CIP2 - Coin Selection Algorithms #2
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This change adds a new informational CIP entitled "Coin Selection
Algorithms for Cardano".
The purpose of this CIP is to describe, in human-readable terms, the
coin selection algorithms currently used by Cardano Wallet and other
parts of the Cardano ecosystem.
The CIP is divided into a number of sections:
The "Background" section:
* introduces the fundamental concepts behind coin selection;
* provides a discussion of why coin selection is a non-trivial problem;
* describes the goals of coin selection algorithms.
The "Interface" section:
* describes the common interface unifying all coin selection algorithms
used within Cardano Wallet;
* describes the standard parameter types, result types, and error types
used by the common interface;
* describes the properties that all conforming implementations are expected
to satisfy.
The "Algorithms" section:
* gives detailed descriptions of each of the individual coin selection
algorithms used in Cardano Wallet;
* gives step-by-step descriptions of the computations involved.
The "Reference Implementations" section:
* provides links to reference implementations of each algorithm in
various languages (currently limited to Haskell, but additional
languages could be added in future revisions).
The "External Resources" section:
* provides links to other resources useful for understanding issues
relating to coin selection.
CIP-JonathanKnowles-CoinSelectionAlgorithms/CIP-JonathanKnowles-CoinSelectionAlgorithms.md
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jonathanknowles
changed the title
This section describes the motivation for writing this document.
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| In particular: | ||
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| * **_v_**<sub>selected</sub> |
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Are you not missing the fee somewhere in here?
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@SebastienGllmt wrote:
Are you not missing the fee somewhere in here?
Hi Sebastien
Many thanks for your comment, and apologies for the long delay in responding. (Until this week, we were focused quite heavily on getting things ready for Shelley.)
To answer your question:
As you might be aware, coin selection (in Cardano Wallet) proceeds in two distinct and consecutive phases:
- Phase 1: Selecting coins from a UTxO set to form an initial coin selection.
The goal of this phase is to produce a selection where sum (inputs) = sum (outputs). - Phase 2: Adjusting the coin selection generated in the previous phase in order to pay for the network fee.
The goal of this phase is to produce a selection where sum (inputs) = sum (outputs) + fee.
Both the former and the latter phases are complex and worthy of discussion in their own right, so we took the decision quite early on to separate the descriptions of these two phases into two different specification documents.
This document deliberately focuses on the former phase: the algorithms involved in creating an initial coin selection. These algorithms are (more or less) independent of any fee adjustment that happens later on. As for fee adjustment, I expect that we'll eventually create another specification document to cover this topic, as soon as time allows.
The fact that you've raised this point is really useful, as it indicates that we ought to make the scope of this document clearer.
I will adjust the abstract and motivation sections and let you know when the document is updated. 👍
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Hi @SebastienGllmt.
As promised, I've revised the abstract and motivation sections, in light of your feedback.
See 5c685d6.
Many thanks again for your feedback!
Of course, let me know if you think we should make further changes.
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| * #### UTxO Balance Insufficient | ||
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| This failure occurs when the total value of the entries within the [initial |
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It may be worth noting that this can also happen in the case that UTXO in your wallet contains less ADA than the cost of including it in the input. The sum of the dust may have a sufficient balance, but each input individually is not worth adding.
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@SebastienGllmt wrote:
It may be worth noting that this can also happen in the case that UTXO in your wallet contains less ADA than the cost of including it in the input. The sum of the dust may have a sufficient balance, but each input individually is not worth adding.
That is indeed true, and a very good point.
However, in Cardano Wallet, dust elimination is mainly the responsibility of the fee adjustment algorithm, whereas this document describes the algorithms involved in making an initial coin selection.
See #2 (comment) for an explanation of these two phases (which are consecutive and distinct from one another).
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| If the [available UTxO list](#available-utxo-list) is _empty_: | ||
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| * Terminate with a [UTxO Balance |
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Again, this is the wrong condition. It should be until the available utxo list has no UTXO with an amount greater than the fee of adding it to the transaction (I know for some currencies this is now straight forward to detect)
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@SebastienGllmt wrote:
Again, this is the wrong condition. It should be until the available utxo list has no UTXO with an amount greater than the fee of adding it to the transaction (I know for some currencies this is now straight forward to detect)
It would be the wrong condition if we were considering fees.
However, fees are handled by a later phase (the fee adjustment phase), which is (currently) outside the scope of this document.
See #2 (comment) for a longer explanation.
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| Represents the set of payments to be made to recipient addresses. | ||
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| * A set of **_change values_** (see [change output](#change-output)), |
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Do we have a minimum size for the value of a change? For example, if the change is just one lovelace, maybe it should just be added to the fee instead.
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Do we have a minimum size for the value of a change? For example, if the change is just one lovelace, maybe it should just be added to the fee instead.
Yes, we do. We have the concept of a dust threshold.
Here's an extract from the API docs for cardano-coin-selection:
A
DustThresholddefines the maximum size of a dust coin.Functions that accept a
DustThresholdargument will generally exclude values that are less than or equal to this threshold from the change sets of generated selections, coalescing such coins together into larger coins that exceed the threshold.
This dust threshold parameter is used in the fee adjustment phase (see #2 (comment)), which is mainly concerned with adjusting a coin selection in order to pay for the network fee, but also concerned with the process of coalescing dust coins into larger coins that exceed the dust threshold.
This change also adds a "Scope" section, to clarify and delineate the scope of this article. In response to review feedback: #2 (comment)
…data-oracle-cddl CDDL files
About this PR
This PR adds a new informational CIP entitled "Coin Selection Algorithms for Cardano".
The purpose of this CIP is to describe, in human-readable terms, the coin selection algorithms currently used by Cardano Wallet and other parts of the Cardano ecosystem.
Reading the Draft
Since the draft is written in Markdown format, see here to read the fully-rendered version.
Section Guide
The draft CIP document is divided into a number of sections:
Background
Interface
Algorithms
Reference Implementations
External Resources