Clustering versus two-phase commit
The two-phase commit protocol, described in detail in Multiphase commit protocols, ensures that transactions are uniformly committed or rolled back across multiple database servers.
In theory, you can take advantage of two-phase commit to replicate data by configuring two database servers with identical data and then defining triggers on one of the database servers that replicate updates to the other database server. However, this sort of implementation has numerous synchronization problems in different failure scenarios. Also, the performance of distributed transactions is inferior to clustering.