event Transfer(address indexed from, address indexed to, uint256 value)- The difference between the indexed and non-indexed parameters in the event is the difference in how they are logged by the EVM.
- The EVM has four opcodes for logging: LOG0, LOG1, LOG3, LOG4. The number represents the number of topics each log can carry along with it.
- To log more data, we must use the data field.
Topics vs Data
-
They are two different ways of logging data out of the EVM.
-
Topics are searchable, but expensive.
-
Data is not searchable, but cheaper.
-
Topics are limited to 128 bytes, while data is not.
-
The first topic is the keccak256 hash of the event name and arguments with all keywords removed. Eg:
keccak256(Transfer(address,address,uint256)) -
If the params to the event are indexed, they will be logged as topics.
-
The signature (topic 0) is omitted when the event is anonymous.
-
The data emitted are the arguments to the event. Eg: in
emit Transfer(from, to, value)the args will be logged as data.
Indexed vs Non-Indexed
- Indexed parameters of an event are logged as additional topics.
- A non-indexed param will be logged as data.
Metadata
- People::
- Related:: EVM, Ethereum
- References:: Understanding event logs on eth
- Status::understood
- Created:: 6th Oct 2022
- Updated::
$=dv.el('t', dv.current().file.mday)