# Designated Fee Delegation

The TX-fee-delegation mechanism (both VIP-191 and MPP) on VeChainThor. The demo is built using tools connex-framework (opens new window) and connex.driver-nodejs (opens new window) that implement the Connex (opens new window) interface in a NodeJS environment.

There are three accounts used in the demo:

SENDER whose address begins with 0xd551,
DELEGATOR (or gas-payer) whose address begins with 0xe466, and
RECIPIENT whose address begins with 0x9143. To demonstrate VIP-191, I created one TX from SENDER to RECIPIENT. The TX was co-signed by SENDER and DELEGATOR to allow TX fee paid by DELEGATOR.

To demonstrate MPP, I created two TXs sent from DELEGATOR to call functions of the built-in contract Prototype to add a payer-user relation and funding plan as described above. I then created one TX from SENDER to DELEGATOR to demonstrate the effect of MPP. Note that DELEGATOR is the master of itself since it is a normal account rather than a contract. The TX can only be sent to DELEGATOR or otherwise it wouldn't be funded by DELEGATOR according to the rules set by MPP.

# CONNEX INTERFACE FOR VIP-191

The Connex interface has been updated to support VIP-191. I want to show you that it is pretty easy to use Connex to construct a VIP-191 enabled TX.

There are two extra things you need to do on top of the normal procedure of constructing a TX:

Create your own function with the following definition:

function (unsignedTx: { raw: string, origin: string }): Promise

This function is typically responsible for send data to the gas-payer, waiting for its response and returns a Promise, if resolved, carrying the gas-payer's digital signature on the corresponding TXID.

Pass the function to the instance of Connex.Vendor.TxSigningService, as you've already created for TX construction, via the delegate method. For instance, you may add a line such as:

signingService.delegate(MyFunc);

That's it!

Note that the function I made in the demo code is NOT a typical function you would see in a real application. It is created purely for this demo and should not be considered as an example of creating such a function.

Results output

--------------------------
VIP-191
--------------------------
TXID       = 0xb58e1d1bf9da3414c24df51926003ebcbac7eb10246dd25548ccfd9202d4276e
From       = 0xd55100eedb61f1e553a38c33a234ce07952c43f2
To         = 0x91436f1E5008B2E6093E114A25842F060012685d
GasPayer   = 0xe4660c72dea1d9fc2a0dc2b3a42107d37edc6327
GasUsed    = 21000
...

The first part shows information of the VIP-191 enabled TX. It can be seen that the actual gas-payer was indeed DELEGATOR rather than SENDER.

--------------------------
MPP - Add User
--------------------------
TXID       = 0x508f50692c88054c5f8df982937b2871cae3cde002a4bc58e975277acca53c87
From       = 0xe4660c72dea1d9fc2a0dc2b3a42107d37edc6327
To         = 0x000000000000000000000050726f746f74797065
GasPayer   = 0xe4660c72dea1d9fc2a0dc2b3a42107d37edc6327
GasUsed    = 25074
...
--------------------------
MPP - Add Credit Plan
--------------------------
TXID       = 0x57329507daadb47a8ea68375577f9699fce3f1329df4703451be36d3804aa853
From       = 0xe4660c72dea1d9fc2a0dc2b3a42107d37edc6327
To         = 0x000000000000000000000050726f746f74797065
GasPayer   = 0xe4660c72dea1d9fc2a0dc2b3a42107d37edc6327
GasUsed    = 44811
...

The second part shows information of the two TXs DELEGATOR sent to the built-in contract Prototype to activate MPP for SENDER. The overhead costs for adding a user and a credit plan are 25k and 45k gas, respectively.

--------------------------
MPP - User Sends TX
--------------------------
TXID       = 0x87b68a65105f2cc5746b88e1e0fd5e1cf4f6d2dbf8459bb8ef2599957ffcb655
From       = 0xd55100eedb61f1e553a38c33a234ce07952c43f2
To         = 0xe4660c72dea1d9fc2a0dc2b3a42107d37edc6327
GasPayer   = 0xe4660c72dea1d9fc2a0dc2b3a42107d37edc6327
GasUsed    = 21000

The last part shows the TX sent from SENDER to DELEGATOR is indeed paid by DELEGATOR under MPP.