> For the complete documentation index, see [llms.txt](https://docs.somnia.network/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.somnia.network/developer/reactivity.md).

# Somnia Reactivity

Reactivity lets applications respond to on-chain events without running a polling loop. It's one of Somnia's standout features.

Somnia provides reactivity in two complementary forms:

* [**On-chain reactivity**](/developer/reactivity/reactivity-onchain.md) stores persistent event subscriptions in chain state and then executes contract handlers when matching events are committed. The reaction is part of chain execution.
* [**Off-chain reactivity**](/developer/reactivity/reactivity-offchain.md) is a WebSocket feature served by the node API. A client subscribes once, receives pushed events, and can ask the node to run read-only simulations for each event so that derived state arrives in the same message — minimising latency. The reaction stays in the client.

## Which one to use

Use on-chain reactivity when the reaction should be part of chain execution, i.e. a guaranteed, public effect. For example: automatic token rebalancing, scheduled settlement, or a callback that must fire even if the original owner is offline.

Use off-chain reactivity when the reaction should stay in your app. For example: a UI that updates when an event lands, an indexer that writes to a database, or a bot that watches a pool. Off-chain subscriptions are more flexible and don't require spending SOMI.

The two kinds of reactivity are independent. You can use both at once.


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