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Making eth_calls
Learn how to perform Contract Calls (eth_calls) in EVM-compatible Substreams
EVM-compatible smart contracts are queryable, which means that you can get real-time data from the contract's internal database. In this tutorial, you will learn how to perform contract calls (
eth_calls) through Substreams.Specifically, you will query the USDT smart contract (
0xdac17f958d2ee523a2206206994597c13d831ec7) to get the number of decimals used by the token. The USDT smart contract exposes a read function called decimals.
USDT contract on EthScan
- 1.First, let's use
substreams initto scaffold a new Substreams project that uses the USDT smart contract:
substreams init
Complete the information required by the previous command, such as name of the project or smart contract to track. In the
Contract address to track step, write 0xdac17f958d2ee523a2206206994597c13d831ec7, the address of the USDT smart contract.Project name (lowercase, numbers, undescores): usdttracker
Protocol: Ethereum
Ethereum chain: Mainnet
Contract address to track (leave empty to use "Bored Ape Yacht Club"): 0xdac17f958d2ee523a2206206994597c13d831ec7
Would you like to track another contract? (Leave empty if not):
Retrieving Ethereum Mainnet contract information (ABI & creation block)
Fetched contract ABI for dac17f958d2ee523a2206206994597c13d831ec7
Fetched initial block 4634748 for dac17f958d2ee523a2206206994597c13d831ec7 (lowest 4634748)
Generating ABI Event models for
Generating ABI Events for AddedBlackList (_user)
Generating ABI Events for Approval (owner,spender,value)
Generating ABI Events for Deprecate (newAddress)
Generating ABI Events for DestroyedBlackFunds (_blackListedUser,_balance)
Generating ABI Events for Issue (amount)
Generating ABI Events for Params (feeBasisPoints,maxFee)
Generating ABI Events for Redeem (amount)
Generating ABI Events for RemovedBlackList (_user)
Generating ABI Events for Transfer (from,to,value)
Writing project files
Generating Protobuf Rust code
- 2.Move to the project folder and build the Substreams.
make build
- 3.Then, verify that the Substreams runs correctly. By default, it will output all the events of the smart contract.
substreams run -e mainnet.eth.streamingfast.io:443 \
substreams.yaml \
map_events \
--start-block 12292922 \
--stop-block +1
The previous command will output the following:
Progress messages received: 0 (0/sec)
Backprocessing history up to requested target block 12292922:
(hit 'm' to switch mode)
----------- BLOCK #12,292,922 (e2d521d11856591b77506a383033cf85e1d46f1669321859154ab38643244293) ---------------
{
"@module": "map_events",
"@block": 12292922,
"@type": "contract.v1.Events",
"@data": {
"transfers": [
{
"evtTxHash": "90e4fd16c989cdc7ecdfd0b6f458eb4be1c538901106bb794bb608f38ac9dd9f",
"evtIndex": 1,
"evtBlockTime": "2021-04-22T23:13:40Z",
"evtBlockNumber": "12292922",
"from": "odjZclYML4FEr4cdtQjwsLEKP78=",
"to": "XmM2sGcWQDHSwcLHo85fcWEdAcw=",
"value": "372200000"
}
]
}
}
all done
The
substreams init command generates Rust structures based on the ABI of the smart contract provided. All the calls are available under the abi::contract::functions namespace of the generated code. Let's take a look.- 1.Open the project in an editor of your choice (for example, VS Code) and navigate to the
lib.rsfile, which contains the main Substreams code. - 2.Create a new function,
get_decimals, which returns aBigIntstruct:
fn get_decimals() -> substreams::scalar::BigInt {
}
- 3.Import the
abi::contract::functions::Decimalsstruct from the generated ABI code.
fn get_decimals() -> substreams::scalar::BigInt {
let decimals = abi::contract::functions::Decimals {};
}
- 4.Next, use the
callmethod to make the actual eth_call by providing the smart contract address:
fn get_decimals() -> substreams::scalar::BigInt {
let decimals = abi::contract::functions::Decimals {};
let decimals_option = decimals.call(TRACKED_CONTRACT.to_vec());
decimals_option.unwrap()
}
In this case, the
call method returns a substreams::scalar::BigInt struct containing the number of decimals used in the USDT token (6).- 5.You can include this function in the
map_eventsmodule just for testing purposes:
#[substreams::handlers::map]
fn map_events(blk: eth::Block) -> Result<contract::Events, substreams::errors::Error> {
let evt_block_time =
(blk.timestamp().seconds as u64 * 1000) + (blk.timestamp().nanos as u64 / 1000000);
// Using the decimals function
let decimals = get_decimals();
substreams::log::info!("Number of decimals for the USDT token: {}", decimals.to_string());
...output omitted...
}
Important: Remember that this tutorial shows how to call the
decimals function, but all the available calls are under the abi::contract::functions namespace, so you should be able to find them just by exploring the auto-generated ABI Rust files.- 6.To see it in action, just re-build and re-run the Substreams:
make build
substreams run -e mainnet.eth.streamingfast.io:443 \
substreams.yaml \
map_events \
--start-block 12292922 \
--stop-block +1
The output should be similar to the following:
Connected (trace ID 6fb1a55ed17001d850d8c6655226ef6f)
Progress messages received: 0 (0/sec)
Backprocessing history up to requested target block 12292922:
(hit 'm' to switch mode)
----------- BLOCK #12,292,922 (e2d521d11856591b77506a383033cf85e1d46f1669321859154ab38643244293) ---------------
map_events: log: Number of decimals for the USDT token: 6
{
"@module": "map_events",
"@block": 12292922,
"@type": "contract.v1.Events",
"@data": {
"transfers": [
{
"evtTxHash": "90e4fd16c989cdc7ecdfd0b6f458eb4be1c538901106bb794bb608f38ac9dd9f",
"evtIndex": 1,
"evtBlockTime": "1619133220000",
"evtBlockNumber": "12292922",
"from": "odjZclYML4FEr4cdtQjwsLEKP78=",
"to": "XmM2sGcWQDHSwcLHo85fcWEdAcw=",
"value": "372200000"
}
]
}
}
all done
Last modified 1mo ago