

    File

    src/run.rs

    use crate::schema;
    use shopify_function::prelude::*;
    use shopify_function::Result;

    #[shopify_function]
    fn run(input: schema::run::Input) -> Result<schema::CartFulfillmentConstraintsGenerateRunResult> {
    // Find deliverable cart line ids of all products with the specific tag.
    let deliverable_line_ids: Vec<String> = input
    .cart()
    .deliverable_lines()
    .iter()
    .filter_map(|deliverable_line| {
    if let schema::run::input::cart::deliverable_lines::Merchandise::ProductVariant(variant) = deliverable_line.merchandise() {
    if *variant.product().has_any_tag() {
    return Some(deliverable_line.id().clone());
    }
    }
    None
    })
    .collect();

    // Short-circuit and return no operations unless we have at least one tagged product.
    if deliverable_line_ids.is_empty() {
    return Ok(no_fulfillment_constraints_result());
    }

    // Find the location representing our Ottawa store.
    let ottawa_location = input
    .locations()
    .iter()
    .find(|location| location.name() == "Ottawa Store");

    // Short-circuit and return no operations if the fulfillment location does not exist.
    if ottawa_location.is_none() {
    return Ok(no_fulfillment_constraints_result());
    }

    // Construct the operations, including our DeliverableLinesMustFulfillFromAddOperation fulfillment constraint.
    let operations = vec![schema::Operation::DeliverableLinesMustFulfillFromAddOperation(schema::DeliverableLinesMustFulfillFromAddOperation {
    deliverable_line_ids: Some(deliverable_line_ids),
    location_ids: vec![ottawa_location.unwrap().id().clone()],
    })];

    // Return the operation.
    Ok(schema::CartFulfillmentConstraintsGenerateRunResult { operations })
    }

    fn no_fulfillment_constraints_result() -> schema::CartFulfillmentConstraintsGenerateRunResult {
    schema::CartFulfillmentConstraintsGenerateRunResult { operations: vec![] }
    }
    // @ts-check

    /**
    * @typedef {import("../generated/api").RunInput} RunInput
    * @typedef {import("../generated/api").CartFulfillmentConstraintsGenerateRunResult} CartFulfillmentConstraintsGenerateRunResult
    */

    /**
    * @type {CartFulfillmentConstraintsGenerateRunResult}
    */
    const NO_CHANGES = {
    operations: [],
    };

    /**
    * @param {RunInput} input
    * @returns {CartFulfillmentConstraintsGenerateRunResult}
    */
    export function run(input) {
    let deliverableLineIds = [];

    // Find deliverable cart line ids of all products with the specific tag.
    for (const deliverableLine of input.cart.deliverableLines) {
    if (deliverableLine.merchandise.__typename == "ProductVariant" &&
    deliverableLine.merchandise.product.hasAnyTag) {

    deliverableLineIds.push(deliverableLine.id);
    }
    }

    // Short-circuit and return no operations unless we have at least three tagged products.
    if (deliverableLineIds.length < 3) { return NO_CHANGES; }

    // Find the location representing our Ottawa store.
    let ottawaLocation = input.locations.find(location => location.name == "Ottawa Store");

    // Short-circuit and return no operations if the fulfillment location does not exist.
    if (ottawaLocation === undefined) { return NO_CHANGES; }

    // Construct the operations, including our DeliverableLinesMustFulfillFromAddOperation fulfillment constraint.
    let operations = [
    {
    deliverableLinesMustFulfillFromAddOperation: {
    deliverableLineIds: deliverableLineIds,
    locationIds: [ottawaLocation.id]
    }
    }
    ];

    // Return the operation.
    return { operations: operations };
    };

  5. If you're using Rust, then build the function's Wasm module:

    Terminal

    cargo build --target=wasm32-wasip1 --release

Anchor to Step 2: Preview the function on a development storeStep 2: Preview the function on a development store

To test your function, you need to make it available to your development store.

  1. If you're developing a function in a language other than JavaScript or TypeScript, ensure you have configured build.watch in your function extension configuration.

  1. Navigate back to your app root:

    Terminal

    cd ../..

  1. Use the Shopify CLI dev command to start app preview:

    Terminal

    shopify app dev

    You can keep the preview running as you work on your function. When you make changes to a watched file, Shopify CLI rebuilds your function and updates the function extension's drafts, so you can immediately test your changes.

  2. Follow the CLI prompts to preview your app, and install it on your development store.

Anchor to Step 3: Test the fulfillment constraint ruleStep 3: Test the fulfillment constraint rule

You can test your fulfillment constraint rule to ensure it's working as expected, and review logs for your function.

  1. Find the ID of your function by executing the following query:

    find-function-query.graphql

    query {
    shopifyFunctions(first: 25) {
    nodes {
    app {
    title
    }
    apiType
    title
    id
    }
    }
    }

    The result contains a node with your function's ID:

    find-function-result.json

    {
    "app": {
    "title": "your-app-name-here"
    },
    "apiType": "fulfillment_constraints",
    "title": "my-fulfillment-constraint-function",
    "id": "YOUR_FUNCTION_ID_HERE"
    }

  1. From within your app, use the ID of your function to invoke the fulfillmentConstraintRuleCreate mutation to register the FulfillmentConstraintRule. The app needs to be the same app that deployed the function and has the write_fulfillment_constraint_rules access scope. You can check out this doc on how to set access scopes for the app.

  2. From your development store's admin, set up the Ottawa Store location along with three or more products with the Promotional candle tag.

  3. Open your development store and build a cart containing three or more of the products you created, and then proceed through checkout.

  4. Open your development store's admin, and find your new order. The products should be assigned to your Ottawa Store location. If you change one of the product's inventory locations, then checkout won't return any shipping rates and buyers won't be able to complete checkout.

    All products have been assigned to the Ottawa Store location

  1. Open your terminal where shopify app dev is running, and review your function executions.

    When testing functions on development stores, the output of dev includes executions of your functions, any debug logging you have added to them, and a link to a local file with the full function execution details.

  2. In a new terminal window, use the Shopify CLI app function replay command to replay a function execution locally, and debug your function without the need to re-trigger the function execution on Shopify.

Terminal

shopify app function replay
  1. Select the function execution from the top of the list. Press q to quit when you are finished debugging.

Anchor to Next stepsNext steps

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