[Ethereum] Indexed Event with string not getting logged


I am facing a strange issue in my solidity smart contract using events. I created the event in 2 ways as below:-

1. event Event1(string assetName, address toAdd);
2. event Event1(string indexed assetName, address indexed toAdd);

Now if I run my smart contract and I have specified approach 1, then the event gets printed in the console logs whereas if I use approach 2, then the event is not printed in the console logs.

Can someone clarify on this?

Note: Not pasting the whole contract code

Best Answer

The combination of string and indexed does not work. To understand why, see how event arguments are stored in the blockchain.

All transactions that are executed generate a transaction receipt, which contains a property called logs. You can look up the receipt using eth.getTransactionReceipt("0x...").

In the transaction receipt, all event arguments that are not indexed are included in the data property. This property can hold values of arbitrary length. However, event arguments that are indexed are not stored in the data property, but end up in topics. This allows filters to work.

For fixed size arguments types this is ok. The size of the entries in topics is long enough to hold all fixed types supported by Solidity. However, string can have an arbitrary length. To still be able to store those values in a topic, Solidity creates a hash of the value, which ends up in the topic.

The upside is that filters can still work, since you can just create the hash of the value you're filtering for. The downside is that web3 is unable to decode the value, since that would require performing the hash function in reverse direction.

To solve this problem, you have two options. You can remove indexed from the argument, so it is stored in the data field and no longer hashed to become part of a topic. Or, you change the type of the argument, e.g. into bytes32 or another. Which is appropriate depends on your case.

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