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.
Pre-requisites
You have some knowledge about Substreams ( and ).
You have the latest version of the installed.
Querying on EthScan
You can query the decimals function by .
USDT contract on EthScan
Initializing the Substreams project
First, let's use substreams init to 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, underscores): 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
Move to the project folder and build the Substreams.
make build
Then, verify that the Substreams runs correctly. By default, it will output all the events of the smart contract.
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.
Open the project in an editor of your choice (for example, VS Code) and navigate to the lib.rs file, which contains the main Substreams code.
Create a new function, get_decimals, which returns a BigInt struct:
Next, use the call method 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).
You can include this function in the map_events module 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.
To see it in action, just re-build and re-run the Substreams:
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
Batching Calls
RPC calls add latency to your Substreams, so you should avoid them as much as possible. However, if you still have to use eth_calls, you should batch them. Batching RPC calls meaning making several calls within the same request.
In the previous USDT example, consider that you want to make three RPC calls: Decimals, Name and Symbol. Instead of creating a request for every call, you can use the substreams_ethereum::rpc::RpcBatch struct to make a single request for all the calls.
In the lib.rs file, create a new function, get_calls() and initialize a batch struct.
fn get_calls() {
let batch = substreams_ethereum::rpc::RpcBatch::new();
}
Add the calls that you want to retrieve by using the ABI of the smart contract: abi::contract::functions::Decimals, abi::contract::functions::Name and abi::contract::functions::Symbol.
The execute() method make the actual RPC call and returns an array of responses. In this case, the array will have 3 responses, one for each call made.
The order used for the response is the same as the order of addition to the request. In this example, responses[0] contains Decimals, responses[1] contains Name, and response[2] contains Symbol.
Decode the Decimals response using the ABI.
fn get_calls() {
let batch = substreams_ethereum::rpc::RpcBatch::new();
let responses = batch
.add(
abi::contract::functions::Decimals {},
TRACKED_CONTRACT.to_vec(),
)
.add(
abi::contract::functions::Name {},
TRACKED_CONTRACT.to_vec(),
)
.add(
abi::contract::functions::Symbol {},
TRACKED_CONTRACT.to_vec(),
)
.execute()
.unwrap()
.responses;
let decimals: u64;
match substreams_ethereum::rpc::RpcBatch::decode::<_, abi::contract::functions::Decimals>(&responses[0]) {
Some(decoded_decimals) => {
decimals = decoded_decimals.to_u64();
substreams::log::debug!("decoded_decimals ok: {}", decimals);
}
None => {
substreams::log::debug!("failed to get decimals");
}
};
}
Then, do the same for Name and Symbol.
fn get_calls() {
let token_address = &TRACKED_CONTRACT.to_vec();
let batch = substreams_ethereum::rpc::RpcBatch::new();
let responses = batch
.add(
abi::contract::functions::Decimals {},
TRACKED_CONTRACT.to_vec(),
)
.add(
abi::contract::functions::Name {},
TRACKED_CONTRACT.to_vec(),
)
.add(
abi::contract::functions::Symbol {},
TRACKED_CONTRACT.to_vec(),
)
.execute()
.unwrap()
.responses;
let decimals: u64;
match substreams_ethereum::rpc::RpcBatch::decode::<_, abi::contract::functions::Decimals>(&responses[0]) {
Some(decoded_decimals) => {
decimals = decoded_decimals.to_u64();
substreams::log::debug!("decoded_decimals ok: {}", decimals);
}
None => {
substreams::log::debug!("failed to get decimals");
}
};
let name: String;
match substreams_ethereum::rpc::RpcBatch::decode::<_, abi::contract::functions::Name>(&responses[1]) {
Some(decoded_name) => {
name = decoded_name;
substreams::log::debug!("decoded_name ok: {}", name);
}
None => {
substreams::log::debug!("failed to get name");
}
};
let symbol: String;
match substreams_ethereum::rpc::RpcBatch::decode::<_, abi::contract::functions::Symbol>(&responses[2]) {
Some(decoded_symbol) => {
symbol = decoded_symbol;
substreams::log::debug!("decoded_symbol ok: {}", symbol);
}
None => {
substreams::log::debug!("failed to get symbol");
}
};
}
Build and run the Substreams.
make build
substreams run -e mainnet.eth.streamingfast.io:443 substreams.yaml map_events --start-block 12292922 --stop-block +1
You should see an output similar to the following:
