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Gas and Fees

Synopsis

This document describes the default strategies to handle gas and fees within a Cosmos SDK application.

Pre-requisite Readings

Introduction to Gas and Fees​

In the Cosmos SDK, gas is a special unit that is used to track the consumption of resources during execution. gas is typically consumed whenever reads and writes are made to the store, but it can also be consumed if expensive computation needs to be done. It serves two main purposes:

  • Make sure blocks are not consuming too many resources and are finalized. This is implemented by default in the Cosmos SDK via the block gas meter.
  • Prevent spam and abuse from end-users. To this end, gas consumed during message execution is typically priced, resulting in a fee (fees = gas * gas-prices). fees generally have to be paid by the sender of the message. Note that the Cosmos SDK does not enforce gas pricing by default, as there may be other ways to prevent spam (e.g. bandwidth schemes). Still, most applications implement fee mechanisms to prevent spam by using the AnteHandler.

Gas Meter​

In the Cosmos SDK, gas is a simple alias for uint64, and is managed by an object called a gas meter. Gas meters implement the GasMeter interface

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/store/types/gas.go#L40-L51

where:

  • GasConsumed() returns the amount of gas that was consumed by the gas meter instance.
  • GasConsumedToLimit() returns the amount of gas that was consumed by gas meter instance, or the limit if it is reached.
  • GasRemaining() returns the gas left in the GasMeter.
  • Limit() returns the limit of the gas meter instance. 0 if the gas meter is infinite.
  • ConsumeGas(amount Gas, descriptor string) consumes the amount of gas provided. If the gas overflows, it panics with the descriptor message. If the gas meter is not infinite, it panics if gas consumed goes above the limit.
  • RefundGas() deducts the given amount from the gas consumed. This functionality enables refunding gas to the transaction or block gas pools so that EVM-compatible chains can fully support the go-ethereum StateDB interface.
  • IsPastLimit() returns true if the amount of gas consumed by the gas meter instance is strictly above the limit, false otherwise.
  • IsOutOfGas() returns true if the amount of gas consumed by the gas meter instance is above or equal to the limit, false otherwise.

The gas meter is generally held in ctx, and consuming gas is done with the following pattern:

ctx.GasMeter().ConsumeGas(amount, "description")

By default, the Cosmos SDK makes use of two different gas meters, the main gas meter and the block gas meter.

Main Gas Meter​

ctx.GasMeter() is the main gas meter of the application. The main gas meter is initialized in FinalizeBlock via setFinalizeBlockState, and then tracks gas consumption during execution sequences that lead to state-transitions, i.e. those originally triggered by FinalizeBlock. At the beginning of each transaction execution, the main gas meter must be set to 0 in the AnteHandler, so that it can track gas consumption per-transaction.

Gas consumption can be done manually, generally by the module developer in the BeginBlocker, EndBlocker or Msg service, but most of the time it is done automatically whenever there is a read or write to the store. This automatic gas consumption logic is implemented in a special store called GasKv.

Block Gas Meter​

ctx.BlockGasMeter() is the gas meter used to track gas consumption per block and make sure it does not go above a certain limit.

During the genesis phase, gas consumption is unlimited to accommodate initialisation transactions. 

app.finalizeBlockState.SetContext(app.finalizeBlockState.Context().WithBlockGasMeter(storetypes.NewInfiniteGasMeter()))

Following the genesis block, the block gas meter is set to a finite value by the SDK. This transition is facilitated by the consensus engine (e.g., CometBFT) calling the RequestFinalizeBlock function, which in turn triggers the SDK's FinalizeBlock method. Within FinalizeBlock, internalFinalizeBlock is executed, performing necessary state updates and function executions. The block gas meter, initialised each with a finite limit, is then incorporated into the context for transaction execution, ensuring gas consumption does not exceed the block's gas limit and is reset at the end of each block.

Modules within the Cosmos SDK can consume block gas at any point during their execution by utilising the ctx. This gas consumption primarily occurs during state read/write operations and transaction processing. The block gas meter, accessible via ctx.BlockGasMeter(), monitors the total gas usage within a block, enforcing the gas limit to prevent excessive computation. This ensures that gas limits are adhered to on a per-block basis, starting from the first block post-genesis.

gasMeter := app.getBlockGasMeter(app.finalizeBlockState.Context())
app.finalizeBlockState.SetContext(app.finalizeBlockState.Context().WithBlockGasMeter(gasMeter))

This above shows the general mechanism for setting the block gas meter with a finite limit based on the block's consensus parameters.

AnteHandler​

The AnteHandler is run for every transaction during CheckTx and FinalizeBlock, before a Protobuf Msg service method for each sdk.Msg in the transaction.

The anteHandler is not implemented in the core Cosmos SDK but in a module. That said, most applications today use the default implementation defined in the auth module. Here is what the anteHandler is intended to do in a normal Cosmos SDK application:

  • Verify that the transactions are of the correct type. Transaction types are defined in the module that implements the anteHandler, and they follow the transaction interface:
https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/types/tx_msg.go#L53-L66

This enables developers to play with various types for the transaction of their application. In the default auth module, the default transaction type is Tx: 

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/proto/cosmos/tx/v1beta1/tx.proto#L15-L28
  • Verify signatures for each message contained in the transaction. Each message should be signed by one or multiple sender(s), and these signatures must be verified in the anteHandler.
  • During CheckTx, verify that the gas prices provided with the transaction are greater than the local min-gas-prices (as a reminder, gas-prices can be deduced from the following equation: fees = gas * gas-prices). min-gas-prices is a parameter local to each full-node and used during CheckTx to discard transactions that do not provide a minimum amount of fees. This ensures that the mempool cannot be spammed with garbage transactions.
  • Verify that the sender of the transaction has enough funds to cover for the fees. When the end-user generates a transaction, they must indicate 2 of the 3 following parameters (the third one being implicit): fees, gas and gas-prices. This signals how much they are willing to pay for nodes to execute their transaction. The provided gas value is stored in a parameter called GasWanted for later use.
  • Set newCtx.GasMeter to 0, with a limit of GasWanted. This step is crucial, as it not only makes sure the transaction cannot consume infinite gas, but also that ctx.GasMeter is reset in-between each transaction (ctx is set to newCtx after anteHandler is run, and the anteHandler is run each time a transactions executes).

As explained above, the anteHandler returns a maximum limit of gas the transaction can consume during execution called GasWanted. The actual amount consumed in the end is denominated GasUsed, and we must therefore have GasUsed <= GasWanted. Both GasWanted and GasUsed are relayed to the underlying consensus engine when FinalizeBlock returns.