How do I find the gas consumed by a contracts extrinsic?

Question

I am querying extrinsic data and I am able to fetch gasLimit and gasPrice out of Extrinsic.method.args. But how do I fetch the amount of gas used from an extrinsic?

Answer 1

Gas and weight are one and the same concept when it comes to pallet-contracts. We merely kept the name gas for historic reasons. The gas limit you supply to the contracts.call extrinsic directly influences the weight of the extrinsic (1to1 translation).

So what you want to index is the system.ExtrinsicSuccess or system.ExtrinsicFailed event emitted by the contracts.call exrinsic. It contains a weight field which tells you the overall weight of the extrinsic. This is essentially the gas that was consumed + static overhead of the extrinsic.

I think things get clearer when you look at the weight annotation of the call extrinsic:

#[pallet::weight(T::WeightInfo::call().saturating_add(*gas_limit))]
pub fn call() { ... }

Answer 2

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With a contracts call extrinsic there will be an associated event ExtrinsicSuccess where you can deduce the gas consumed. You may want to look at using an indexer.

Answer 3

If you are using the Polkadot API, you need to check the Contract section (https://polkadot.js.org/docs/api-contract/start/contract.read/):

const { gasConsumed, result, output } = await contract.query.get(alicePair.address, { value, gasLimit });

Interacting with your contract (querying or executing functions) will return information like the gasConsumed in the execution, as shown above.