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An enterprise-grade Java-based, Apache 2.0 licensed Ethereum client https://wiki.hyperledger.org/display/besu
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195 lines
7.4 KiB
195 lines
7.4 KiB
# Events and Logs
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When a transaction is mined, smart contracts emit events and write logs to the blockchain.
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Logs are associated with the contract address and included in the blockchain but logs are not accessible
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from within contracts. Log storage is cheaper than contract storage (that is, it costs less gas) so if the required data can
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be stored in and accessed from logs, the cost is reduced. For example, you can use logs to display all
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transfers made using a specific contract but not the current state of the contract.
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A Dapp front end can either request logs using the [JSON-RPC API filter methods](Accessing-Logs-Using-JSON-RPC.md)
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or subscribe to logs using the [RPC Pub/Sub API](RPC-PubSub.md#logs).
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## Topics
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Log entries contain up to four topics. The first topic is the [event signature hash](#event-signature-hash) and up to three topics
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are the indexed [event parameters](#event-parameters).
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!!! example
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Log entry for an event with one indexed parameter:
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```json
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{
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"logIndex": "0x0",
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"removed": false,
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"blockNumber": "0x84",
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"blockHash": "0x5fc573d76ec48ec80cbc43f299ebc306a8168112e3a4485c23e84e9a40f5d336",
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"transactionHash": "0xcb52f02342c2498df82c49ac26b2e91e182155c8b2a2add5b6dc4c249511f85a",
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"transactionIndex": "0x0",
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"address": "0x42699a7612a82f1d9c36148af9c77354759b210b",
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"data": "0x",
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"topics": [
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"0x04474795f5b996ff80cb47c148d4c5ccdbe09ef27551820caa9c2f8ed149cce3",
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"0x0000000000000000000000000000000000000000000000000000000000000001"
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]
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}
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```
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## Event Parameters
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Up to three event parameters can have the `indexed` attribute. Indexed parameters are stored as `topics`
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in the logs. Indexed parameters can be searched and filtered.
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Topics are 32 bytes. If an indexed argument is an array (including `string` and `byte` datatypes),
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the keccak-256 hash of the paramater is stored as a topic.
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Non-indexed parameters are included in the logs `data` but cannot be easily searched or filtered.
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!!! example
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A Solidity contract that stores one indexed and one non-indexed parameter and has an event that emits the value of each parameter:
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```solidity
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pragma solidity ^0.5.1;
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contract Storage {
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uint256 public valueIndexed;
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uint256 public valueNotIndexed;
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event Event1(uint256 indexed valueIndexed, uint256 valueNotIndexed);
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function setValue(uint256 _valueIndexed, uint256 _valueNotIndexed) public {
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valueIndexed = _valueIndexed;
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valueNotIndexed = _valueNotIndexed;
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emit Event1(_valueIndexed, _valueNotIndexed);
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}
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}
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```
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!!! example
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Log entry created by invoking the contract above with `valueIndexed` set to 5 and `valueNotIndexed` set to 7:
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```json
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{
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"logIndex": "0x0",
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"removed": false,
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"blockNumber": "0x4d6",
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"blockHash": "0x7d0ac7c12ac9f622d346d444c7e0fa4dda8d4ed90de80d6a28814613a4884a67",
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"transactionHash": "0xe994022ada94371ace00c4e1e20663a01437846ced02f18b3f3afec827002781",
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"transactionIndex": "0x0",
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"address": "0x43d1f9096674b5722d359b6402381816d5b22f28",
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"data": "0x0000000000000000000000000000000000000000000000000000000000000007",
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"topics": [
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"0xd3610b1c54575b7f4f0dc03d210b8ac55624ae007679b7a928a4f25a709331a8",
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"0x0000000000000000000000000000000000000000000000000000000000000005"
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]
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}
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```
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## Event Signature Hash
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The first topic in a log entry is always the the event signature hash. The event signature hash is a keccak-256
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hash of the event name and input argument types. Argument names are ignored. For example, the event `Hello(uint256 worldId)`
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has the signature hash `keccak('Hello(uint256)')`. The signature identifies to which event log topics belong.
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!!! example
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A Solidity contract with two different events:
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``` solidity
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pragma solidity ^0.5.1;
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contract Storage {
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uint256 public valueA;
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uint256 public valueB;
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event Event1(uint256 indexed valueA);
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event Event2(uint256 indexed valueB);
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function setValue(uint256 _valueA) public {
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valueA = _valueA;
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emit Event1(_valueA);
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}
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function setValueAgain(uint256 _valueB) public {
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valueB = _valueB;
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emit Event2(_valueB);
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}
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}
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```
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The event signature hash for event 1 is `keccak('Event1(uint256)')` and the event signature hash for event
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2 is `keccak('Event2(uint256)')`. The hashes are:
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* `04474795f5b996ff80cb47c148d4c5ccdbe09ef27551820caa9c2f8ed149cce3` for event 1
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* `06df6fb2d6d0b17a870decb858cc46bf7b69142ab7b9318f7603ed3fd4ad240e` for event 2
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!!! tip
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You can use a library keccak (sha3) hash function such as provided in [Web3.js](https://github.com/ethereum/wiki/wiki/JavaScript-API#web3sha3)
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or an an online tool such as https://emn178.github.io/online-tools/keccak_256.html to generate event signature hashes.
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!!! example
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Log entries from invoking the Solidity contract above:
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```json
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[
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{
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"logIndex": "0x0",
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"removed": false,
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"blockNumber": "0x84",
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"blockHash": "0x5fc573d76ec48ec80cbc43f299ebc306a8168112e3a4485c23e84e9a40f5d336",
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"transactionHash": "0xcb52f02342c2498df82c49ac26b2e91e182155c8b2a2add5b6dc4c249511f85a",
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"transactionIndex": "0x0",
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"address": "0x42699a7612a82f1d9c36148af9c77354759b210b",
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"data": "0x",
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"topics": [
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"0x04474795f5b996ff80cb47c148d4c5ccdbe09ef27551820caa9c2f8ed149cce3",
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"0x0000000000000000000000000000000000000000000000000000000000000001"
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]
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},
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{
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"logIndex": "0x0",
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"removed": false,
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"blockNumber": "0x87",
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"blockHash": "0x6643a1e58ad857f727552e4572b837a85b3ca64c4799d085170c707e4dad5255",
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"transactionHash": "0xa95295fcea7df3b9e47ab95d2dadeb868145719ed9cc0e6c757c8a174e1fcb11",
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"transactionIndex": "0x0",
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"address": "0x42699a7612a82f1d9c36148af9c77354759b210b",
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"data": "0x",
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"topics": [
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"0x06df6fb2d6d0b17a870decb858cc46bf7b69142ab7b9318f7603ed3fd4ad240e",
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"0x0000000000000000000000000000000000000000000000000000000000000002"
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]
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}
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]
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```
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## Topic Filters
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[Filter options objects](../Reference/JSON-RPC-API-Objects.md#filter-options-object) have a `topics` key to filter logs by topics.
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Topics are order-dependent. A transaction with a log containing topics `[A, B]` is matched with the following topic filters:
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* `[]` - Match any topic
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* `[A]` - Match A in first position
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* `[[null], [B]]` - Match any topic in first position AND B in second position
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* `[[A],[B]]` - Match A in first position AND B in second position
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* `[[A, C], [B, D]]` - Match (A OR C) in first position AND (B OR D) in second position
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!!! example
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The following filter option object returns log entries for the [Event Parameters example contract](#event-parameters) where `valueIndexed` is set to
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5 or 9:
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```json
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{
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"fromBlock":"earliest",
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"toBlock":"latest",
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"address":"0x43d1f9096674b5722d359b6402381816d5b22f28",
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"topics":[
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["0xd3610b1c54575b7f4f0dc03d210b8ac55624ae007679b7a928a4f25a709331a8"],
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["0x0000000000000000000000000000000000000000000000000000000000000005", "0x0000000000000000000000000000000000000000000000000000000000000009"]
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]
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}
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```
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