Transaction mining causes smart contracts to emit events and write logs to the blockchain.
The smart contract address is the link to the logs and the blockchain includes the logs, but contracts cannot access logs. Log storage is cheaper than contract storage (that is, it costs less gas) so storing and accessing the required data in logs reduces the cost. For example, use logs to display all transfers made using a specific contract, but not the current state of the contract.
A log entry for an event with one indexed parameter:
```json
{
"logIndex": "0x0",
"removed": false,
"blockNumber": "0x84",
"blockHash": "0x5fc573d76ec48ec80cbc43f299ebc306a8168112e3a4485c23e84e9a40f5d336",
"transactionHash": "0xcb52f02342c2498df82c49ac26b2e91e182155c8b2a2add5b6dc4c249511f85a",
"transactionIndex": "0x0",
"address": "0x42699a7612a82f1d9c36148af9c77354759b210b",
"data": "0x",
"topics": [
"0x04474795f5b996ff80cb47c148d4c5ccdbe09ef27551820caa9c2f8ed149cce3",
"0x0000000000000000000000000000000000000000000000000000000000000001"
]
}
```
Event parameters
Up to three event parameters can have the indexed attribute. Logs store these indexed parameters as topics. Indexed parameters are searchable and filterable.
Topics are 32 bytes. If an indexed argument is an array (including string and byte datatypes), the log stores the keccak-256 hash of the parameter as a topic.
Log data includes non-indexed parameters but is difficult to search or filter.
!!! example
A Solidity contract storing one indexed and one non-indexed parameter and has an event emitting
the value of each parameter:
```solidity
pragma solidity ^0.5.1;
contract Storage {
uint256 public valueIndexed;
uint256 public valueNotIndexed;
event Event1(uint256 indexed valueIndexed, uint256 valueNotIndexed);
function setValue(uint256 _valueIndexed, uint256 _valueNotIndexed) public {
valueIndexed = _valueIndexed;
valueNotIndexed = _valueNotIndexed;
emit Event1(_valueIndexed, _valueNotIndexed);
}
}
```
!!! example
A log entry created by invoking the contract in the previous example with `valueIndexed` set to
5 and `valueNotIndexed` set to 7:
```json
{
"logIndex": "0x0",
"removed": false,
"blockNumber": "0x4d6",
"blockHash": "0x7d0ac7c12ac9f622d346d444c7e0fa4dda8d4ed90de80d6a28814613a4884a67",
"transactionHash": "0xe994022ada94371ace00c4e1e20663a01437846ced02f18b3f3afec827002781",
"transactionIndex": "0x0",
"address": "0x43d1f9096674b5722d359b6402381816d5b22f28",
"data": "0x0000000000000000000000000000000000000000000000000000000000000007",
"topics": [
"0xd3610b1c54575b7f4f0dc03d210b8ac55624ae007679b7a928a4f25a709331a8",
"0x0000000000000000000000000000000000000000000000000000000000000005"
]
}
```
Event signature hash
The first topic in a log entry is always the event signature hash. The event signature hash is a keccak-256 hash of the event name and input argument types, with argument names ignored. For example, the event Hello(uint256 worldId) has the signature hash keccak('Hello(uint256)'). The signature identifies to which event log topics belong.
!!! example
A Solidity contract with two different events:
```solidity
pragma solidity ^0.5.1;
contract Storage {
uint256 public valueA;
uint256 public valueB;
event Event1(uint256 indexed valueA);
event Event2(uint256 indexed valueB);
function setValue(uint256 _valueA) public {
valueA = _valueA;
emit Event1(_valueA);
}
function setValueAgain(uint256 _valueB) public {
valueB = _valueB;
emit Event2(_valueB);
}
}
```
The event signature hash for event 1 is keccak('Event1(uint256)') and the event signature hash for event 2 is keccak('Event2(uint256)'). The hashes are:
04474795f5b996ff80cb47c148d4c5ccdbe09ef27551820caa9c2f8ed149cce3 for event 1
06df6fb2d6d0b17a870decb858cc46bf7b69142ab7b9318f7603ed3fd4ad240e for event 2.
!!! tip
You can use a library keccak (sha3) hash function, such as provided in
[Web3.js](https://web3js.readthedocs.io/en/v1.2.11/web3-utils.html?highlight=sha3#sha3), or an online tool,
such as https://emn178.github.io/online-tools/keccak_256.html, to generate event signature
hashes.
Filter options objects have a topics key to filter logs by topics.
Topics are order-dependent. A transaction with a log containing topics [A, B] matches with the following topic filters:
[] - Match any topic
[A] - Match A in first position
[[null], [B]] - Match any topic in first position AND B in second position
[[A],[B]] - Match A in first position AND B in second position
[[A, C], [B, D]] - Match (A OR C) in first position AND (B OR D) in second position.
!!! example
The following filter option object returns log entries for the
[Event Parameters example contract](#event-parameters) with `valueIndexed` set to 5 or 9:
```json
{
"fromBlock":"earliest",
"toBlock":"latest",
"address":"0x43d1f9096674b5722d359b6402381816d5b22f28",
"topics":[
["0xd3610b1c54575b7f4f0dc03d210b8ac55624ae007679b7a928a4f25a709331a8"],
["0x0000000000000000000000000000000000000000000000000000000000000005", "0x0000000000000000000000000000000000000000000000000000000000000009"]
]
}
```
