"Distributed locks aren't real," "Anyone who's trying to sell you a distributed lock is selling you sawdust and lies." This may sound rather bleak, but it doesn't say that locking, itself, is impossible in a distributed system: It's just that some like to remind us that all of the system's components must participate in the protocol. This blog post is the story of how we implemented a distributed locking protocol that gives your components a straightforward way of joining in.
The coordinating component of the protocol is an object that extends the semantics of java.util.concurrent.locks.Lock. We call it FencedLock, following the naming used in Martin Kleppmann's 2016 post "How to Do Distributed Locking."