Polkadot

On Polkadot Asset Hub, H rides in a remark the payer signs, batched with the transfer: utility.batch_all([assets.transfer_keep_alive(asset, Id(payTo), amount), system.remark_with_event(R)]), where R is the 77 ASCII bytes lcp:sha256: followed by H. The payer pays every fee, in DOT or in an asset its wallet selects.

The facilitator needs an RPC node that serves JSON-RPC over HTTP, including state_call, author_submitExtrinsic, chain_getBlock, chain_getFinalizedHead and state_getStorage. It loads the network's runtime metadata on the first payment, and again whenever the node's runtime version changes.

What /verify checksCopy Anchor Link

1. The network is configured, and the requirements are ones the profile admits, else invalid_network or invalid_payment_requirements.
2. payload.extrinsic and payload.call are lowercase hex; the call is exactly utility.batch_all([assets.transfer_keep_alive(asset, Id(payTo), amount), system.remark_with_event(R)]) for this asset, payTo and amount, and R carries an LCP sha256 string, else invalid_payload.
3. With the network's runtime metadata, the extrinsic decodes as a signed v4 extrinsic whose call is payload.call byte for byte, with a mortal era, else invalid_payload. The node must serve the chain the network names (its genesis hash), else unexpected_verify_error.
4. TaggedTransactionQueue_validate_transaction at the best block returns Ok, else invalid_transaction.

The answer is {isValid: true, payer}, with payer the signer in SS58.

What /settle doesCopy Anchor Link

1. Repeats every check of /verify.
2. Reads the stored answer for the extrinsic's BLAKE2b-256 hash, and returns a final one as it is.
3. Claims the hash, with the first block to scan: the block after the finalized head read before validation, never before the era's birth.
4. Submits with author_submitExtrinsic. A refusal that says the node has already seen the extrinsic or its nonce is read as possibly included, and the scan runs.
5. Scans every second: first the finalized blocks, then the blocks up to the head. In the block that holds the extrinsic, success needs System.ExtrinsicSuccess, System.Remarked for R's BLAKE2b-256 and Assets.Transferred for asset. A failure counts only in a finalized block. Once every block to the era's last is finalized without the extrinsic, it can never be included: invalid_transaction_state.
6. Stores the answer and returns it.

Success is {success: true, transaction, network, payer}, where transaction is <block hash>-<extrinsic index>. While the extrinsic is broadcast but not yet in a block, the answer is settlement_pending with transaction set to the extrinsic's hash.

Where H is on chainCopy Anchor Link

R is inside the extrinsic the payer signed, and the System.Remarked event carries its BLAKE2b-256. The extrinsic is read by block and index. No index searches remarks.