cap-0046-07.md

CAP: 0046-07 (formerly 0055)
Title: Fee and resource model in smart contracts
Working Group:
    Owner: MonsieurNicolas
    Authors: dmkozh
    Consulted:
Status: Final
Created: 2022-06-03
Discussion:
Protocol version: 20

Simple Summary

This CAP defines the mechanism used to determine fees when using smart contracts on the Stellar network.

Motivation

With the introduction of smart contracts on the network, the existing fee model of the "classic" transaction system is too simplistic: it requires careful design of the code that runs "on chain" as to ensure that all operations have a similar cost and performance profile, which is not possible with arbitrary code running in contracts.

Goals Alignment

Goals of the updated fee model are to:

• ensure fairness between users and use cases.
• promote scalable patterns on the network, doing more with the same amount of overall resources.
• ensure that the network operates in a sustainable way, network operators should be in control of their operating cost.

Abstract

This CAP proposes various network level parameters (voted on by validators), and fee structure for the different kinds of resources involved on the network.

The fee structure is designed to discourage "spam" traffic and overall waste of infrastructure capacity.

Specification

XDR changes

See the full XDR diffs in the Soroban overview CAP.

Fee and resource limit configuration is specified via the following network parameters (in some cases increments are used to mitigate for rounding errors):

// General “Soroban execution lane” settings
struct ConfigSettingContractExecutionLanesV0
{
    // maximum number of Soroban transactions per ledger
    uint32 ledgerMaxTxCount;
};

// "Compute" settings for contracts (instructions and memory).
struct ConfigSettingContractComputeV0
{
    // Maximum instructions per ledger
    int64 ledgerMaxInstructions;
    // Maximum instructions per transaction
    int64 txMaxInstructions;
    // Cost of 10000 instructions
    int64 feeRatePerInstructionsIncrement;

    // Memory limit per transaction. Unlike instructions, there is no fee
    // for memory, just the limit.
    uint32 txMemoryLimit;
};

// Ledger access settings for contracts.
struct ConfigSettingContractLedgerCostV0
{
    // Maximum number of ledger entry read operations per ledger
    uint32 ledgerMaxReadLedgerEntries;
    // Maximum number of bytes that can be read per ledger
    uint32 ledgerMaxReadBytes;
    // Maximum number of ledger entry write operations per ledger
    uint32 ledgerMaxWriteLedgerEntries;
    // Maximum number of bytes that can be written per ledger
    uint32 ledgerMaxWriteBytes;

    // Maximum number of ledger entry read operations per transaction
    uint32 txMaxReadLedgerEntries;
    // Maximum number of bytes that can be read per transaction
    uint32 txMaxReadBytes;
    // Maximum number of ledger entry write operations per transaction
    uint32 txMaxWriteLedgerEntries;
    // Maximum number of bytes that can be written per transaction
    uint32 txMaxWriteBytes;

    int64 feeReadLedgerEntry;  // Fee per ledger entry read
    int64 feeWriteLedgerEntry; // Fee per ledger entry write

    int64 feeRead1KB;  // Fee for reading 1KB

    // The following parameters determine the write fee per 1KB.
    // Write fee grows linearly until bucket list reaches this size
    int64 bucketListTargetSizeBytes;
    // Fee per 1KB write when the bucket list is empty
    int64 writeFee1KBBucketListLow;
    // Fee per 1KB write when the bucket list has reached `bucketListTargetSizeBytes` 
    int64 writeFee1KBBucketListHigh;
    // Write fee multiplier for any additional data past the first `bucketListTargetSizeBytes`
    uint32 bucketListWriteFeeGrowthFactor;
};

// Historical data (pushed to core archives) settings for contracts.
struct ConfigSettingContractHistoricalDataV0
{
    int64 feeHistorical1KB; // Fee for storing 1KB in archives
};

// Contract event-related settings.
struct ConfigSettingContractEventsV0
{
    // Maximum size of events that a contract call can emit.
    uint32 txMaxContractEventsSizeBytes;
    // Fee for generating 1KB of contract events.
    int64 feeContractEvents1KB;
};

// Bandwidth related data settings for contracts.
// We consider bandwidth to only be consumed by the transaction envelopes, hence
// this concerns only transaction sizes.
struct ConfigSettingContractBandwidthV0
{
    // Maximum sum of all transaction sizes in the ledger in bytes
    uint32 ledgerMaxTxsSizeBytes;
    // Maximum size in bytes for a transaction
    uint32 txMaxSizeBytes;

    // Fee for 1 KB of transaction size
    int64 feeTxSize1KB;
};

Soroban resources are provided in a SorobanTransactionData extension of transaction:

// Resource limits for a Soroban transaction.
// The transaction will fail if it exceeds any of these limits.
struct SorobanResources
{   
    // The ledger footprint of the transaction.
    LedgerFootprint footprint;
    // The maximum number of instructions this transaction can use
    uint32 instructions; 

    // The maximum number of bytes this transaction can read from ledger
    uint32 readBytes;
    // The maximum number of bytes this transaction can write to ledger
    uint32 writeBytes;
};

// The transaction extension for Soroban.
struct SorobanTransactionData
{
    ExtensionPoint ext;
    SorobanResources resources;
    // Portion of transaction `fee` allocated to resource fees.
    int64 resourceFee;
};

Semantics

Fee model overview

The approach taken in this proposal is to decompose the total transaction fee into the following additive components:

• competitiveResourcesFee - the fee for 'competitive' network resources (defined below) and non-refundable resources, based on the values declared in transaction and network-defined fee rates.
• refundableResourcesFee - the maximum fee for resources that don't need to be strictly restricted per ledger and thus are charged based on the actual usage.
• inclusionFeeBid - this is the "social value" part of the fee, it represents the intrinsic value that the submitter puts on that transaction.

The 'competitive' resources are resources that have to be limited per ledger in order to ensure reasonable close time and prevent network from overloading. These resources are bounded on different dimensions, i.e. there is no single 'proxy' resource that could be used to restrict them. On a high level, these resources are:

• instructions (virtual CPU instructions to execute)
• ledger data access (ledger IO metrics)
• network propagation (bandwidth usage)

Soroban transaction fee has to cover all three components, but only inclusionFeeBid is used for transaction prioritization.

TransactionSet semantics

All Soroban transactions must be present in phase 1 of GeneralizedTransactionSet (all the remaining 'classic' transactions must be in phase 0). The Soroban phase must contain only a single TXSET_COMP_TXS_MAYBE_DISCOUNTED_FEE component. Refer to CAP-0042 for details on GeneralizedTransactionSet and phases.

While transactions bid specific inclusionFeeBid, the effective bid may be lowered within a transaction set component by setting baseFee in txsMaybeDiscountedFee component.

When set:

• all transactions within the component must bid not less than baseFee, i.e. for each transaction inclusionFeeBid >= baseFee
• the effective inclusion bid for transactions in that group is baseFee

The total resource consumption for every one the 'competitive' resources must not exceed the ledger-wide limits. The specific limits are specified in sections below on per-resource basis.

The usual GeneralizedTransactionSet validity and comparison rules also apply to Soroban corresponding to the semantics described in CAP-0042.

Transaction validation

All Soroban transactions must have ext.sorobanData() extension present and populated.

resources contain the declared values of resources that the transaction is paying the fee for. These values have to not exceed the limits specified by the network settings.

resourceFee is computed based on the resources declared in tx and transaction envelope size:

resourceFee(tx) = Instructions_fee(resources.instructions) + LedgerDataAccess_fee(resources) + NetworkData_fee(size(txEnvelope)) + Historical_flat_fee(size(txEnvelope))

Note, that Historical_flat_fee is a 'competitive' resource, but it's constant for any transaction execution result and thus is a part of non-refundable fee (as its refund is always 0).

resourceFee corresponds to the sum of competetiveResourcesFee and refundableResourcesFee components.

The rules for limits and fee computation per-resource are specified in dedicated sections below.

At validation time total transaction fee (tx.fee) has to cover the fee components based only on the values declared in transaction:

tx.fee = sorobanData.resourceFee + inclusionFeeBid

Minimum valid inclusionFeeBid value is 100 stroops, thus the following condition has to be true:

tx.fee >= sorobanData.resourceFee + 100

sorobanData.resourceFee value has to cover the 'competetive' resource fee computed based on the declared resource values specified in sorobanData and transaction envelope size:

sorobanData.resourceFee >= resourceFee(tx)

The remaining value of sorobanData.resourceFee - resourceFee(tx) is considered to be a refundable part of the resource fee and has to cover the refundable resources consumed at apply time.

Similarly to 'classic' transactions, source account must be able to pay for the total fee (tx.fee) for the transaction.

Fee computation while applying transactions

As in classic, total fees are taken from the source account balance before applying transactions.

Total fee charged is equal to tx.fee if baseFee is not set in the transaction set component, and tx.fee - inclusionFeeBid + baseFee if baseFee is set in the transaction set component.

During transaction execution the resource limits declared by transaction are enforced and exceeding any one of the limits leads to transaction failure with <OP_NAME>_RESOURCE_LIMIT_EXCEEDED operation error code (every Soroban operation defines a separate error for this, such as INVOKE_HOST_FUNCTION_RESOURCE_LIMIT_EXCEEDED).

The per-resource failure conditions are specified in the sections below.

At the end of the transaction execution, compute the final refundable fee for successful transaction as follows:

effectiveRefundableFee = Events_fee(emittedContractEventsSizeBytes) + Rent_fee

where emittedContractEventsSizeBytes is the size of the emitted contract events and invocation return value, and Rent_fee is the fee for the rent bumps performed by the transaction (if any). If effectiveRefundableFee > sorobanData.resourceFee - resourceFee(tx) (i.e. if actual required refundable fee is greater than the refundableResourcesFee component defined above), the transaction fails.

In case if transaction fails effectiveRefundableFee is set to 0.

After executing the transaction, the refund amount is computed as sorobanData.resourceFee - resourceFee(tx) - effectiveRefundableFee. Protocol refunds that amount (when non-zero) to the transaction source account. The ledger modification due to refund is reflected under txChangesAfter in the meta.

Note, that refund happens for the failed transactions as well.

Per-resource specifications

This section describes the fee contributions, per-transaction/per-ledger maximum limits and apply-time enforcement for all the transaction resources.

Instructions

Instructions bound the execution time of the transactions in the ledger.

A transaction contains:

• maximum number of CPU instructions that transaction may use sorobanData.resources.instructions

All the configuration values come from ConfigSettingContractComputeV0.

Fee: Instructions_fee(instructions) = round_up(instructions * feeRatePerInstructionsIncrement / 10000)

Validity constraints:

• per transaction
  • resources.instructions <= txMaxInstructions.
• ledger wide (GeneralizedTransactionSet)
  • sum of all resources.instructions <= ledgerMaxInstructions.

Apply-time enforcement: instructions metered during the contract execution may not exceed instructions declared in the transaction. Refer to CAP-0046-10 for metering details.

Ledger data

Ledger data resources bounds the amount and size of ledger reads and writes.

A transaction contains:

• the read sorobanData.resources.footprint.readOnly and read/write sorobanData.resources.readWrite sets of ledger keys.
• the maximum total amount of data that can be read from the ledger in bytes sorobanData.resources.readBytes
• the maximum total amount of data that can be written to the ledger in bytes sorobanData.resources.writeBytes

All the configuration values come from ConfigSettingContractLedgerCostV0.

Fee:

LedgerDataAccess_fee(resources) =
  (length(resources.footprint.readOnly)+length(resources.footprint.readWrite))*feeReadLedgerEntry + // cost of reading ledger entries
  length(resources.footprint.readWrite)*feeWriteLedgerEntry + // cost of writing ledger entries
  round_up(resources.readBytes * feeRead1KB / 1024) + // cost of processing reads
  round_up(write_fee_per_1kb(BucketListSize)* resources.writeBytes / 1024) // cost of adding to the bucket list

where BucketListSize is the average size of the bucket list over the moving window. Refer to the State Archival CAP for details, and write_fee_per_1kb is a function that determines the ledger write fee per 1024 bytes based on the bucket list size and is defined as follows:

// this is the fee rate slope
// feeRate1KB = (writeFee1KBBucketListHigh - writeFee1KBBucketListLow)/bucketListTargetSizeBytes

// in all cases, rate is clamped as to not fall under MINIMUM_WRITE_FEE_PER_1KB in case
// writeFee1KBBucketListLow or writeFee1KBBucketListHigh are too low

// if s < bucketListTargetSizeBytes,
//   grow by feeRate1KB until we reach writeFee1KBBucketListHigh
write_fee_per_1kb(s) = max(MINIMUM_WRITE_FEE_PER_1KB,
    (writeFee1KBBucketListHigh - writeFee1KBBucketListLow)*s/bucketListTargetSizeBytes)

// else (s >= bucketListTargetSizeBytes),
//   grow by bucketListWriteFeeGrowthFactor*feeRate1KB from writeFee1KBBucketListHigh
write_fee_per_1kb(s) = max(MINIMUM_WRITE_FEE_PER_1KB,
    writeFee1KBBucketListHigh +
    bucketListWriteFeeGrowthFactor*(writeFee1KBBucketListHigh - writeFee1KBBucketListLow)*
        (s-bucketListTargetSizeBytes)/bucketListTargetSizeBytes)

Validity constraints:

• per transaction
  • length(resources.footprint.readOnly) + length(resources.footprint.readWrite) <= txMaxReadLedgerEntries.
  • resources.readBytes <= txMaxReadBytes.
  • length(resources.footprint.readWrite) <= txMaxWriteLedgerEntries.
  • resources.writeBytes <= txMaxWriteBytes.
• ledger wide (GeneralizedTransactionSet)
  • sum(length(resources.footprint.readOnly) + length(resources.footprint.readWrite)) <= ledgerMaxReadLedgerEntries.
  • sum(length(resources.footprint.readWrite)) <= ledgerMaxWriteLedgerEntries.
  • sum(resources.readBytes) <= ledgerMaxReadBytes.
  • sum(resources.writeBytes) <= ledgerMaxWriteBytes.

Apply-time enforcement:

• Before executing the transaction logic all the entries in the footprint (both read-only and read-write) are read from the ledger and the total read size is computed by adding the size of the key and size of the entry read (if any) to the total value. If total read size exceeds resources.readBytes, transaction fails.
• During the host function execution any read/write of a ledger key outside of the footprint (or write of a read-only entry) leads immediately to a transaction failure.
• After the execution the total size of the writes is computed by adding sizes of the keys and values of the non-removed entries. If the total write size exceeds resources.writeBytes, transaction fails. Entry deletion is 'free' and not counted towards the total write size.

Bandwidth related

Bandwidth utilization is bounded by the total size of the transactions flooded and included to the ledger.

All the configuration values come from ConfigSettingContractBandwidthV0.

A transaction contains:

• implicitly, its impact in terms of bandwidth utilization, the size (in bytes) of the TransactionEnvelope

Fee: NetworkData_fee(txEnvelope) = round_up(size(txEnvelope) * feeTxSize1KB / 1024)

Validity constraints:

• per transaction
  • size(txEnvelope) <= txMaxSizeBytes
• ledger wide
  • sum of all size(txEnvelope) <= ledgerMaxTxsSizeBytes.

Apply-time enforcement: None

Historical storage

Historical storage is utilized for any transaction result and hence the fee has to be paid unconditionally. The fee depends on TransactionEnvelope size.

All the configuration values come from ConfigSettingContractHistoricalDataV0.

Fee: Historical_flat_fee(txEnvelope) = round_up((size(txEnvelope)+TX_BASE_RESULT_SIZE) * feeHistorical1KB / 1024)

Where TX_BASE_RESULT_SIZE is a constant approximating the size in bytes of transaction results published to archives and is set to 300.

Validity constraints: None

Apply-time enforcement: None

Contract events and return value

Contract events are a 'side' output of the transaction that is written to metadata and not to ledger. Invocation return value has the same properties and thus is included into this as well.

Note, that ledger changes are also emitted in metadata for transaction, but their size is bounded by proxy with ledger access limits and we can consider write fees to also cover metadata writes as well.

All the configuration values come from ConfigSettingContractEventsV0.

Fee: Events_fee(eventsBytes) = round_up(eventsBytes * feeContractEvents1KB / 1024)

Validity constraints: None

Apply-time enforcement:

• compute the consumed events size as the sum of events emitted during the host function invocation and its return value. If total size exceeds ConfigSettingContractEventsV0.txMaxContractEventsSizeBytes, the transaction fails

Rent fee

Rent fee has to be paid if operation increases the lifetime of the ledger entries and/or increases entry size.

Rent fee is computed only at transaction application time and it depends on the state of the ledger entries before and after the transaction has been applied.

Fee: Rent_fee = sum(rent_fee_per_entry_change(entry_before, entry_after)) + ttl_write_fee for all the ledger entry changes.

Entry rent fee consists of two components: fee for renting new ledgers with the new entry size and fee for renting the old ledgers with increased size. If entry_before does not exist, we treat its size as 0 and live_until_ledger as 0 for the sake of this formula.

rent_fee_per_entry_change(entry_before_entry_after) = 
  if (entry_after.live_until_ledger > entry_before.live_until_ledger,
      rent_fee_for_size_and_ledgers(
          entry_after.is_persistent,
          size(entry_after),
          new_live_until_ledger - max(entry_before.live_until_ledger, current_ledger - 1)),
      0) +
  if (exists(entry_before) && size(entry_after) > size(entry_before),
      rent_fee_for_size_and_ledgers(
          entry_after.is_persistent,
          size(entry_after) - size(entry_before),
          entry_before.live_until_ledger - current_ledger + 1),
      0)

rent_fee_for_size_and_ledgers is the main rent primitive that computes the fee for renting S bytes of ledger space for the period of L ledgers:

rent_fee_for_size_and_ledgers(is_persistent, S, L) = round_up(
    S * L * write_fee_per_1kb(BucketListSize) /
    (1024 *
     if (is_persistent, persistentRentRateDenominator, tempRentRateDenominator))
)

Settings values come from StateArchivalSettings.

Additionally, we charge for the TTLEntry writes of entries that had liveUntilLedgerSeq changed using the same rate as for any other entry write:

ttl_write_fee =  
  num_ttl_updates * feeWriteLedgerEntry +
  round_up(write_fee_per_1kb(BucketListSize) * TTL_ENTRY_SIZE / 1024)

where num_ttl_updates is the number of ledger entries that had live_until_ledger updated and TTL_ENTRY_SIZE is size of TTLEntry with its key and is set to 68 bytes.

Validity constraints: None

Apply-time enforcement: None

Operations

Every Soroban transaction must contain exactly 1 operation. There is no fee for operations, but there is a ledger-wide limit on transactions (and thus operations) defined by ConfigSettingContractExecutionLanesV0.ledgerMaxTxCount.

'Fee bump' semantics

Soroban transactions are compatible with the 'fee bump' mechanism via FeeBumpTransactionEnvelope. Total transaction fee can be increased in this way in order to account for the higher network contention. However, fee bump transactions can only modify the overall fee of transaction and their semantics is independent of the inner ('bumped') transaction. This leads to the following of the Soroban 'fee bumps':

• sorobanData.resourceFee can not be increased via FeeBumpTransactionEnvelope, so only the inclusion fee can be raised
• sorobanData.resources can not be modified either, which is why the fee bump envelope is transparent for the resource accounting, i.e. it is not accounted for when computing the transaction size for the sake of enforcing limits/charging the fees
• The point former also applies to the TransactionSet validation: ledgerMaxTxsSizeBytes limit enforcement only includes sizes of the inner envelopes of the fee bump transactions

The relation between the resouce and inclusion fees for Soroban 'fee bumps' is defined in the same fashion as for regular Soroban transactions:

feeBumpTx.fee = feeBumpTx.innerTx.sorobanData.resourceFee + fullInclusionFee

Protocol treats 'fee bump' as an additional operation. Thus the effective inclusion fee bid used for transaction prioritization is defined as follows:

inclusionFeeBid = fullInclusionFee / 2 = (feeBumpTx.fee - feeBumpTx.innerTx.sorobanData.resourceFee) / 2

Soroban transactions might fail at apply time due to too low declared resource values or too low refundable fee. We don't provide any built-in way for re-using the failed transactions in the first version of Soroban. However, the user experience can be significantly improved by decoupling the transaction signature from the signatures used for the host function invocation itself, specifically by using the Soroban Authorization Framework (CAP-0046-11). If all the signatures are decoupled, then any party can pay the transaction fees and sign new transactions in case of failure and there is no need to use FeeBumpTransactionEnvelope at all (which is cheaper). Soroban nonces will only be consumed on transaction success, so the signatures can be re-used as many times as needed until the transaction succeeds.

Future work

Initial implementation of 'fee bumps' follows the 'classic' rules, which simplifies the protocol design, but comes with a number of shortcomings:

• It's not possible to increase the resource fee
• It's not possible to increase the declared resources
• The inclusion fee has to be 2x of the inclusion fee for the regular transactions

Future protocol versions may fix these shortcomings by introducing the new type of the 'fee bump' transaction envelope that addresses these shortcomings. The envelope will need to have the SorobanData extension that overrides the SorobanData of the inner transaction, so that every relevant value can be increased. The new envelope may also have a different inclusion fee semantics that wouldn't count the 'fee bump' as an additonal operation.

Design Rationale

Fee estimation

This proposal relies heavily on the existence of a "preflight" mechanism to determine all parameters needed to compute fees.

Additional logic (not covered in this CAP), will be needed to determine the market rate of resources based for example on historical data (see below).

Resources

Fees are used to ensure fair and balanced utilization of resources.

For each resource type, we're assuming a model where we can define:

• the maximum resource consumption for a transaction, as to protect the network.
• a reasonable price for any given transaction, as to ensure that there are no broken markets
• additional constraints may include
  • a "ledger wide" maximum as to protect the network and downstream systems when producing blocks.
  • "execution lane" maximum, as to ensure that execution lanes (executed in parallel), are balanced. This CAP does not attempt to define actual semantics or fee models related to parallel execution, and is mentioned here for context.

We’re also assuming that resource allocation is done independently of “classic” transactions (ie: the amount of resources allocated to smart contract execution is independent of other traffic). This points to “smart contract transactions” being managed as their own “phase” (in GeneralizedTransactionSet terminology) and having its own dedicated capacity expressed in terms of transactions (ledgerMaxTxCount).

Reasonable fees should be more than some minimum (on top of "on chain market dynamics") both to combat "spam" transactions and ensure that there is no strange incentive to perform certain operations on chain instead of performing them on other systems with worse properties (like centralized cloud infrastructure).

Validators are expected to vote regularly (once a quarter for example) to ensure that fees are set correctly for the broader ecosystem. The exact way fee parameters are established is outside the scope of this document.

Compute

CAP-0046: WebAssembly Smart Contract Runtime Environment introduces the notion of virtual instructions. In the context of this CAP, the only thing that matters is that an "instruction" represents an arbitrary base unit for "execution time".

As a consequence, the "goal" for validators is to construct a GeneralizedTransactionSet that uses up to lcl.ConfigSettingContractComputeV0.ledgerMaxInstructions.

Ledger data

Read traffic

Reads are logically performed before transaction execution.

When performing reads of a ledger entry:

• The ledger entry needs to be located via some index in the ledger and the entry loaded. Depending on the underlying database technology, this translates to at least 1 disk operation.
• The bucket entry needs to be xdr decoded.

The resources to allocate in this context are therefore:

• a maximum number of ledger entry read operations in a ledger ledgerMaxReadLedgerEntries.
• a maximum number of bytes that can be read in a ledger ledgerMaxReadBytes.

The cost of a "ledger entry read" is fairly open ended, and depends on many variables. In this proposal, we give it a "base cost" for simplicity even if it translates to multiple disk operations (which is typically the case when using B-Trees for example, or if the ledger entry is retrieved by lookup over multiple buckets).

That "base cost" is defined by validators as feeReadLedgerEntry. This proposal does not let transactions compete directly on the number of ledger entry read operations, therefore the cost of a read operation is feeReadLedgerEntry (validators must still construct transaction sets that keep the number of reads below a maximum).

Transactions contain the total number of bytes that they will read from the bucket list as well at a fee bid for reading those bytes.

The number of bytes read corresponds to the size of the latest BucketEntry for that ledger entry (and does not take into account the possibility that an implementation may read stale entries in buckets or may have to read other entries from a bucket).

The fee is determined based on the rate feeRead1KB expressed for reading 1 KB (1024 bytes) worth of data.

As transactions compete for the total read capacity ledgerMaxReadBytes for a given ledger, the inclusion fee goes up.

Write traffic and ledger size

Writes are performed after transaction execution, and are blocking the actual closing of a ledger.

When writing a ledger entry:

• The bucket entry is marshaled to binary.
• The bucket entry is appended to the topmost bucket serially.
• The bucket entry is read, hashed and written back with every level merge operation.

In this proposal, we're modeling "worst case": a bucket entry gets added to the bucket list and has to travel all the way to the bottom bucket, contributing as many bytes as the bucket entry itself.

In that case, the overhead is dominated by the size of buckets and bucket entries, and the number of bucket entries is not really a factor when merging.

Consequently, we can model the cost of a write as an append to the overall bucket list and charge a "base rate" for adding a bucket entry.

For allocating ledger entry writes, the model is analogous to "reads": a ledger is constructed as to not exceed ledgerMaxWriteLedgerEntry writes and each write contributes feeWriteLedgerEntry to the overall fee for that transaction (no market dynamics here).

As for "bytes written", the model that was chosen is:

• use the total bucket list size as the main resource to track.
• a cost function, allows to price the cost of expanding ledger size.
• ledger size, and therefore price of storage, goes down as bucket entries get merged/deleted.

The cost function that was selected is similar to what was proposed in Ethereum's make EIP 1559 more like an AMM curve.

The main point being that the fee for adding b bytes to a bucket list of size s is calculated as fee(b,s) = lfee(s + b) - lfee(s), where lfee is the "total cost to build a bucket list of a given size". When designing for specific properties of that function, it's useful to see the "fee rate": fee_rate(s) = lim b->0, fee(b, s)/ b = (lfee(s+b) - lfee(b))/b, is the derivative of lfee, ie fee_rate(s) = lfee'(s).

Properties that we're looking for:

• validators should be able to pick parameters such that total bucket list size can grow to size M_base (that is deemed manageable by the ecosystem), but puts up a lot of resistance to grow to size M_base+M_buffer and beyond.
• fee_rate(s) should provide enough feedback for users and use cases to self-correct. It would not be desirable at the extreme to have very low fees up to M_base and suddenly "hit a wall" where fees shoot up to extremely high numbers after that.

Given those, the choice for fee_rate is constructed as the superposition of the following 2 functions (integrating yields the respective lfee component):

• (feeRateM - feeRate)*s/M_base + feeRate --> (feeRateM - feeRate)*s^2/(2*M_base) + feeRate*s
• if s > M_base, exp(K*(s-M_base)/B_buffer) --> exp(K*(s-M_base)/B_buffer)*B_buffer/K

Where feeRate and feeRateM are the fee rate at size 0 and M_base respectively.

Which together yields:

lfee(s) = (feeRateM - feeRate)*s^2/(2*M_base) + feeRate*s + (if s > M_base, exp(K*(s-M_base)/B_buffer), 0).

With K picked such that fee(1, M_base+M_buffer) is orders of magnitude larger than what the market would be willing to pay.

We simplify those functions further by charging fees linearly to the number of bytes within a ledger (see rationale below).

As a consequence the final formula looks like this:

fee(b) = round_up(b*fee_rate(s))

With fee_rate(s) = (feeRateM - feeRate)*s/M_base + feeRate + if (s > M_base, exp(K*(s-M_base)/B_buffer), 0)

We can simplify this even further by replacing the exponential component by a steep linear slope that causes fees to be "extremely high" at M_buffer, which turns the formula into what is specified above:

fee_rate(s) = (feeRateM - feeRate)*s/M_base + feeRate + if (s > M_base, K*(s-M_base)/B_buffer, 0)

where K >= 1.

Ledger size averaging

Tracking the ledger size for every ledger introduces unnecessary noise that leads to the following issues:

• flooding might be somewhat imprecise due to fees changing every ledger with a risk of transaction becoming invalid
• wrong incentives, such as trying to pay the rent for a long time period right after the bucket list merge ledger
• fee estimations are harder for the clients

To alleviate all of these issues, instead of using the current ledger size, this proposal uses the average of the ledger size over the sliding window, that is large enough to average out most of the noise coming from short-term merges and rather representing the ledger size change trends rather than actual size at any moment.

Putting it together

"read/write" operations need to first read data before writing it. The amount of data written back can be larger or smaller than what was read, as consequence:

• The number of ledger entry reads is the size of ledger entries referenced in ledger footprints (both read and read/write).
• The number of bytes to read is the size of bucket entries from both the read and read/write footprints.
• The number of bytes to write is the number of bytes associated with bucket entries referenced by the readWrite footprint.
• The number of ledger entries to write is the size of the read/write footprint.

Ledger size reduction

So far we've established a model for deriving fees based on the bucket list size, but there needs to be a mechanism to ensure that the cost of storage does not grow indefinitely, hurting usability of the network.

Core ideas and principles:

• Ledger space is a shared public resource, policies should be set to ensure fair use.
• cost of using ledger space should converge towards market rate over time
  • in particular creating spam ledger entries should cost market rate over the long term.
• abandoned entries should not cost anything to network participants over the long term.

This proposal therefore depends on a solution with the following high level properties:

• ledger entries have to periodically pay for "rent", where the rent amount is adjusted on a per period basis (as to approximate "market rate")
• ledger entries that do not want to pay for rent anymore should be purged from the ledger, freeing up space for other entries (and lowering the overall price of storage)
  • purged entries may be recoverable by relying on external recovery nodes that can reconstruct proofs that validators can verify.

Historical storage

Historical storage corresponds to data that needs to be persisted by full validators outside of the bucket list.

This includes transactions and their result.

As the data is stored only once but for "eternity", it has to be priced accordingly (at a minimum, this data has to be made available as to allow validators to catch up to the network).

The model retained in the context of this CAP is to just have the validators set a flat rate per byte for this kind of data (updated on a regular basis as to track cost of storage over time).

Transaction Result

In order to reduce the base cost of transactions, the "result" published to archive is fixed size and the actual detailed transaction result is emitted in the meta and accounted for in the same way as contract events. See CAP-0046: Smart Contract Events for more details.

Extended meta data

Extended meta data here refers to parts of the meta data (produced when closing ledgers) that are not related to ledger changes:

• Smart contracts generate "events"
• TransactionResult

See CAP-0046: Smart Contract Events for more details.

Fees are needed to control for the overhead in those systems.

The model retained in this CAP is a flat rate per byte model for simplicity. It is expected that this fee would be orders of magnitude smaller than what is needed to persist data on chain.

Bandwidth

Transactions need to be propagated to peers on the network.

At the networking layer, transactions compete for bandwidth on a per ledger basis (ledgerMaxPropagateSizeBytes).

Note that validators may apply additional market dynamics due to implementation constraints, especially when trying to balance propagating large transactions vs smaller ones. See CAP-0042: Multi-Part Transaction Sets.

Ephemeral payload

In the future, it may be possible to attach a ephemeralPayload (Hash + size), that gets cleared before applying transactions (used in the context of proofs of availability).

Further reading: blob transactions in Ethereum.

Refunds on “flat rate” resources

Some resources are priced determined on a per ledger basis, independently of transaction set composition.

For such resources, a transaction gets charged the “worst case” utilization at the beginning of the transaction execution, and gets refunded based on actual usage at the end of the execution.

No refund for unused capacity on market based resources

If a transaction declares that it wants to use up to X units of a given resource, nominators assemble a transaction set with that information, potentially excluding other transactions because of this.

As a consequence, there should not be any refund for unused capacity. Specifically, if a resource was priced at a given rate by validators, the fee charged will be for the entire capacity (note that this still lets validators provide discounts on the rate).

Transaction fees and prioritization

This proposal assumes that fees charged for resources based on network settings are "fair", and that market dynamics should be shifted towards the "intent" of any given transaction (also called "social value" of a transaction).

This implies that:

• transactions are flooded/included purely based on their social value.
• additional throttling at the overlay layer may occur when some resources are scarce (similar to how in classic, the rate of operations that can be flooded is capped).

Note that the inclusion fee is not related to the amount of work that a transaction does. In other words, a transaction performing twice as much work than another but with the same inclusion fee bid are considered to have the same priority.

This simplification allows to remove entirely the need to model on chain a "synthetic universal resource" that can be used to represent the amount of work a given transaction performs (such as "gas" in Ethereum for example).

The following notable properties are expected with this model:

• adjustment to fee rates can be done using arbitrary models based on historical data, outside of the network
• in the future, additional logic can be added to have some price adjustment based on historical usage (similar to what is done for ledger space)
• validators (via CAP-0042 components) can still group similar transactions together.

Alternate fee model considered: multidimensional and uniform fees

Another way considered at some point was to try to dynamically price resources as to attain some sort of market rate as quickly as possible. This section goes over the approaches to implement "resource markets".

Note that we’re excluding “flat rate” resources where there is no competition from this section.

There are two ways to do it:

• have a separate market for each dimension. Transactions need to explicitly bid on each dimension.
  • This allows accurate price discovery for all resources. For example, if there is a lot of contention on "Instructions", this allows to discover the price of an instruction.
  • Relative priority between transactions is flexible, this is good (more room for innovation by nominators) and bad (harder for clients to know what to do to “get ahead”).
• transactions just specify a "fee bid" for multiple dimensions at once (potentially all markets at once)
  • there needs to be a function that computes the "minimum fee" for a given transaction, mixing all dimensions somehow (polynomial of sorts for example). Effectively creating a "synthetic universal resource".
  • comparing transactions can be done by comparing the ratio between the fee bid and the minimum fee, which is simple.
  • There is no price discovery of individual dimensions as people automatically bid more on all dimensions at once. That said, nominators can just pick "market prices" for each dimension that fits recent network conditions.

Both solutions require nominators to price resources (in much the same way that CAP-0042 allows nominators to price operations in the classic protocol).

The bidding is more complicated with the first approach. In order to come up with a reasonable bid, clients need not only to have 'market prices' for every resource, but also need to take into account the comparison algorithm used during transaction set building. For example, validators may consider ordering transactions by a tuple of bid-to-min-fee ratios for every resource (e.g. (instructions, IO, bandwidth)) and in order to prevent abuse of the fixed order, they would dynamically come up with that order depending on the current contents of the transaction queue. It's not obvious how to bid optimally for such an algorithm, as every ledger priorities might change several times.

For the second approach the bidding is comparable with the classic transactions: there is just a single 'market rate' for the smart contract transactions, that can be both used as a part of the bidding strategy and for comparison. The downside is that it requires maintaining parameters used to give different weights to the various resources as to come up with a "synthetic universal resource" that the network can reason about.

Related work:

• Ethereum Multidimensional EIP-1559.

Protocol Upgrade Transition

None, this fee model will only apply to smart contract transactions.

A subsequent CAP may update the fee model for the existing classic transaction subsystem as to be more consistent with this CAP.

Resource Utilization

There are no significant resource utilization changes compared to the classic fee model.

Security Concerns

The resource fees and limits are introduced to maintain network health and therefore the all the risks are around the network liveness and DOS possibility, but not necessarily security.

Incorrect configuration or incorrect enforcement calibration might lead to high ledger close times or spam.

Test Cases

The fees are covered in most of the Soroban-related test cases.

Implementation

TransactionFrame::validateSorobanResources enforces the limts at transaction validation time.

InvokeHostFunctionOpFrame::doApply performs most of the apply-time resource limit enforcement.

[fees.rs][https://github.com/stellar/rs-soroban-env/blob/d92944576e2301c9866215efcdc4bbd24a5f3981/soroban-env-host/src/fees.rs] file of Soroban host contains all the fee computation logic specified here.