Glossary
Webhook
A webhook is a message Shopify sends to an app the moment something happens in your store, so the app does not have to keep asking.
An order is placed, and within a second Shopify has told every app that asked to know.
Why it matters to a stockroom
The alternative is polling: an app asking “anything new?” every minute, forever. That is slower, wastes your Admin API limit, and still misses things between checks.
With webhooks, a change to an inventory item reaches a warehouse system while the person who made it is still standing at the shelf.
Expect it twice
Shopify guarantees delivery at least once, not exactly once. A network blip means the same message arrives again.
Any integration worth trusting handles that, usually by recording the message id and ignoring one it has already seen. An app that does not will book the same delivery into your stock twice.
Where it goes wrong
The receiving end goes down for an hour. Shopify retries for a while and then gives up, and those messages are gone. A good integration reconciles on a schedule as well as listening, so a missed hour is caught by the next sweep rather than never.
The other one is reaching for a webhook when Shopify Flow would do. If the action is an email or a tag, Flow needs no server and nothing to keep running.
Related terms
Admin API
The Admin API is how apps read and change things in your Shopify store, including the stock levels a scanner updates.
Shopify Flow
Also called: Flow
Shopify Flow is Shopify's own automation tool. It watches for something happening in your store and runs an action when it does.
Inventory item
An inventory item is the countable half of a variant. Stock levels attach to it, one per location, not to the variant itself.