Events — the only way to query contract history

solidity
event Transfer(address indexed from, address indexed to, uint256 value);

function transfer(address to, uint256 value) public returns (bool) {
    balances[msg.sender] -= value;
    balances[to] += value;
    emit Transfer(msg.sender, to, value);  // log to blockchain's event index
    return true;
}

Up to 3 params per event can be indexed. Indexed params go into a special log bloom filter — clients can filter by them efficiently:

typescript
const filter = token.filters.Transfer(null, myAddress);  // indexed `to` filter
const logs = await token.queryFilter(filter, fromBlock, toBlock);

Non-indexed params are stored as ABI-encoded data. Still readable, just not filterable.

• Gas: 375 gas per byte of log data. Storing in a mapping costs 20,000 gas per slot. Events are ~100× cheaper.
• Historical queries: state only gives you the CURRENT value. Events give you a full history.
• Frontend + indexer integration: The Graph, Dune, your app — they all read events.

Every ERC-20 token emits Transfer on every balance change. Want to know the top 100 holders of a token? Scan Transfer events, aggregate. No contract modification needed.

Add a Deposited event to a vault contract that emits on every deposit.