Synchronous Composability

Synchronous composability describes a situation in which smart contracts can interact with each other instantly, inside the same transaction and the same block, with a single combined outcome. If any step fails, the whole transaction is reversed, so users never end up halfway through. This is what makes flash loans possible: a user can borrow, trade across several exchanges and repay in one atomic transaction. It is a key reason DeFi applications on one chain can be combined like building blocks, an idea known more broadly as composability. Synchronous composability is lost when applications sit on different chains or rollups, because messages between them take time and can fail separately, which is called asynchronous composability. Restoring it across rollups is an active research area, with approaches such as shared sequencing and the based rollup model.