Shopify inventory automation that survives a flash sale

Most Shopify inventory bugs are not really inventory bugs. They are ordering bugs. Webhooks are delivered at least once and in no guaranteed order, so a naive handler that writes whatever it receives will happily overwrite a newer count with an older one.

The fix is to treat every webhook as a hint that something changed, not as the change itself. When a webhook lands, enqueue a reconciliation job for that variant. The job reads the current value from the Admin API and writes the result once. Duplicated webhooks then collapse into a single write instead of a race.

Rate limits are the second half of the problem. Shopify uses a leaky bucket, so bursts are fine until they are not. Route every write through a single queue with a token budget, and let the queue absorb the spike. During a flash sale the queue drains a little slower; nothing fails.

Finally, reconcile on a schedule regardless of webhooks. A nightly full pass costs very little and catches anything the event stream missed. When drift does happen, you find it before your customers do.