12.41. DD 41: Wallet Balance and Amount Definitions

12.41.1. Summary

This design document discusses terminology and concepts used in the wallet for balances and amounts.

12.41.2. Motivation

There are many different types of balances and amounts, and they need to have a clear definition.

Furthermore, the user in some situations needs to know/decide whether an amount that the user chooses includes fees or not.

12.41.3. Proposed Solution

Amounts

• “effective amount”: An effective amount always represents the direct effect on the wallet’s balance of the same currency.
• “raw amount”: The raw amount always refers to the amount with fees applied. The exact interpretation of that depends on the transaction type.
• “instructed amount”: An instructed amount always refers to the amount that a user has explicitly specified as an input. It is not directly a property of transactions, but might be added as metadata to transactions for informational purposes. How the instructed amount is interpreted differs based on the “instructed amount mode” that is specified together with the amount. By default, the instructed amount is assumed to translate to the raw amount of the corresponding transaction.
• “counter-party effective amount”: An amount that estimates the effect of the transaction of the balance (either wallet or bank account) of the other party. This is usually a conservative estimate, i.e. when sending money, this is the lower bound for the funds that the other party will obtain after fees.

Transaction types

WITHDRAW

raw amount is the total to be wired in exchange bank account

coins = select-coin(withdraw, mode, instructed_amount)

if instructed_amount mode = raw

raw_amount = instructed_amount

effective_amount = instructed_amount - coins.withdrawal_fee

if instructed_amount mode = effective

raw_amount = instructed_amount + coins.withdrawal_fee

effective_amount = instructed_amount

DEPOSIT

raw amount is the total wire transfer in the bank account

coins = select-coin(deposit, mode, instructed_amount)

if instructed_amount mode = raw

raw_amount = instructed_amount

effective_amount = instructed_amount + coins.deposit_fee + coins.refresh_fee + wire.transfer_fee

if instructed_amount mode = effective

raw_amount = instructed_amount - coins.deposit_fee - coins.refresh_fee - wire.transfer_fee

effective_amount = instructed_amount

PULL CREDIT

raw amount is the purse_value in the exchange that counter-party need will pay

coins = select-coin(pull, mode, instructed_amount)

if instructed_amount mode = raw

raw_amount = instructed_amount

effective_amount = instructed_amount - coins.withdrawal_fee - purse_fee

if instructed_amount mode = effective

raw_amount = instructed_amount + coins.withdrawal_fee + purse_fee

effective_amount = instructed_amount

if instructed_amount mode = counter-party

raw_amount = instructed_amount - coins.counter-party_deposit_fee

effective_amount = instructed_amount - coins.counter-party_deposit_fee - coins.withdrawal_fee - purse_fee

counter-party_raw_amount = raw_amount

counter-party_effective_amount = raw_amount + coins.counter-party_deposit_fee

Note

counter-party_effective_amount is an estimation since refresh fee is not included. Refresh fee can’t be calculated because depends on the coins available in the wallet of the counter-party

Note

coins.counter-party_deposit_fee is the minimum deposit_fee that can be calculated for the given exchange. Counter-party may pay more if it have different preferences doing the coin selection.

PUSH DEBIT

raw amount is the purse_value in the exchange to be withdrawn that counter-party will withdraw

coins = select-coin(push, mode, instructed_amount)

if instructed_amount mode = raw

raw_amount = instructed_amount

effective_amount = instructed_amount + coins.deposit_fee + purse_fee

if instructed_amount mode = effective

raw_amount = instructed_amount - coins.deposit_fee - purse_fee

effective_amount = instructed_amount

if instructed_amount mode = counter-party

raw_amount = instructed_amount + coins.counter-party_withdraw_fee

effective_amount = instructed_amount - coins.counter-party_withdraw_fee - coins.withdrawal_fee - purse_fee

counter-party_raw_amount = raw_amount

counter-party_effective_amount = raw_amount - coins.counter-party_withdraw_fee

Note

coins.counter-party_withdraw_fee is the minimum withdraw_fee that can be calculated for the given exchange. Counter-party may pay more if it have different preferences doing the coin selection.

Note

Next transaction types are not initiated in the wallet so instructed amount is not defined by the wallet user.

It may be helpful to calculate effective_amount to check if the wallet is able to perform the operation

contract_wire_fee = min(wire.transfer_fee / contractTerms.amortization_factor, contractTerms.max_wire_fee)

max_merchant_fee = min(contractTerms.max_fee, contract_wire_fee)

PAYMENT

raw amount is the net value of the order without fees

instructed_amount = contractTerms.amount

coins = select-coin(deposit, mode, raw_amount)

raw_amount = instructed_amount - max_merchant_fee

effective_amount = instructed_amount + coins.deposit_fee + coins.refresh_fee + wire.transfer_fee - max_merchant_fee

Note

The current coin-selection algorithm the order_price is neither raw_amount nor effective_amount. We can calculate the raw_amount of the payment as (contractTerms.amount - max_merchant_fee) and then this operation becomes equivalent than a deposit (in terms of fee calculation).

PUSH CREDIT

raw amount is the purse_value in the exchange to be withdrawn

instructed_amount = p2pContract.amount

coins = select-coin(withdraw, mode, raw_amount)

raw_amount = instructed_amount

effective_amount = instructed_amount - coins.withdrawal_fee

Note

In the case that the withdrawal_fee of the coin selection for the push-credit amount is higher than the wire_fee of the exchange, can the wallet ask the exchange to make a wire transfer of the purse instead of proceeding?

PULL DEBIT

raw amount is the net value of the invoice without fees

instructed_amount = p2pContract.amount

coins = select-coin(deposit, mode, raw_amount)

raw_amount = instructed_amount

effective_amount = instructed_amount + coins.deposit_fee + coins.refresh_fee + wire.transfer_fee

REFUND

raw amount is the amount that the merchant refunded

instructed_amount = refund.amount

raw_amount = instructed_amount

effective_amount = instructed_amount - refund_fee - refresh_fee

Note

There may be the case that the merchant should refund all the value of the purchase and that may include paying for the refund_fee.

Is there a way that the merchant can initiate a refund of purchase + refund_fee so the wallet will get the same effective_amount?

TIP

raw amount is the amount that the merchant send as tip

instructed_amount = tip.amount

raw_amount = instructed_amount + withdrawal_fee

effective_amount = instructed_amount

Note

We should not show fee for tips in the wallet since the merchant is the one choosing the exchange and we can assume that those tips are paid by the merchant. So the wallet only care about the effective.

Coin selection algorithm

Is an optimization algorithm that will choose coins given a denomination list until amount is reached and the coins selected minimize the fee spent.

select-coin will receive 3 parameters:

• operation type: tell for which fee is doing the optimization (can be withdraw, deposit, push, pull)
• amount: how much value value the coins need to sum up
• mode: the interpretation of the amount parameter and affect the denom selection
  • effective: the amount is the expected balance change
  • raw: the amount include fee of the operation paid by this wallet
  • counter-party: only valid for push and pull, the amount include fee of the operation and fee of the counter-party operation

Given the operation type, the fees taking into account when doing the calculation

withdraw:

• withdrawal_fee of denominations until amount

deposit:

• deposit_fee for every coin until total
• refresh_fee of (total-amount)
• wire_transfer for every exchange

pull:

• withdraw_fee of the amount
• purse_fee of the exchange
• if the mode is counter-party it should also take into account the deposit_fee of selected exchange

push:

• deposit_fee for every coin until total
• refresh_fee of (total - amount)
• purse_fee of the exchange
• if the mode is counter-party it should also take into account the withdrawal_fee of selected exchange

Note

select-coin must be predictable (select the same coins for the same parameters) and if the difference between two amounts are the fee for a given operation:

operation: withdrawal

withdrawal_fee = amount1 - amount2

then the wallet should select the same coins using the correct mode

select-coin(withdraw, raw, amount1) == select-coin(withdraw, effective, amount2)

Instructed Amount Modes

The interpretation and possible choices of the instructed amount mode depends on which transaction is initiated.

For withdrawal:

• raw-mode (default): instructed amount is what is subtracted from the reserve balance (i.e. it’s the raw amount)
• effective-mode: instructed amount is what the user wants to have as material balance in the wallet

FIXME(dold): However, that does not really cover the user case where the merchant charges fees and the user has to pay for that. So in theory we could have a mode that withdraws enough to pay for some particular claimed order, but IMHO that’s overkill.

For deposits (where there is no contract that already specifies an amount):

• max-mode: The instructed amount is ignored (can be zero in the request), and all coins are spent (note that the calculation should be made available when the user is asked to specify an amount when using a template)
• raw-mode (default): The instructed amount is what will be paid to the “merchant” (or the customer’s bank account), ignoring wire fees
• effective-mode: The instructed amount is how the wallet’s balance will be affected. Difficult to compute accurately because refresh is involved. Note that the calculation should ideally again be made available when the user is asked to specify an amount when using a template.

For peer-push-debit:

• raw-mode (default): The instructed amount is what will be paid, deposit fees are covered by the sender, withdrawal fees from the reserve by the receiver
• effective-mode: Instructed amount is the effect on the material balance. Difficult to compute accurately because refresh is involved.
• counter-party-effective-mode: Instructed amount is the effect on the counterparty’s wallet balance. Difficult to compute accurately because coin selection by receiver may not match our expectations.

FIXME(dold): Should we also have a mode where withdrawal fees are covered by the side that does peer-push-debit? However, that assumes the other party has the same withdrawal coin selection algorithm. FIXME(grothoff): isn’t this the counterparty-effective-mode you described above, and that would seem to exactly have this issue?

For peer-pull-credit:

• raw-mode (default): Amount that the other party has to put in the reserve. The payer has to pay any deposit fees on top. The receiver balance is increased by the specified amount minus any withdraw fees.
• effective-mode: Amount by which the initiator’s balance is increased. Difficult to compute as the receiver has to simulate different coin selections and their effect on withdraw fees to arrive at the minimum total amount that must be deposited into the reserve.

Illustrative Example

To explain the differences between raw, effective and instructed amounts, consider the following scenario: Alice wants to send money to Bob via a P2P push payment.

Example 1:

• Alice starts a withdrawal of KUDOS:10 from her bank’s web interface into her Taler wallet. The instructed amount is KUDOS:10 and (by default for bank-integrated withdrawals), the mode is raw-mode. After fees, KUDOS:9.8 arrive in her Taler wallet.

Example 3:

• Alice wants to pay for a KUDOS:10 monthly magazine subscription. Her Taler wallet is empty though.
• She starts withdrawal through her Android wallet app, where she selects KUDOS:10 as the instructed amount with mode=effective-mode. This translates to amountEffective=KUDOS:10 and amountRaw=KUDOS:10.10.
• Alice is redirected to her banking app where she transfers KUDOS:10.10 to the exchange.
• Her Taler wallet balance will be KUDOS:10.10 after the withdrawal completes.

Note that on the amount she chooses and the fees / denom structure of the exchange, the amountEffective might be higher than the instructed amount.

FIXME(dold): That flow does not work if withdrawal starts in the bank. Maybe there needs to be a mechanism where the wallet tells the bank the adjusted amount that needs to be transferred? That would be a new feature in the bank integration API.

Example 4:

• Alice has KUDOS:10 in her wallet.
• Alice wants to initiate a peer-push payment with amountInstructed=KUDOS:8 and mode=effective-mode. That means that after the payment, she expects exactly KUDOS:2 to remain in her wallet.
• Due to the fee configuration, her wallet computes amountRaw=KUDOS:7.5 and amountEffective=KUDOS:7.8. The effective amount in this case does not equal the instructed amount, despite the mode=effective-mode. That’s because there no amount that can be spend so that the spend amount with resulting refresh fees equal KUDOS:8.
• Alice confirms the peer-push payment initiation, and exactly KUDOS:7.5 are credited to the purse that her wallet creates.
• Bob merges the purse into his reserve. Bob’s wallet automatically withdraws from the reserve, and his wallet balance increases by KUDOS:7.1, since withdrawal fees are deducted.

Balances

The following types of balances are defined:

• available: Balance that the wallet believes will certainly be available for spending, modulo any failures of the exchange or double spending issues. This includes available coins not allocated to any spending/refresh/… operation. Pending withdrawals are not counted towards this balance, because they are not certain to succeed. Pending refreshes are counted towards this balance. This balance type is nice to show to the user, because it does not temporarily decrease after payment when we are waiting for refreshes
• material: Balance that the wallet believes it could spend right now, without waiting for any operations to complete. This balance type is important when showing “insufficient balance” error messages.
• age-acceptable: Subset of the material balance that can be spent with age restrictions applied.
• merchant-acceptable: Subset of the material balance that can be spent with a particular merchant (restricted via min age, exchange, auditor, wire_method).
• merchant-depositable: Subset of the merchant-acceptable balance that the merchant can accept via their supported wire methods.

Balance Mismatch

The wallet uses the following terminology when an operation can’t succeed because the balance is too low, even though the instructed amount

• “fee-gap-estimate”: Additional (material) balance that the wallet estimates it still needs for the operation to succeed.
  • This value is an estimated, because newly withdrawn coins might have different fees.
  • This value is specified per exchange, because each exchange has different fees.

FIXME(dold): Should we specify an upper-bound fee-gap-estimate to simplify it for the UIs?

12.41.4. Discussion / Q&A

(This should be filled in with results from discussions on mailing lists / personal communication.)