Gabarito C
Clustering model
Asynchronous cascading master-slave replication, row-based, using triggers and queueing in the database.
Slony-I can be combined with a pooler and load-balancer, such as pgPool2 or DBD::Multiplex, to form a complete clustering system.
Use-case
Cascading replication (master-->slave-->slave)
Partial replication
Upgrades of PostgreSQL and server platform
Failover and slave promotion
Online provisioning
Limited detach/reattach slaves
Maturity and field-testing (in production for 5 years)
Slony-I is a "master to multiple slaves" replication system supporting cascading (e.g. - a node can feed another node which feeds another node...) and failover. The big picture for the development of Slony-I is that it is a master-slave replication system that includes all features and capabilities needed to replicate large databases to a reasonably limited number of slave systems. Slony-I is a system designed for use at data centers and backup sites, where the normal mode of operation is that all nodes are available.
Drawbacks
Not a complete clustering solution
Complexity of setup
Limitations on schema changes
Performance of large bulk loads and large object replication
Write overhead and associated maintenance (vacuum etc.)
Multiple points of monitoring required
Master-slave, replication lag
The main drawback to Slony-I even as a replication system is the complexity of its setup and administration. The design of the system, with the database itself being used for queueing row updates, also significantly increases the amount of data writing and I/O done by the DBMS.
Also, since Slony-I is asynchronous master-slave, all writes have to be segregated to the master. Additionally, there is a noticeable lag (1-3 seconds) between the master and the slaves which may cause users to have an inconsistent view of the data.
"Retroceder Nunca Render-se Jamais !"
Força e Fé !
Fortuna Audaces Sequitur !