This presentation covers an open source PostgreSQL cluster called Postgres-XC (Extensible Cluster) developed by NTT and EnterpriseDB. Postgres-XC provides for write-scalable performance and cluster-wide transaction consistency. Updates through one coordinator are visible from any other coordinator immediately after the commit. At present, Postgres-XC is focusing on the transactional use case of the cluster. With a DBT-1-based benchmark, Postgres-XC has achieved a 3.4 scalability for five servers and 6.4 scalability for ten servers. We will explain the main features of Postgres-XC, key algorithms and implementation, as well as the latest performance results. Further technical issues and the development roadmap will also be presented.