The next pooling resource we will explore is named pgpool-II, but we'll refer to it simply as pgpool. This is another popular connection proxy, but it predates PgBouncer by almost a year, having been available since late 2006. The scope of pgpool is also much larger, providing functionality such as query-based replication, connection pooling, load balancing, parallel-query, and more.

Perhaps surprisingly, we won't discuss most of these features in this book. Interesting as they may be, these advanced features don't directly apply to building a highly available PostgreSQL cluster. Of course, we always encourage experimentation.

One feature pgpool exposes, which is directly relevant to this book, is server pooling. What does this mean? If we have two PostgreSQL servers, we can make use of a virtual IP address so that clients need not modify configuration files when we switch the primary database server. However, in order to move the IP address between servers, it must first...