13.1. Introduction

Postgres-XC inherits a rich set of tools for developers to manage concurrent access to data from PostgreSQL. Internally, data consistency is maintained by using a multiversion model (Multiversion Concurrency Control, MVCC). This means that while querying a database each transaction sees a snapshot of data (a database version) as it was some time ago, regardless of the current state of the underlying data. This protects the transaction from viewing inconsistent data that could be caused by (other) concurrent transaction updates on the same data rows, providing transaction isolation for each database session. MVCC, by eschewing explicit locking methodologies of traditional database systems, minimizes lock contention in order to allow for reasonable performance in multiuser environments.

The main advantage of using the MVCC model of concurrency control rather than locking is that in MVCC locks acquired for querying (reading) data do not conflict with locks acquired for writing data, and so reading never blocks writing and writing never blocks reading. PostgreSQL maintains this guarantee even when providing the strictest level of transaction isolation through the use of an innovative Serializable Snapshot Isolation (SSI) level.

As in PostgreSQL, table- and row-level locking facilities are also available in Postgres-XC for applications that cannot adapt easily to MVCC behavior. However, proper use of MVCC will generally provide better performance than locks. In addition, application-defined advisory locks provide a mechanism for acquiring locks that are not tied to a single transaction.