In PostgreSQL 9.4, a major new feature called "replication slots" has been introduced. The idea is to give users and tools alike a chance to connect to the transaction log stream in a standard way and consume data.
Basically, two types of replication slots exist:
Physical replication slots
Logical replication slots
The following two sections describe those two types of replication slots in detail.
Physical replication slots are an important new feature of PostgreSQL 9.4. The idea is that a client can create a replication slot to make sure that the server only discards what has really made it to the client. Remember that in PostgreSQL, the transaction log is normally recycled as soon as a certain amount of new XLOG has been created. To the streaming replication slaves or some other clients, this can turn out to be disastrous, because if the slave/client cannot consume the transaction log fast enough or if there is simply not enough bandwidth...