[QUESTION] - Using the Cardano API to compute minimum UTxO values.

[jonathanknowles]

Internal/External

Internal

Area

Using the Cardano API function Cardano.Api.Fees.calculateMinimumUTxO to compute minimum UTxO values.

Context

We're currently attempting to use the Cardano.Api.Fees.calculateMinimumUTxO function to replace cardano-wallet's hand-coded minimum UTxO calculation. Our main use cases of this function are:

• assigning minimum ada quantities to auto-generated change outputs during coin selection;
• assigning minimum ada quantities to user-specified outputs without ada;
• verifying minimum ada quantities of all outputs.

However, while testing this function, we've discovered some interesting behaviour:

• Suppose that:
  • We have an arbitrary TxOut t with arbitrary tokens and an arbitrary address, whose ada quantity we can adjust with the function setLovelace :: Lovelace -> TxOut -> TxOut.
  • We call calculateMinimumUTxO with t0 = setLovelace 0 t, and the function returns ada quantity a.
  • We call calculateMinimumUTxO with ta = setLovelace a t, and the function returns ada quantity b.
• In some situations, it's possible for the following to be true: b > a.
• In other words, calculateMinimumUTxO appears to not reach a fixed point before returning its final value.
• This only applies to the Alonzo and Babbage eras: earlier eras are unaffected.
• See our test suite for a demonstration of the behaviour. (We currently use QuickCheck coverage checks to verify that this happens.)

Question

Is this behaviour expected?

More generally:

• What properties should callers expect the calculateMinimumUTxO function to satisfy?
• Should callers be expected to compute a fixed point themselves (i.e., repeatedly calling the function until the result is stable)?
• Is there a test suite that verifies the expected behaviour and usage?

Many thanks in advance! 🙏🏻

Jonathan

[JaredCorduan]

I'm less familiar with the use cases of the Cardano API, but it looks like calculateMinimumUTxO mostly just wraps the ledger function evaluateMinLovelaceOutput, which only computes the minimum and does not attempt to compute the fixed point.

But I think such a fixed point function would be very helpful for many folks, so I made this: https://github.com/input-output-hk/cardano-ledger/issues/2905

[JaredCorduan]

@jonathanknowles - setMinCoinSizedTxOut introduced in https://github.com/input-output-hk/cardano-ledger/pull/3008 computes the fixed point.