The BDR (Bi-Directional Replication) project adds multi-master replication to PostgreSQL 9.4. This chapter explains what BDR is, how it works, and where to best apply it. See Concepts to learn about what BDR does and how it does it.
Because BDR requires a modified PostgreSQL 9.4 there is also a UDR (unidirectional replication) variant available. This version doesn't require a modified version of PostgreSQL but has some limitations that don't apply to BDR. See Section 1.5.