# Defining Sync Composability

## Making Ethereum Rollups Feel Like One Chain
Ethereum’s rollup-centric roadmap has unlocked scalability but at the cost of fragmentation. Each rollup maintains its own state, sequencer, and user environment, making it difficult for applications to interact across them. Developers have lost the seamless experience that made Ethereum powerful — where contracts could freely call each other within a single transaction.

**Sync composability** brings that power back. It defines how multiple rollups can behave as one unified execution environment, allowing transactions to flow between domains instantly, atomically, and in order.

It’s based on three core properties: composability, atomicity, and synchronicity.

## Composability — Many Interactions, One Transaction
Composability is what makes DeFi and Ethereum so expressive. It allows multiple smart contract calls to be combined into one transaction — for example, swapping on Uniswap, depositing on Aave, and minting an NFT all in a single click.

Sync composability extends that concept across rollups.
A user can, for instance, deposit collateral on Base, swap on Arbitrum, and open a leveraged position on Optimism — all in one transaction.

The entire cross-rollup flow is treated as a single logical operation. There’s no waiting for bridges, no multi-step processes, and no fragmented user experience. Everything happens as one coherent execution, even though different rollups are involved.

## Atomicity — All or Nothing
Atomicity ensures that the entire operation succeeds or nothing happens at all.
If any part of a cross-rollup transaction fails — say a swap on Arbitrum doesn’t go through or a vault on Base runs out of liquidity — then the whole transaction reverts.

This prevents partial states or inconsistencies across rollups.
Just like on Ethereum L1, users and protocols can rely on deterministic outcomes — either everything executes successfully, or it all rolls back safely.

This property is essential for protecting users from risk when transactions span multiple rollups, ensuring a level of reliability equal to L1.

## Synchronicity — Instant, Ordered Execution
Synchronicity means all parts of a cross-rollup transaction execute immediately and in sequence, within a single transaction window.
There’s no need to wait for messages or proofs to finalize between domains — actions on one rollup can instantly trigger others on different rollups.

In other words, if a user performs three actions across three rollups, those actions are executed one after another in real time — not spread over minutes or hours.

This synchronous behavior restores Ethereum’s native “one-block feel” across all rollups. It enables use cases that simply aren’t possible with asynchronous bridging — such as flash loans, cross-rollup arbitrage, or real-time composable DeFi flows.

## Putting It All Together
Sync composability happens when these three properties — composability, atomicity, and synchronicity — hold together.
A transaction can bundle multiple cross-domain actions, ensure they either all succeed or none do, and execute them sequentially and instantly.

The result is a unified experience across the Ethereum rollup ecosystem — one where rollups behave not as separate chains but as parts of a single synchronous machine.

## Why It Matters
Sync composability restores what made Ethereum unique: composability and expressiveness.
Developers regain the ability to build complex, interconnected systems across multiple rollups without sacrificing safety or user experience.

This unlocks:
* **Cross-rollup flash loans** — borrow on one rollup, use it on another, and repay instantly
* **Atomic swaps and lending flows** — trade and collateralize across domains without risk
* **Unified liquidity** — fragmented assets can finally interact as if they live on the same chain

Sync composability doesn’t just connect rollups — it fuses them into one coherent network, turning Ethereum’s modular future into a single composable economy.

## Visualizing the Difference
The diagram below captures the difference clearly:

On the left, async composability requires multiple steps — bridging, switching networks, swapping, and bridging back.
On the right, sync composability compresses all of it into a single “bridge, swap, and return” action — one transaction, one flow, instant execution.