The standard lightning use case is running a standalone node on one's laptop. Here's some other use cases that LDK supports.
Mobile devices with lightning have unique requirements often not well served by today's lightning ecosystem. Not only do they need to operate with minimal footprint, they also have intermittent data access and cannot shutdown safely. More importantly, many existing wallets already have business logic to handle blockchain data, keys, and storage, and do not wish to duplicate much of that logic to integrate lightning (at worst fetching the blockchain twice). LDK offers a flexible API to allow users to integrate lightning with their own keys, blockchain data, and storage. To allow full flexibility in footprint, the API supports routing data being fetched via the Lightning P2P protocol, an external service, or routes can be calculated off-device. It also provides cross-platform compatibility for free, allowing synchronization of lightning state across devices and, as long as there is protection from simultaneous-updates, users to access their wallet on any device. See the Overview page for more details on the interfaces LDK provides for integration.
LDK Supports various HSM configurations. In conjunction with the Lightning Signer project , an external HSM can be used to verify most protocol details about states before signing, ensuring host compromise cannot steal funds by broadcasting revoked states. For nodes seeking a higher level of assurance, the entire Rust-Lightning channel state machine can be run on an offline device, communicating with the outside world via a proxy host which maintains TCP connections with peers. Such a configuration ensures all details of the lightning protocol are enforced without consideration of host compromise.
Many large Bitcoin transactors have large amounts of custom-built infrastructure for interacting with the Bitcoin blockchain. Such tight integration with business logic may be difficult with existing lightning implementations focusing on standalone operation. For such transactors, LDK offers the possibility of integrating a library in their native runtime, storing and handling lightning data and events in the same way they do blockchain events.