Add UMAD-07: Post-Transaction Hooks

umad-07-post-tx-hooks.md

@@ -1 +1,30 @@
 # UMAD-07: Post-transaction compliance hooks
+
+UMA offers a mechanism by which VASPs can notify each other of the UTXOs used to complete a transaction. This is useful
+for compliance purposes, as it allows VASPs to submit those UTXOs to their compilance providers for transaction
+monitoring.
+
+**Note:** VASPs only need to receive post-transaction hooks from their counterparty VASP for the UTXO-based compliance
+flow, i.e. for cases where their Compliance Provider does not support lookups and registration via node public key. If
+the compliance provider supports using the node public key instead, VASPs can omit the `utxoCallback` field. However,
+VASPs do still need to send post-transaction hooks for their counter-party if the other VASP has provided a
+`utxoCallback` themselves.
+
+As mentioned in [UMAD-05](/umad-05-payreq-request.md), in the payreq request, the sending VASP specifies a
+`payerdata.compliance.utxoCallback` field which is a URL that the receiving VASP should call on transaction completion
+to notify the sending VASP of the UTXOs used to complete the transaction. Similarly, in the payreq response, the
+receiving VASP specifies a `compliance.utxoCallback` field which is a URL that the sending VASP should call on
+transaction completion to notify the receiving VASP of the UTXOs used to complete the transaction. This request from
+each VASP to the other is called a "post-transaction hook". It is a POST request with the following structure:
+
+```http
+POST <utxoCallback>
+
+{
+  "utxos": { "utxo": string, "amountMsats": number }[]
+}
+```
+
+The request body is JSON with a single field, `utxos`, which is an array of objects containing the UTXO and the
+corresponding amount in millisats that went over each channel used to complete the transaction. Both VASPs can
+register the incoming UTXOs with their Compliance Provider for transaction monitoring.