Hook Fees

What to expect when your Hook runs.

Hook Creation Fees

SetHook transactions are charged per byte of created webassembly. The rate is 500 drops per byte. Thus a 1kib Hook will cost 0.5 XAH to create.

Hook Execution Fees

When Hooks are Strongly Executed the originating transaction must pay for the Strong Executions in the originating transaction's fee.

Hook Execution fees are charged at a rate of 1 drop per web assembly instruction in the worst-case execution of the function hook (or cbak in the case of a callback). Thus a small Hook with a lot of looping may end up attracting high runtime fees.

Fee RPC Helper

Transaction fees on a ledger with the Hooks Amendment enabled become non-trivial to compute for end-users and/or wallet applications. This is because strong hooks must be paid for by the originator of a transaction, and there may be as many as 4 strong hooks on the sending account and 4 on the receiving account, as well as any other strong transactional stakeholders involved (as can be the case with some exotic transaction types). Further, if the transaction is a SetHook then the size of the parameters, the size of the code and whether it is a create operation or an install operation all determine the size of the fee.

Therefore it is highly recommended that all transactions be run through the updated fee RPC call before they are submitted to the ledger.

To invoke the RPC call:

1. Open a websocket connection to the Hooks node you will be working with.

2. Compose the serialized transaction you wish to know the fee for with the following:

• Fee: "0"

• SigningPubKey: "" (That is: 0 byte VL of type 0x73. In hex:0x7300.)

• Do not sign the transaction.

1. Submit it as a hex blob to the RPC as follows:

{"command":"fee", "tx_blob":"<hex blob>"}

For HTTP POST RPC submit it as follows:

{"method":"fee", "params": [{"tx_blob":"<hex blob>"}] }

The response should look something like

{
  result: {
    drops: {
      base_fee: '130520',
    },
  //...
  },
  type: 'response'
}

Take the base fee and set it as the Fee field in the transaction. Now sign and submit it as per the normal transaction submission process.

If there is an invalid value for tx_blob or tx_blob is missing, a regular JSON result will be returned with a base_fee of 10.

Emission Fees

Hooks have access to the same computation the Fee RPC Helper does. To use this simply call etxn_fee_base with a buffer containing the serialised transaction as the arguments. As with the RPC call, you must ensure that the Fee field is present in the serialised transaction. The value is irrelevant.

When etxn_fee_base returns the recommended fee you may use sto_emplace to emplace it into the serialised transaction before emission. The relevant field is sfFee.