PostgreSQL is designed to be easily extensible. For this reason, extensions loaded into the database can function just like features that are built in. The contrib/ directory shipped with the source code contains several extensions, which are described in Appendix F. Other extensions are developed independently, like PostGIS . Even PostgreSQL replication solutions can be developed externally. For example, Slony-I is a popular primary/standby replication solution that is developed independently from the core project.