Imagine a situation where one day you have to set up a distributed cluster with the use of Solr. The amount of data is just too much for a single server to handle. Of course, only you can set up a second server or go for another master database with another set of data. But before Solr 4.0, you would have to take care of the data distribution yourself. In addition to this, you would also have to take care of setting up replication, thinking about data duplication, and so on. You don't have to do this now because Solr 4.0 can do it for you. Let's see how.
Before continuing, I advise you to read the Installing standalone ZooKeeper recipe in Chapter 1, Apache Solr Configuration. This recipe shows how to set up a ZooKeeper cluster ready for production use. However, if you already have ZooKeeper running, you can skip that recipe.