PL/Proxy is a PostgreSQL procedural language (PL) handler that allows to do remote procedure calls between PostgreSQL databases, with optional sharding.
PL/Proxy was developed in Skype as a scalability solution for PostgreSQL database. At first it was very thin PL component that connected to C++ server that handled all sharding decisions. This proved to be too complex setup and in version 2 it was re-written to current archtecture of PL that handles sharding and generic pooler process (PgBouncer) that is able to handle large amount of standard PostgreSQL connections that the PL makes.
Older releases can be downloaded from pgfoundry archive.