EnterpriseDB Supplemental Guide: Statement-Based Replication Middleware

2.4.4 Statement-Based Replication Middleware

With statement-based replication middleware, a program intercepts every SQL query and sends it to one or all servers. Each server operates independently. Read-write queries are sent to all servers, while read-only queries can be sent to just one server, allowing the read workload to be distributed.

If queries are simply broadcast unmodified, functions like random(), CURRENT_TIMESTAMP, and sequences would have different values on different servers. This is because each server operates independently, and because SQL queries are broadcast (and not actual modified rows).

If this is unacceptable, either the middleware or the application must query such values from a single server and then use those values in write queries. Also, care must be taken that all transactions either commit or abort on all servers, perhaps using two-phase commit (PREPARE TRANSACTION and COMMIT PREPARED). Pgpool and Sequoia are an example of this type of replication.