You can use location rules to rank the possible locations for a line item during checkout. In this tutorial, you'll use [Shopify Functions](/docs/apps/build/functions) to create a function that prioritizes locations in a particular country.

> Beta:
> Location rules is a new feature that's only available by request. Reach out to [Shopify Plus Support](https://help.shopify.com/en/support/plus) to know more about your eligibility and the requirements for the beta program.


## What 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.
- Use the order routing settings to create a location rule in a Shopify store.
- Review logs for your function.

## Requirements

Refer to [Build location rules](/docs/apps/build/orders-fulfillment/order-routing-apps/location-rules#requirements).

## Step 1: Create the location rule function

To create your location rule function, you can use Shopify CLI to [generate](/docs/api/shopify-cli/app/app-generate-extension) a starter function, specify the inputs for your function using an [input query](/docs/apps/build/functions/input-output/metafields-for-input-queries), and implement your function logic using Rust.

1. Navigate to your app directory:

    <p>
    <div class="react-code-block" data-preset="terminal">
    <div class="react-code-block-preload ThemeMode-dim">
    <div class="react-code-block-preload-bar "></div>
    <div class="react-code-block-preload-placeholder-container">
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    </div>

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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTcd <directory>
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


1. Run the following command to create a new location rule extension:

    <p>
        <div class="react-code-block" data-preset="terminal">
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        <script type="text/plain" data-language="bash">
        RAW_MD_CONTENTshopify app generate extension --template order_routing_location_rule --name location-rule
        END_RAW_MD_CONTENT</script>

        </div>
        </p>


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](/docs/apps/build/functions/programming-languages).


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    </div>


    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENT?  What would you like to work in?
    > (1) Rust
      (2) JavaScript
      (3) TypeScript
      (4) Wasm
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


    > 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](/docs/apps/build/functions/programming-languages/webassembly-for-functions).

1. Navigate to `extensions/location-rule`:

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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTcd extensions/location-rule
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


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

    `run.graphql` defines the input for the function. You need the fulfillment groups, with the available locations for the items in the group.

    The query differs slightly in Rust and JavaScript due to code generation requirements.

    <p>
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    </div>
    </div>

    <script data-option="filename" data-value="run.graphql"></script>

    <script type="text/plain" data-language="graphql" data-title="Rust input query" data-filename="src/run.graphql">
    RAW_MD_CONTENTquery Input {
      fulfillmentGroups {
        handle
        inventoryLocationHandles
      }
      locations {
        handle
        address {
          countryCode
        }
      }
    }
    END_RAW_MD_CONTENT</script>
<script type="text/plain" data-language="graphql" data-title="JavaScript input query" data-filename="src/run.graphql">
    RAW_MD_CONTENTquery RunInput {
      fulfillmentGroups {
        handle
        inventoryLocationHandles
      }
      locations {
        handle
        address {
          countryCode
        }
      }
    }
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


    > Tip:
    > If a store has many locations, then running this query can result in a big input to parse. To optimize performance, consider loading the location in the input and querying the inventory locations by ID.

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

    <p>
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    <div class="react-code-block-preload-bar "></div>
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    </div>

    </div>
    </div>


    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTshopify app function typegen
    END_RAW_MD_CONTENT</script>

    </div>
    </p>



1. Replace the `src/run.rs` or `src/run.js` file with the following code.

    The function logic sorts the locations in your store, putting locations in Canada first.

    <p>
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    <script type="text/plain" data-language="rust" data-title="Rust" data-filename="src/run.rs">
    RAW_MD_CONTENTuse run::output::FulfillmentGroupRankedLocations;
    use shopify_function::prelude::*;
    use shopify_function::Result;
    
    #[shopify_function_target(query_path = "src/run.graphql", schema_path = "schema.graphql")]
    fn run(input: input::ResponseData) -> Result<output::FunctionRunResult> {
        // Load the fulfillment groups and generate the rank operations for each one
        let operations = input
            .fulfillment_groups
            .into_iter()
            .map(|fulfillment_group| output::Operation {
                rank: build_rank_operation(fulfillment_group, &input.locations),
            })
            .collect();
        // Return the operations
        Ok(output::FunctionRunResult { operations })
    }
    
    fn build_rank_operation(
        input: run::input::InputFulfillmentGroups,
        locations: &[run::input::InputLocations],
    ) -> FulfillmentGroupRankedLocations {
        FulfillmentGroupRankedLocations {
            fulfillment_group_handle: input.handle,
            rankings: prioritize_canadian_locations(input.inventory_location_handles, locations),
        }
    }
    
    fn prioritize_canadian_locations(
        handles: Vec<Handle>,
        locations: &[run::input::InputLocations],
    ) -> Vec<run::output::RankedLocation> {
        // Load the inventory locations for the fulfillment group
        handles
            .into_iter()
            .map(|location_handle| {
                let location = locations.iter().find(|&loc| loc.handle == location_handle);
                // Rank the location as 0 if the country code is CA, otherwise rank it as 1
                run::output::RankedLocation {
                    location_handle,
                    rank: match location {
                        Some(location) => {
                            if location.address.country_code == Some("CA".to_string()) {
                                0
                            } else {
                                1
                            }
                        }
                        None => 1,
                    },
                }
            })
            .collect()
    }
    END_RAW_MD_CONTENT</script>
<script type="text/plain" data-language="javascript" data-title="JavaScript" data-filename="src/run.js">
    RAW_MD_CONTENT// @ts-check
    
    /**
    * @typedef {import("../generated/api").RunInput} RunInput
    * @typedef {import("../generated/api").RunInput["fulfillmentGroups"][0]} FulfillmentGroup
    * @typedef {import("../generated/api").RunInput["locations"][0]} Location
    * @typedef {import("../generated/api").FunctionRunResult} FunctionRunResult
    * @typedef {import("../generated/api").Operation} Operation
    * @typedef {import("../generated/api").FulfillmentGroupRankedLocations} FulfillmentGroupRankedLocations
    * @typedef {import("../generated/api").RankedLocation} RankedLocation
    */
    
    /**
    * @param {RunInput} input
    * @returns {FunctionRunResult}
    */
    export function run(input) {
        // Load the fulfillment groups and generate the rank operations for each one
        let operations = input.fulfillmentGroups
          .map(fulfillmentGroup => /** @type {Operation} */(
            {
              rank: buildRankOperation(fulfillmentGroup, input.locations)
            }
          ));
    
        // Return the operations
        return { operations: operations };
    };
    
    /**
    * @param {FulfillmentGroup} fulfillmentGroup
    * @param {Location[]} locations
    * @returns {FulfillmentGroupRankedLocations}
    */
    function buildRankOperation(fulfillmentGroup, locations) {
      return {
        fulfillmentGroupHandle: fulfillmentGroup.handle,
        rankings: prioritizeCanadianLocations(fulfillmentGroup.inventoryLocationHandles, locations),
      };
    };
    
    /**
    * @param {string[]} locationHandles
    * @param {Location[]} locations
    * @returns {RankedLocation[]}
    */
    function prioritizeCanadianLocations(locationHandles, locations) {
      // Load the inventory locations for the fulfillment group
      return locationHandles.map(locationHandle => {
        const location = locations.find((loc) => loc.handle == locationHandle);
        return {
          locationHandle,
          // Rank the location as 0 if the country code is CA, otherwise rank it as 1
          rank: location?.address.countryCode === "CA" ? 0 : 1,
        }
      });
    };
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


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

    <p>
      <div class="react-code-block" data-preset="terminal">
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      <script type="text/plain" data-language="bash">
      RAW_MD_CONTENTcargo build --target=wasm32-wasip1 --release
      END_RAW_MD_CONTENT</script>

      </div>
      </p>


    If you encounter any errors, then ensure that you've [installed Rust and the `wasm32-wasip1` target](#requirements).

## Step 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](/docs/api/functions/configuration#properties).




1. Navigate back to your app root:

    <p>
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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTcd ../..
    END_RAW_MD_CONTENT</script>

    </div>
    </p>



1. Use the Shopify CLI [`dev` command](/docs/api/shopify-cli/app/app-dev) to start app preview:

    <p>
    <div class="react-code-block" data-preset="terminal">
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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTshopify app dev 
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


    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.

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


## Step 3: Test the location rule

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

Before you test the location rule, make sure that you have the following:

- Two [locations](https://help.shopify.com/manual/locations/setting-up-your-locations) in your store in different countries, one of them in Canada.
- One product that is stocked on multiple locations.

1. From the Shopify admin, go to **Settings** > **Shipping and delivery**.
1. In the **Order routing** section, click **Manage**.
1. Click **Add rule**.

    Your new rule should be an available option.

1. Add your new rule and drag it to the top to make it your highest priority location rule.
1. Open your development store.
1. Add products to your cart. At least one product should be stocked at multiple locations, including your Canadian location.
1. Proceed through checkout.
1. Open your development store's admin, and find your new order. This should be assigned to your Canadian location.
1. Open your terminal where `shopify app dev` is running, and review your function executions.

    When [testing functions on development stores](/docs/apps/build/functions/test-debug-functions#test-your-function-on-a-development-store), 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.
1. In a new terminal window, use the Shopify CLI [`app function replay`](/docs/api/shopify-cli/app/app-function-replay) command to [replay a function execution locally](/docs/apps/build/functions/test-debug-functions#execute-the-function-locally-using-shopify-cli), and debug your function without the need to re-trigger the function execution on Shopify.

    <p>
    <div class="react-code-block" data-preset="terminal">
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    <div class="react-code-block-preload-codeline"></div>
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    <script type="text/plain" data-language="bash">
    RAW_MD_CONTENTshopify app function replay
    END_RAW_MD_CONTENT</script>

    </div>
    </p>


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


## Next steps

- [Add configuration](/docs/apps/build/orders-fulfillment/order-routing-apps/location-rules/add-configuration) to your location rules using metafields.