What are Smart Contract Events?

Smart contract events broadcast new data coming from a contract.

1.1 Why are Smart Contract Events Important?

Smart contract event listening notifies applications and users in real-time that something new has happened in a smart contract.

1.2 Where are Events Used in Smart Contract Applications?

1.2.1 Frontend

If you swap tokens on a website using an AMM DEX smart contract, the token balances will know when to be updated on the front end almost instantly using events.

1.2.2 Backend

Trading bots can listen to AMM DEX swap contract events and find potential arbitrage opportunities. IoT robotics applications in the physical world connected to a smart contract can be controlled with events. Chainlink oracle nodes also depend on smart contract events to know when to communicate between the real world and blockchains.

1.3 How to Listen to Smart Contract Events on Shardeum?

1.3.1 WebSockets

We are planning to add WebSocket RPC URL support in the future to subscribe to smart contract events. This way, users will be able to save API calls instead of polling for new data every new block/bundle/cycle.

1.3.2 Polling

Reading smart contract events can also be done using Shardeum cycles (we listen to blocks/bundles to do this).

1.4 Reading events with Shardeum Cycles:

To get the current cycle: get the latest block, then divide by 10, and round down.
Build the JSON URL with:

Example with:

startCycle = endCycle = 49
address = 0x23FF65f07cAbAd1643440a0114d71260F2Bb6352

https://explorer-dapps.shardeum.org/api/transaction?startCycle=19020&endCycle=19045&address=0x245E2395712F888CeD1033C924115105dC32e388

Filter for transactions per page [note, 10 transactions per page]:

https://explorer-dapps.shardeum.org/api/transaction?startCycle=19020&endCycle=19045&address=0x245E2395712F888CeD1033C924115105dC32e388&page=1

1.5 JSON URL Filter Variables

?startCycle=lastestCycle
&endCycle=lastestCycle
&address=addressToListenTo
&page=1

1.6 Event Listening With Polling Examples:

Reading transaction events from the null address (address(0)) from cycle 0 to 1000:

1.6.1 Javascript

const axios = require('axios');

let baseUrl = "https://explorer-dapps.shardeum.org/api/transaction?startCycle=19020&endCycle=19045&address=0x245E2395712F888CeD1033C924115105dC32e388"

getTransactionsToAddressCycleRange(baseUrl)

async function readJSONLoop(totalTransactions) {

    let total = totalTransactions;
    let pageIndex = 1

    while ( total > 0 ) {

        let filterUrl = baseUrl + "&page=" + pageIndex
        console.log(filterUrl)

        let responseRawJSON = await axios.get(filterUrl);
        responseRawJSON = responseRawJSON.data;
        console.log(responseRawJSON);

        total -= 10;
        pageIndex++;
    }

}

async function getTransactionsToAddressCycleRange(baseUrl) {

    let responseRawJSON = await axios.get(baseUrl);
    let responseDataJSON = responseRawJSON.data;
    let totalTransactions = responseDataJSON.totalTransactions
    console.log(totalTransactions);

    readJSONLoop(totalTransactions)

}

1.6.2 Python

from urllib.request import urlopen
import json

transactionsInCycleRangeUrlString = "https://explorer-dapps.shardeum.org/api/transaction?startCycle=19020&endCycle=19045&address=0x245E2395712F888CeD1033C924115105dC32e388"
transactionsInCycleRangeUrlOpened = urlopen(transactionsInCycleRangeUrlString)
transactionsInCycleRangeUrlJSON = json.loads(transactionsInCycleRangeUrlOpened.read())
totalTransactions = transactionsInCycleRangeUrlJSON["totalTransactions"]
print(totalTransactions)
pageIndex = 1

while totalTransactions > 0:
    print(pageIndex)
    print(totalTransactions)
    pageIndexIncrementUrlString = transactionsInCycleRangeUrlString + "&page=" + str(pageIndex)
    pageIndexIncrementUrlOpened = urlopen(pageIndexIncrementUrlString)
    rawTransactionDataPage = json.loads(pageIndexIncrementUrlOpened.read())
    print(rawTransactionDataPage)
    totalTransactions -= 10
    pageIndex += 1

1.6.3 Go

package main

import (
   "io/ioutil"
   "log"
   "net/http"
   "encoding/json"
   "strconv"
)

func main() {

   baseUrl := "https://explorer-dapps.shardeum.org/api/transaction?startCycle=";
   transactionCount := getTransactionCount(6928, baseUrl)
   log.Println(transactionCount)

   readJsonLoop(transactionCount, baseUrl)

}

type transactionCountJson struct {
    Success bool `json:"success"`
   TotalTransactions int `json:"totalTransactions"`
}

func getTransactionCount(cycleNumber int, baseUrl string) (x int) {

   getRequestUrl := 
      baseUrl +
      strconv.Itoa(cycleNumber)+
      "&endCycle="+
      strconv.Itoa(cycleNumber);
   log.Println(getRequestUrl)

   resp, err := http.Get(getRequestUrl)
   if err != nil {
      log.Fatalln(err)
   }

   rawBodyBytes, err := ioutil.ReadAll(resp.Body)
   if err != nil {
      log.Fatalln(err)
   }

   var transactionCountJsonInstance transactionCountJson

   err = json.Unmarshal(rawBodyBytes, &transactionCountJsonInstance)
   if err != nil {
      log.Fatalln(err)
   }

   return transactionCountJsonInstance.TotalTransactions;
}

func readJsonLoop(totalTransactions int, baseUrl string) {

   total := totalTransactions;
   pageIndex := 1;

   for total > 0 {

      getRequestUrl := 
         baseUrl+
         strconv.Itoa(6928)+
         "&endCycle="+
         strconv.Itoa(6928)+
         "&page=" + 
         strconv.Itoa(pageIndex)
      log.Println(getRequestUrl)

      resp, err := http.Get(getRequestUrl)
      if err != nil {
         log.Fatalln(err)
      }

      rawBodyBytes, err := ioutil.ReadAll(resp.Body)
      if err != nil {
         log.Fatalln(err)
      }

      log.Printf(string(rawBodyBytes))

      total -= 10;
      pageIndex++;
   }

}

1.6.4 Rust

use serde_json;

#[tokio::main]
async fn main() -> Result<(), reqwest::Error> {

    let base_url = String::from("https://explorer-dapps.shardeum.org/api/transaction?startCycle=");
    let cycle_number = 6928;

    let transaction_count : u64 = get_transaction_count(cycle_number.clone(), base_url.clone()).await;    
    println!("transaction_count: {:#?}", transaction_count);

    read_json_loop(cycle_number.clone(), base_url.clone(), transaction_count).await;

    Ok(())
}

async fn get_transaction_count(cycle_number: u64, base_url: String) -> u64   {

    let get_request_url = 
        base_url +
        &cycle_number.to_string() +
        "&endCycle=" +
        &cycle_number.to_string();
    println!("getRequestUrl: {:#?}", get_request_url);

    let new_todo: serde_json::Value = reqwest::Client::new()
        .get(get_request_url)
        .send()
        .await.unwrap()
        .json()
        .await.unwrap();

    println!("JSON raw response: {:#?}", new_todo);
    println!("{:#?}", new_todo["success"]);
    println!("{:#?}", new_todo["totalTransactions"]);
    println!("{:#?}", new_todo["totalTransactions"].as_u64().unwrap());

    return new_todo["totalTransactions"].as_u64().unwrap();

}

async fn read_json_loop(cycle_number: u64, base_url: String, total_transactions: u64) {

   let mut total : i64 = total_transactions as i64; //Convert value to be signed so we do not have an underflow error when the value become negative.
   let mut page_index = 1;

   while total > 0 {

        let get_request_url = 
            base_url.clone() +
            &cycle_number.to_string() +
            "&endCycle=" +
            &cycle_number.to_string() +
            "&page=" + 
            &page_index.to_string();
        println!("getRequestUrl: {:#?}", get_request_url);

        let new_todo: serde_json::Value = reqwest::Client::new()
            .get(get_request_url)
            .send()
            .await.unwrap()
            .json()
            .await.unwrap();

        println!("JSON raw response: {:#?}", new_todo);

        total -= 10;
        page_index += 1;
    }   

}

1.7 Listening for the latest cycle

Listening for the latest cycle, which might contain transaction events from an address:

1.7.1 Javascript

const axios = require('axios');
const ethers = require('ethers')

const rpcURL = "https://dapps.shardeum.org/"

const provider = new ethers.providers.JsonRpcProvider(rpcURL)

const timeMilliSec = 1000;

listenForCycle()

function timeout(ms) {
    return new Promise(resolve => setTimeout(resolve,ms));
}

async function listenForCycle() {
  while (true){

    console.log("Current cycle (1 cycle = 10 blocks [bundles]) ")
    let cycle = await provider.getBlockNumber();
    console.log(Math.floor(cycle/10))

    let baseUrlCycleAddress = "https://explorer-dapps.shardeum.org/api/transaction?startCycle=" + cycle + "&endCycle=" + cycle + "&address=0x0000000000000000000000000000000000000000" 
    console.log(baseUrlCycleAddress)

    let responseRawJSON = await axios.get(baseUrlCycleAddress);
    let responseDataJSON = responseRawJSON.data;
    let totalTransactions = responseDataJSON.totalTransactions
    console.log(totalTransactions);

    readJSONLoopLatestCycle(totalTransactions,baseUrlCycleAddress)

    await timeout(60*timeMilliSec)

  }
}

async function readJSONLoopLatestCycle(totalTransactions,baseUrl) {

    let total = totalTransactions;
    let pageIndex = 1

    while ( total > 0 ) {

        let filterUrl = baseUrl + "&page=" + pageIndex
        console.log(filterUrl)

        let responseRawJSON = await axios.get(filterUrl);
        responseRawJSON = responseRawJSON.data;
        console.log(responseRawJSON);

    total -= 10;
        pageIndex++;
    }

}

1.7.2 Python

from web3 import Web3
import time
import math
from urllib.request import urlopen
import json

ShardeumConnectionHTTPS = "https://dapps.shardeum.org/";
web3 = Web3(Web3.HTTPProvider(ShardeumConnectionHTTPS))

print("Connected to Web3? ")
print(web3.isConnected())

print("Chain ID? ")
print(web3.eth.chain_id)

addressToSubscribeTo = "0x0000000000000000000000000000000000000000"

while True:
print("Current cycle (1 cycle = 10 blocks [bundles]) ")
cycle =  (math.floor(web3.eth.blockNumber/10))  #Divide current bundle [block] by 10, then round down to get cycle.
print(cycle)

transactionsInCycleRangeUrlString = "https://explorer-dapps.shardeum.org/api/transaction?startCycle=" + str(cycle) + "&endCycle=" + str(cycle) + "&address=" + addressToSubscribeTo
print(transactionsInCycleRangeUrlString)
transactionsInCycleRangeUrlOpened = urlopen(transactionsInCycleRangeUrlString)
transactionsInCycleRangeUrlJSON = json.loads(transactionsInCycleRangeUrlOpened.read())
totalTransactions = transactionsInCycleRangeUrlJSON["totalTransactions"]
print(totalTransactions)
pageIndex = 1

while totalTransactions > 0:
    print(pageIndex)
    print(totalTransactions)
    pageIndexIncrementUrlString = transactionsInCycleRangeUrlString + "&page=" + str(pageIndex)
    pageIndexIncrementUrlOpened = urlopen(pageIndexIncrementUrlString)
    rawTransactionDataPage = json.loads(pageIndexIncrementUrlOpened.read())
    print(rawTransactionDataPage)
    totalTransactions -= 10
    pageIndex += 1

time.sleep(60)

1.7.3 Go

package main

import (
   "io/ioutil"
   "log"
   "net/http"
   "encoding/json"
   "context"
   "math/big"
   "strconv"
   "time"

   "github.com/ethereum/go-ethereum/ethclient"
)

func main() {

   client, chainID := clientSetup("https://dapps.shardeum.org/")

   log.Println("chainID: ", chainID)


   for {

      header, err := client.HeaderByNumber(context.Background(), nil)
      if err != nil {
         log.Fatal(err)
      }

      blockNumber := header.Number
      bigNumberTenBundlesPerCycle, _ := new(big.Int).SetString("10", 10)  //Value 10, decimal units also 10. 
      cycleNumber := new(big.Int).Div(blockNumber, bigNumberTenBundlesPerCycle)
      bigNumberOne, _ := new(big.Int).SetString("1", 10)  //Value 1, decimal units 10. 
      cycleNumberMinusOne := new(big.Int).Sub(cycleNumber, bigNumberOne)
      log.Println(cycleNumberMinusOne)

      baseUrl := "https://explorer-dapps.shardeum.org/api/transaction?startCycle=";
      transactionCount := getTransactionCount(cycleNumberMinusOne, baseUrl)
      log.Println(transactionCount)

      readJsonLoop(cycleNumberMinusOne,baseUrl,transactionCount)

      time.Sleep(60 * time.Second)

   }

}

type transactionCountJson struct {
    Success bool `json:"success"`
   TotalTransactions int `json:"totalTransactions"`
}

func getTransactionCount(cycleNumber *big.Int, baseUrl string) (x int) {

   getRequestUrl := 
      baseUrl +
      cycleNumber.String()+
      "&endCycle="+
      cycleNumber.String();
   log.Println(getRequestUrl)

   resp, err := http.Get(getRequestUrl)
   if err != nil {
      log.Fatalln(err)
   }

   rawBodyBytes, err := ioutil.ReadAll(resp.Body)
   if err != nil {
      log.Fatalln(err)
   }

   var transactionCountJsonInstance transactionCountJson

   err = json.Unmarshal(rawBodyBytes, &transactionCountJsonInstance)
   if err != nil {
      log.Fatalln(err)
   }

   return transactionCountJsonInstance.TotalTransactions;
}

func readJsonLoop(cycleNumber *big.Int, baseUrl string, totalTransactions int) {

   total := totalTransactions;
   pageIndex := 1;

   for total > 0 {

      getRequestUrl := 
         baseUrl+
         cycleNumber.String()+
         "&endCycle="+
         cycleNumber.String()+
         "&page=" + 
         strconv.Itoa(pageIndex)
      log.Println(getRequestUrl)

      resp, err := http.Get(getRequestUrl)
      if err != nil {
         log.Fatalln(err)
      }

      rawBodyBytes, err := ioutil.ReadAll(resp.Body)
      if err != nil {
         log.Fatalln(err)
      }

      log.Printf(string(rawBodyBytes))

      total -= 10;
      pageIndex++;
   }

}

func clientSetup(wssConnectionURL string) (client *ethclient.Client, chainID *big.Int) {

   client, err := ethclient.Dial(wssConnectionURL) //Also works with HTTPS connections. 
   if err != nil {
       log.Fatal(err)
   }

   chainID, err = client.NetworkID(context.Background())
   if err != nil {
      log.Fatal(err)
   }
   return
}

1.7.4 Rust

use serde_json;
use ethers_providers::{Middleware, Provider, Http};
use std::thread::{sleep};
use std::time::{Duration};

#[tokio::main]
async fn main() {

    let rpc_shardeum_https = "https://dapps.shardeum.org/";

    let provider = Provider::<Http>::try_from(rpc_shardeum_https).expect("could not instantiate HTTP Provider");

    let chainid = provider.get_chainid().await.unwrap();
    println!("Got chainid: {}", chainid);

    loop{

        let current_block_number = provider.get_block_number().await.unwrap();
        println!("current_block_number: {:?}", current_block_number);

        let current_cycle_number = current_block_number/10;
        println!("current_cycle_number: {:?}", current_cycle_number);

        let current_cycle_number_minus_one = current_cycle_number-1;
        println!("current_cycle_number: {:?}", current_cycle_number_minus_one);

        let base_url = String::from("https://explorer-dapps.shardeum.org/api/transaction?startCycle=");

        let transaction_count : u64 = get_transaction_count(current_cycle_number_minus_one.as_u64().clone(), base_url.clone()).await;    
        println!("transaction_count: {:#?}", transaction_count);

        read_json_loop(current_cycle_number_minus_one.as_u64().clone(), base_url.clone(), transaction_count).await;

        sleep(Duration::from_millis(60000)); // Sleep for 60 seconds.

    }

}

async fn get_transaction_count(cycle_number: u64, base_url: String) -> u64   {

    let get_request_url = 
        base_url +
        &cycle_number.to_string() +
        "&endCycle=" +
        &cycle_number.to_string();
    println!("getRequestUrl: {:#?}", get_request_url);

    let new_todo: serde_json::Value = reqwest::Client::new()
        .get(get_request_url)
        .send()
        .await.unwrap()
        .json()
        .await.unwrap();

    println!("JSON raw response: {:#?}", new_todo);
    println!("{:#?}", new_todo["success"]);
    println!("{:#?}", new_todo["totalTransactions"]);
    println!("{:#?}", new_todo["totalTransactions"].as_u64().unwrap());

    return new_todo["totalTransactions"].as_u64().unwrap();

}

async fn read_json_loop(cycle_number: u64, base_url: String, total_transactions: u64) {

   let mut total : i64 = total_transactions as i64; //Convert value to be signed so we do not have an underflow error when the value become negative.
   let mut page_index = 1;

   while total > 0 {

        let get_request_url = 
            base_url.clone() +
            &cycle_number.to_string() +
            "&endCycle=" +
            &cycle_number.to_string() +
            "&page=" + 
            &page_index.to_string();
        println!("getRequestUrl: {:#?}", get_request_url);

        let new_todo: serde_json::Value = reqwest::Client::new()
            .get(get_request_url)
            .send()
            .await.unwrap()
            .json()
            .await.unwrap();

        println!("JSON raw response: {:#?}", new_todo);

        total -= 10;
        page_index += 1;
    }   

}

What are Polling Cycles in Shardeum?

Table of contents

What are Smart Contract Events?

1.1 Why are Smart Contract Events Important?

1.2 Where are Events Used in Smart Contract Applications?

1.2.1 Frontend

1.2.2 Backend

1.3 How to Listen to Smart Contract Events on Shardeum?

1.3.1 WebSockets

1.3.2 Polling

1.4 Reading events with Shardeum Cycles:

1.5 JSON URL Filter Variables

1.6 Event Listening With Polling Examples:

1.6.1 Javascript

1.7 Listening for the latest cycle

Subscribe to my newsletter

Shreya Bhardwaj

Shreya Bhardwaj

What are Polling Cycles in Shardeum?

Table of contents

What are Smart Contract Events?​

1.1 Why are Smart Contract Events Important?​

1.2 Where are Events Used in Smart Contract Applications?​

1.2.1 Frontend​

1.2.2 Backend​

1.3 How to Listen to Smart Contract Events on Shardeum?​

1.3.1 WebSockets​

1.3.2 Polling​

1.4 Reading events with Shardeum Cycles:​

1.5 JSON URL Filter Variables

1.6 Event Listening With Polling Examples:​

1.6.1 Javascript

1.7 Listening for the latest cycle

Subscribe to my newsletter

Shreya Bhardwaj

Shreya Bhardwaj

What are Smart Contract Events?

1.1 Why are Smart Contract Events Important?

1.2 Where are Events Used in Smart Contract Applications?

1.2.1 Frontend

1.2.2 Backend

1.3 How to Listen to Smart Contract Events on Shardeum?

1.3.1 WebSockets

1.3.2 Polling

1.4 Reading events with Shardeum Cycles:

1.6 Event Listening With Polling Examples: