Shopify Functions
Input query
The input query is the GraphQL file that declares what data a Shopify Function receives at run time. A Function only sees what its input query asks for, and because the input is capped at 128 kB, deciding which fields go in is as much work as the logic itself.
Also called: run.graphql, Function input, input GraphQL
This is the most underestimated file when writing a Function. The logic is usually short; what decides whether the Function works on a large cart is what you asked for.
The 128 kB limit is not theoretical
With a 10-line cart there is room for everything. With a 200-line wholesale order pad, every extra field is multiplied by 200. Asking for every variant’s title, description and images “just in case” is the fastest way to make a Function that passed its tests fail on a customer’s real order.
The rule I follow: the input query asks for exactly the fields the code reads, and when I add a field to the code I add it here too, in the same commit.
Aliases are part of the design
A metafield arrives with its namespace and key, which makes for ugly code. With an alias it arrives under the name that makes sense to the logic:
query RunInput {
cart {
lines {
quantity
merchandise {
__typename
... on ProductVariant {
id
title
casePack: metafield(namespace: "b2b", key: "case_pack") { value }
}
}
}
}
}That casePack is the same metafield under a name that reads. And __typename is there because a cart line is not always a product variant, and discarding what is not one before touching it avoids the most common error.
A complete example with its function and its tests is in shopify-checkout-lab.