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Gas and Fees

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

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 read 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 will be finalized. This is implemented by default in the SDK via the block gas meter.
  • Prevent spam and abuse from end-user. 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 SDK does not enforce gas pricing by default, as there may be other ways to prevent spam (e.g. bandwidth schemes). Still, most applications will implement fee mechanisms to prevent spam. This is done via 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/v0.40.0-rc3/store/types/gas.go#L34-L43

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.
  • 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.
  • 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 BeginBlock via setDeliverState, and then tracks gas consumption during execution sequences that lead to state-transitions, i.e. those originally triggered by BeginBlock, DeliverTx and EndBlock. At the beginning of each DeliverTx, 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. A new instance of the BlockGasMeter is created each time BeginBlock is called. The BlockGasMeter is finite, and the limit of gas per block is defined in the application's consensus parameters. By default Cosmos SDK applications use the default consensus parameters provided by Tendermint:

+++ https://github.com/tendermint/tendermint/blob/v0.34.0-rc6/types/params.go#L34-L41

When a new transaction is being processed via DeliverTx, the current value of BlockGasMeter is checked to see if it is above the limit. If it is, DeliverTx returns immediately. This can happen even with the first transaction in a block, as BeginBlock itself can consume gas. If not, the transaction is processed normally. At the end of DeliverTx, the gas tracked by ctx.BlockGasMeter() is increased by the amount consumed to process the transaction:

ctx.BlockGasMeter().ConsumeGas(
	ctx.GasMeter().GasConsumedToLimit(),
	"block gas meter",
)

AnteHandler

The AnteHandler is run for every transaction during CheckTx and DeliverTx, before a Protobuf Msg service method for each sdk.Msg in the transaction. AnteHandlers have the following signature:

// AnteHandler authenticates transactions, before their internal messages are handled.
// If newCtx.IsZero(), ctx is used instead.
type AnteHandler func(ctx Context, tx Tx, simulate bool) (newCtx Context, result Result, abort bool)

The anteHandler is not implemented in the core SDK but in a module. This gives the possibility to developers to choose which version of AnteHandler fits their application's needs. 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 transaction 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/v0.40.0-rc3/types/tx_msg.go#L49-L57 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/v0.40.0-rc3/proto/cosmos/tx/v1beta1/tx.proto#L12-L25
  • 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 is greater than the local min-gas-prices (as a reminder, gas-prices can be deducted 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 ensure 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 extremely important, as it not only makes sure the transaction cannot consume infinite gas, but also that ctx.GasMeter is reset in-between each DeliverTx (ctx is set to newCtx after anteHandler is run, and the anteHandler is run each time DeliverTx is called).

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 DeliverTx returns.

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