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 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 are finalized. This is implemented by default in the Cosmos SDK via the block gas meter.
- Prevent spam and abuse from end-user. To this end,
gasconsumed duringmessageexecution is typically priced, resulting in afee(fees = gas * gas-prices).feesgenerally have to be paid by the sender of themessage. Note that the Cosmos SDK does not enforcegaspricing by default, as there may be other ways to prevent spam (e.g. bandwidth schemes). Still, most applications implementfeemechanisms to prevent spam by using theAnteHandler.
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
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.0if the gas meter is infinite.ConsumeGas(amount Gas, descriptor string)consumes the amount ofgasprovided. If thegasoverflows, it panics with thedescriptormessage. If the gas meter is not infinite, it panics ifgasconsumed 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()returnstrueif the amount of gas consumed by the gas meter instance is strictly above the limit,falseotherwise.IsOutOfGas()returnstrueif the amount of gas consumed by the gas meter instance is above or equal to the limit,falseotherwise.
ctx, and consuming gas is done with the following pattern:
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 CometBFT:
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:
AnteHandler
TheAnteHandler is run for every transaction during CheckTx and DeliverTx, 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:
auth module, the default transaction type is Tx:
- Verify signatures for each
messagecontained in the transaction. Eachmessageshould be signed by one or multiple sender(s), and these signatures must be verified in theanteHandler. - During
CheckTx, verify that the gas prices provided with the transaction is greater than the localmin-gas-prices(as a reminder, gas-prices can be deducted from the following equation:fees = gas * gas-prices).min-gas-pricesis a parameter local to each full-node and used duringCheckTxto 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,gasandgas-prices. This signals how much they are willing to pay for nodes to execute their transaction. The providedgasvalue is stored in a parameter calledGasWantedfor later use. - Set
newCtx.GasMeterto 0, with a limit ofGasWanted. This step is crucial, as it not only makes sure the transaction cannot consume infinite gas, but also thatctx.GasMeteris reset in-between eachDeliverTx(ctxis set tonewCtxafteranteHandleris run, and theanteHandleris run each timeDeliverTxis called).
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.