So far in this book, we have been studying large-scale data processing. This chapter is different: it is about building general distributed applications using Hadoop’s distributed coordination service, called ZooKeeper.
Writing distributed applications is hard. It’s hard primarily because of partial failure. When a message is sent across the network between two nodes and the network fails, the sender does not know whether the receiver got the message. It may have gotten through before the network failed, or it may not have. Or perhaps the receiver’s process died. The only way that the sender can find out what happened is to reconnect to the receiver and ask it. This is partial failure: when we don’t even know if an operation failed.
ZooKeeper can’t make partial failures go away, since they are intrinsic to distributed systems. It certainly does not hide partial failures, either. But what ZooKeeper does do is give you a set of tools to build distributed applications that can safely handle partial failures.
ZooKeeper also has the following characteristics:
ZooKeeper is, at its core, a stripped-down filesystem that exposes a few simple operations and some extra abstractions, such as ordering and notifications.
The ZooKeeper primitives are a rich set of building blocks that can be used to build a large class of coordination data structures and protocols. Examples include distributed queues, distributed locks, and leader ...