[TASK] Calculate estimated fee for substrate based transactions

[salman01zp]

Overview

Calculate the estimated fee for relaying substrate based transactions/extrinsic. Fees on substrate based chains are calculated based on three parameters

• Base Fee
• Length Fee
• Network congestion

You can read more about fee details here

Here is an open issue on subxt Return fee details for an Extrinsic #527

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Task Checklist

Similar to EVM Fees Enpoint, we will need one for Substrate.

• Add an API endpoint that takes in the (TypedChainId, VAnchorTreeId, estimatedXtFees) and returns the relayer estimated fees.
• Calculate the estimated fee for relaying transaction (see below for the calculation)
• Add validation check for fee submitted by a user.
• Write tests to ensure that the above functionality works correctly

Estimate the Extrinsic fees

There is an RPC method that you can call with the signed extrinsic call and it will return the fees details, this RPC is payment_queryFeeDetails. Here is an example:

• Request Payload:

{"id":1,"jsonrpc":"2.0","method":"payment_queryInfo","params":["0xad0184008eaf04151687736326c9fea17e25fc5287613693c912909cb226aa4794f26a48010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010124010000000004306d000000"]}

• Response:

{"jsonrpc":"2.0","result":{"weight":1307595,"class":"normal","partialFee":"1130000000100090506"},"id":1}

On the client side, the user will use this method to estimate the fees so they can send to the relayer.

You will notice the following information in the result:

partialFee = "1130000000100090506";

The final fee is like the following:

// Calculates the fees in how much tokens
fee = parseInt(partialFee) / (10 ** tokenDecimals)

Example:

tokenDecimals = 18; // for tTNT (Tangle Standalone Test Network)
partialFee = "1130000000100090506";
fee = parseInt(partialFee) / (10 ** tokenDecimals) // => 1.1300 tTNT

// The calculated fees above, is the base transaction fees that the relayer will pay for sending this transaction.

If you are using polkadot API, you can see the docs here about estimating the transaction fees.

Wrapped Token to the Native token Conversion

When working on the VAnchors, we are working on wrapped tokens, each VAnchor as an associated assetId that can be later used to get that asset information, for example say we have assetId = 1, by calling assetRegistry.assets we will get the following:

{
  name: webbtTNT
  assetType: PoolShare
  existentialDeposit: 100,000,000,000,000,000
  locked: false
}

You will notice the name is webbtTNT and it is a wrapped token over the native tTNT token, so it should be 1:1 conversion in this case.

Please Notice that since currently there is no way for us to fetch prices of Substrate tokens, we will (for now) hardcode the price of the 1 tTNT = $0.10. That also could change in the future.

[shekohex]

Related to tangle-network/tangle#151