vbank

Virtual Bank

This module manages the integration between the Cosmos x/bank module and the VM.

"Bank purses" connect Cosmos-level bank module accounts with Agoric-level ERTP purses and payments. A "bank purse" is created at the ERTP level which be observed and manipulated as other purses, but modifications to it are sent over the bridge to act upon the bank account. Updates to the bank account balance are sent over the bridge to update the bank purse, whose balance can be queried entirely at the ERTP level.

Parameters

• feeCollectorName: the module which handles fee distribution to stakers.
• reward_epoch_duration_blocks: the duration (in blocks) over which fees should be given to the fee collector.

State

The Vbank module maintains no significant state, but will access stored state through the bank module.

Protocol

Purse operations which change the balance result in a downcall to this module to update the underlying account. A downcall is also made to query the account balance.

Upon an EndBlock() call, the module will scan the block for all MsgSend and MsgMultiSend events (see cosmos-sdk/x/bank/spec/04_events.md) and perform a VBANK_BALANCE_UPDATE upcall for all denominations held in only the mentioned module accounts.

The following fields are common to the Vbank messages:

• "address", "recipient", "sender": account address as a bech32-encoded string
• "amount": either amount to transfer or account balance, as an integer string
• "denom": denomination as a string
• "nonce": unique integer, as a number
• "type": string of the type of message (enumerated below)
• "updated": list of objects with the fields "address", "denom", "amount".

Downcalls from JS to Cosmos (by type):

• VBANK_GET_BALANCE (type, address, denom): gets the account balance in the given denomination from the bank. Returns the amount as a string.
• VBANK_GIVE (type, recipeient, denom, amount): adds amount of denomination to account balance to reflect a deposit to the virtual purse. Returns a VBANK_BALANCE_UPDATE message restricted to the recipient account and denomination.
• VBANK_GIVE_TO_FEE_COLLECTOR (type, denom, amount): stores rewards which will be gradually sent to the fee collector
• VBANK_GRAB (type, sender, denom, amount): burns amount of denomination from account balance to reflect withdrawal from virtual purse. Returns a VBANK_BALANCE_UPDATE message restricted to the sender account and denomination.

Upcalls from Cosmos to JS: (by type)

• VBANK_BALANCE_UPDATE (type, nonce, updated): inform virtual purse of change to the account balance (including a change initiated by VBANK_GRAB or VBANK_GIVE).

Testing

Test the following transfer scenarios:

• Purse-to-bank-purse
• bank-purse-to-purse
• bank-purse-to-bank-purse

The initial BLD and IST purses are bank purses, but newly-created purses will not be bank purses by default.

Governance

To use Cosmos governance to change the reward_epoch_duration_blocks value:

$ agd query vbank params
reward_epoch_duration_blocks: "720"
$ cat <<EOF > epoch-duration-proposal.json
{
  "title": "Vbank param-change test",
  "description": "Decrease the fee disbursal epoch parameter to 30 blocks.",
  "changes": [
    {
      "subspace": "vbank",
      "key": "reward_epoch_duration_blocks",
      "value": "30"
    }
  ],
  "deposit": "1000000ubld"
}
EOF
$ agd tx gov submit-proposal param-change epoch-duration-proposal.json --from=mykey --chain-id=agoric
# Then vote on the proposal.
$ agd tx vote ...
# After passing,
$ agd query vbank params
reward_epoch_duration_blocks: "30"
$