[FIXED] What are Solidity Events

Issue

I have been struggling for quite some time with finding an explanation to what events are in Solidity (or in the Blockchain context). As far as I understand, they are a way of storing (or logging) information on the particular contract that can then be updated throughout the life of that contract. But how is this different than a plain ol' variable? Why can't I just create a variable that is then simply updated with new information?

Solution

From the docs:

Solidity events give an abstraction on top of the EVM’s logging functionality. Applications can subscribe and listen to these events through the RPC interface of an Ethereum client.

It's easier for an off-chain app to subscribe to new event logs than to a variable change. Especially when the variable is not public.

Same goes for querying historical event logs (easy through the JSON-RPC API and its wrappers such as Web3 or Ethers.js), vs. historical changes of the variable (complicated, would need to query a node for each block and look for the changes proactively).

Example: The ERC-20 token standard defines the Transfer() event. A token contract emits this event each time a transfer (of its tokens) occurs. This allows a blockchain explorer (or any other off-chain app) to react to this event - for example to update their own database of token holders. Without the event, they would have no way (or a very complicated way at least) to learn about the transfer.

Answered By - Petr Hejda

Answer Checked By - Katrina (PHPFixing Volunteer)