Stateless Smart Contracts

  1. The Dumb Contract
  2. The Filter

Dumb Contract

contract DataStore {
mapping(address => mapping(bytes32 => string)) public store;

event Save(address indexed from, bytes32 indexed key, string value);

function save(bytes32 key, string value) {
store[msg.sender][key] = value;
Save(msg.sender, key, value);
contract DataStore {
function save(bytes32 key, string value) {}


const abi = [
constant: false,
inputs: [
{ name: "key", type: "bytes32" },
{ name: "value", type: "string" }
name: "save",
outputs: [],
payable: false,
type: "function"
const decoder = new InputDataDecoder(abi);
const decodeInput = input => decoder.decodeData(input);
const processArgs = input =>, i) => {
const type = input.types[i];
if (type === "string") {
return arg;
if (type === "bytes32") {
const toHex = `0x${arg.toString("hex")}`;
return web3.toUtf8(toHex);
return arg;
const run = async () => {
const tx = "0xc9fdf51d...";
const transaction = await web3.eth.getTransaction(tx);
const input = decodeInput(transaction.input);
if ( === "save") {
const args = processArgs(input);
const address = transaction.from;
const key = args[0];
const value = args[1];
// save the address / key / value to a database

Extras Details

function registerUsername(bytes32 username) external {
var hasUsername = usernames[msg.sender];
var isOwned = addresses[username];
if (isOwned != 0) throw; // prevents registration of existing username
if (hasUsername != 0) throw; // prevents registered address from registering another username
if (!isLowercase(username)) throw; // username must be lowercase
usernames[msg.sender] = username;
addresses[username] = msg.sender;
function post(string data) external {
var username = usernames[msg.sender];
if (username == 0) throw; // user must be registered



  1. A stateless design requires a mix of decentralization and centralization to provide the best user experience / gas cost.
  2. Other contracts cannot access your data.
  3. If your contract has no events you must processes every block to find relevant transactions. This can be mitigated by firing off empty events with marginal extra gas costs.




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James Childs-Maidment

James Childs-Maidment

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