Composability is a general term in software engineering, and the analog of it in blockchains can be seen as: The ability for individual units of the system to interact with one another.
The two most common forms of this are:
- Cross-contract composability: The ability for two contracts to call into each other’s code while being executed.
- Example: Imagine a smart contract that is a DEX to be able to read the price of an asset from another “price oracle” smart contract. Or, the the DEX contract to update the price in a price oracle.
- Within a single smart contract chain, these interactions are often called synchronous, as they happen immediately.
- Cross-chain composability: The ability for two independent blockchains (either an L1 or L2 — see The Layers Terminology) to interact with one another, such as Bridges and Cross Chain Messaging.
- Imagine the Ethereum blockchain sending a message to Solana to inform it that some tokens should be moved to Solana.
- Across different blockchains, these interactions are often called asynchronous, as a message is sent and no immediate feedback is given to the sender. It is “fire and forget”.