Events (Smart Contract Logging)

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Pronunciation

[ee-vents smart kon-trakt log-ing]

Analogy

Events are like headlines in a newspaper—they summarize important occurrences for readers (off‑chain listeners) without storing them in the main story.

Definition

logs emitted by contracts to record indexed data and facilitate efficient off‑chain monitoring and filtering.

Key Points Intro

Events enhance via:

Key Points

Indexed topics: Up to 3 indexed parameters for efficient filtering.

Data field: Stores non‑indexed event arguments.

Gas efficiency: Cheaper than storage writes for audit trails.

Off‑chain listeners: Tools like The Graph subscribe to events.

Example

``` event Transfer(address indexed from, indexed to, uint256 value); function transfer(...) { ...; emit Transfer(msg.sender, to, amt); } ```

Technical Deep Dive

When `emit` is called, the creates a log entry with `topics[0]=keccak256(eventSignature)`, `topics[1..]` for indexed args, and `data` for others. Logs are stored in the receipt trie, not contract storage, and do not affect root. Clients retrieve logs via `eth_getLogs` RPC.

Security Warning

Relying solely on events for critical can be unsafe—events are not accessible to contracts and may be pruned in archive nodes.

Caveat

Events are not accessible on‑chain; do not use them for contract logic decisions.

Related Terms

ABI

Blockchain & Cryptocurrency Glossary

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