Cloudera Developer Blog · ZooKeeper Posts
With the recent release of CDH3b2, many users are more interested than ever to try out Cloudera’s Distribution for Hadoop (CDH). One of the questions we often hear is, “what does it take to migrate?”.
If you’re not familiar with CDH3b2, here’s what you need to know.
CDH3 beta 2 is the first version of CDH to incorporate Apache ZooKeeper. ZooKeeper is a highly reliable and available coordination service for distributed processes. It is a proven technology and a well established open source project at Apache (sub-project of Hadoop).
ZooKeeper is distributed coordination
Often distributed applications need some way to coordinate across processes; locking resources, managing queues of events, electing a “leader” process, configuration, etc… Coordination operations such as these are notoriously hard to get right. ZooKeeper provides a relatively simple API which allows clients to correctly implement these and many other coordination mechanisms.
In my first few weeks here at Cloudera, I’ve been tasked with helping out with the Apache ZooKeeper system, part of the umbrella Hadoop project. ZooKeeper is a system for coordinating distributed processes. In a distributed environment, getting processes to act in any kind of synchrony is an extremely hard problem. For example, simply having a set of processes wait until they’ve all reached the same point in their execution – a kind of distributed barrier – is surprisingly difficult to do correctly. ZooKeeper offers an API to facilitate this sort of distributed coordination. For example, it is often used to serve locks to client processes – locks are just another kind of coordination primitive – in the form of small files that ZooKeeper tracks.
In order to be useful, ZooKeeper must be both highly reliable and available as systems will rely upon it as a critical component. For example, if locks cannot be taken, processes cannot make progress and the whole system will grind to a halt. ZooKeeper is built on a suite of reliable distributed systems techniques and protocols, and is typically run on a cluster of machines so that if some should fail, the remaining ones can continue to provide service. Under the hood, ZooKeeper is responsible for ordering calls made by clients so that each request is processed atomically and in a fixed and firm order.