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Filters in Ethereum-PHP

It is a good practice to include events in your contracts, so that if a function is called you can react in your PHP application on it.

Solidity example

pragma solidity ^0.4.24;

contract TestEventHandling {
    event CalledTrigger(address indexed from, uint256 value);

    function triggerEvent() public {
       uint256 myVal = 9999;
       emit CalledTrigger2(msg.sender, myVal);
    }

}

In this solidity example CalledTrigger is emitted if a user calls the triggerEvent() method.

The logs of the TX will include the data of the CalledTrigger event in

  • topics
    • topics[0] Kessac256 of the Event Signature. E.g: sha3('CalledTrigger(address, uint256)') --> 32bit hash.
    • topics[1-n] Indexed values in the order of definition (n <=4)
  • data remaining non indexed Event parameters which need to be ABI decoded

See eth_getfilterchanges.

Compatibility

There are a lot of issues with filtering in Ethereum, especially considering you may want to use a service provider like Infura.io to connect to Ethereum.

Infura does not support any non SSH filter Methods. So if you depend Infura, you need to consider it in your architecture.

In the future EthQl might be a alternative approach.

Ethereum-PHP tries to be compatible with ANY client, so I don't consider Filters consistently working.

Architectural considerations

The concept of Event driven filters does not really match PHP.

Using filters

Using Filters with a Symfony EventDispatcher might be a solution if you are actually using a Ethereum node which supports filters, but you still need to poll for your events every time a new Block is created. This might be an option if you want to search for Data in any new Block, but you will need to set up a cronjob to call getFilterChanges in the frequency of block time.

User triggered events

Currently suggesting a simplified Ajax based Approach:

  • User submits a TX in the frontend and submits the TX receipt hash/id to your PHP app
  • Use eth_getTransactionReceipt() to get the logs data and handle the emitted events on the PHP side

More below.

Event Listener

In some cases you might want to index all events in a Blockchain or listen continuously to contract events whenever a new Block is generated.

Natively PHP is very bad at this. I implemented a Block/Event listener based on ReactPHP which allows Block processing and Event listening.

Truffle integration: This approach is very easy to integrate if you are using Truffle for your Contract code.

// Extend a \Ethereum\SmartContract with EventHandlers
class CallableEvents extends SmartContract {
  public function onCalledTrigger1 (EthEvent $event) {
    echo '### ' . substr(__FUNCTION__, 2) . "(\Ethereum\EmittedEvent)\n";
    var_dump($event);
  }
  public function onCalledTrigger2 (EthEvent $event) {
    echo '### ' . substr(__FUNCTION__, 2) . "(\Ethereum\EmittedEvent)\n";
    var_dump($event);
  }
}

$web3 = new Ethereum('http://192.168.99.100:8545');
$networkId = '5777';

// Contract Classes must have same name as the solidity classes for this to work.
$contracts = SmartContract::createFromTruffleBuildDirectory(
  'YOUR/truffle/build/contracts',
   $web3,
   $networkId
);

// process any Transaction from current Block to the future.
new ContractEventProcessor(
  $web3,
  $contracts,
  'latest',
  'latest'
);

See integration-with-truffle-and-contract-events.

Note on weird return values

The return of eth_getFilterChanges() depends on which method you use to setup the filter. Filter added with

  • eth_newBlockFilter (block hashes) -> returns [D32]
  • eth_newPendingTransactionFilter (transaction hashes) -> returns [D32]
  • eth_newFilter -> returns [FilterChange]

This is also not correctly reflected in ethjs-schema.

Source Ethereum Wiki

Current recommendation

  1. Get the TX-hash from frontend interaction
  2. Query the results in PHP using eth_getTransactionReceipt(TX-hash)
  3. The TransactionReceipt contains an array "logs" which contains FilterChange objects
  4. Use contractInstance->processLog(FilterChange) to get the decoded Contract event data

Example tests/TestEthClient/Unit/CallableEventsTest.php