Smart contracts announce what happens through events: a transfer, a trade, a new order, a role change. An event listener is a service that watches for those events and passes them to your own systems the moment they happen, so a payment can be credited, an order fulfilled, a dashboard updated or an alert sent. It sounds simple; doing it without missing or duplicating events is where the work is.
How a Reliable Listener Works
- Subscribe to new logs over WebSockets, filtered by contract address and event signature, with polling as a fallback when the connection drops.
- Decode each log with the contract ABI into typed data: who, what, how much.
- Wait for enough confirmations for the value at stake, and handle reorgs: events from blocks that disappear are rolled back or never delivered.
- Checkpoint the last processed block, so after a restart or outage the listener catches up without gaps.
- Deliver through webhooks, a message queue or directly into your database, at least once, with idempotency keys so receivers can safely ignore duplicates.
Typical Uses
- Crediting deposits and payments in a back office or exchange.
- Triggering business processes: fulfilment, notifications, KYC checks, accounting entries.
- Feeding live dashboards and trading bots.
- Security monitoring: alerts on admin actions, large transfers or paused contracts.
Listener or Indexer?
A listener reacts to new events as they happen. When you also need to query history, aggregate or join data from many contracts, combine it with data indexing. Hosted notification services from node providers are a quick start; a self-run listener gives full control over latency, filtering and costs. We build both setups, usually in Rust or TypeScript.
Frequently Asked Questions
How fast are events delivered?
Within moments of the block, plus the confirmation time you choose. For payments, waiting for a few confirmations is usually wise.
What happens if our server is down?
The listener resumes from its last checkpoint and delivers everything it missed, in order.
Which chains are supported?
All EVM chains; Solana uses a different mechanism, which we support separately.
Who owns the code?
Ownership of the source code is set out in the contract, and the code can be fully yours, including documentation and tests.
Part of our EVM development.
React to Your Contracts in Real Time
Tell us which events matter and where they need to go. We will build a listener that never misses one.