Event Log
Structured data emitted during smart-contract execution and stored in transaction receipts for efficient off-chain indexing.
Explain Like I'm 12
Structured data emitted during smart-contract execution and stored in transaction receipts for efficient off-chain indexing.
Why It Matters
Smart-contract concepts are essential for understanding programmable blockchain applications.
How It Works
In Solidity, an event is declared and then 'emitted' during a function call. The EVM takes the parameters, hashes them if necessary, and writes them into the transaction log (bloom filters) within the block. External tools like The Graph index these logs, allowing for fast searches and querying.
Real-World Example
Decentralized exchanges like Uniswap emit a 'Swap' event every time a trade occurs, which trackers use to display real-time trading volumes and price data.
Advantages
- Significantly cheaper than state storage
- Enables easy history tracking
- Supports asynchronous data retrieval
Limitations
- Not accessible by smart contracts themselves
- Can grow large in volume
- Requires indexing for efficient search
Common Misconceptions
- Users often think smart contracts can read their own logs, but logs are actually invisible to on-chain logic.
- Many assume logs are saved in the permanent state, but they reside in transaction receipts only.
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Related Terms
ABI
An Application Binary Interface defines how external software encodes calls to and decodes results from a smart contract.
EVM
The Ethereum Virtual Machine (EVM) is a decentralized, Turing-complete computation engine that executes smart contracts on the Ethereum network. It functions as a global state machine, where the state is updated based on code execution on the blockchain. Because the EVM is implemented by every node in the network, it ensures that smart contracts run exactly as intended across all devices, regardless of local hardware differences. This allows for complex, programmable logic to be embedded into the blockchain.
Smart Contract
A smart contract is a self-executing program stored on a blockchain that automatically runs when predetermined conditions are met. These contracts eliminate the need for intermediaries by encoding terms directly into lines of code, ensuring that the agreement is enforced exactly as written without human interference. Because they reside on an immutable ledger, the execution results are verifiable, transparent, and impossible to tamper with once deployed.
Smart Contract
A self-executing program stored on a blockchain that automatically enforces and executes the terms of an agreement when predetermined conditions are met. Smart contracts are deterministic, immutable once deployed, and form the backbone of decentralized applications.
Solidity
Solidity is the primary high-level, object-oriented programming language used for writing smart contracts that execute on the Ethereum Virtual Machine (EVM). It is statically typed and supports inheritance, libraries, and complex user-defined types. Solidity was designed specifically for blockchain environments to facilitate the creation of self-executing contracts with contract-based accountability. It is heavily influenced by C++, Python, and JavaScript, making it relatively accessible to developers coming from traditional software engineering backgrounds who are transitioning into the Web3 ecosystem.