Azure Functions: configure blob trigger only for new events

There is no way to do this currently. Internally we track which blobs we have processed by storing receipts in our control container azure-webjobs-hosts. Any blob not having a receipt, or an old receipt (based on blob ETag) will be processed (or reprocessed). That's why your existing blobs are being processed - they don't have receipts. BlobTrigger is currently designed to ensure that ALL blobs in a container matching the path pattern are eventually processed, and reprocessed any time they are updated.

If you feel passionately about this, you can log a feature request in our repo here with details on your scenario.

The way I get around this is to set Metadata on a processed Blob (e.g Status = Complete). When the trigger gets fired I first check for this piece of Metadata and return the function if it is already set.

The downside to this is that the update of the Metadata will trigger an additional execution of the function