In PostgreSQL, the streaming replication data flows in one direction only. The XLOG is provided by the master to a handful of slaves, which consume the transaction log and provide you with a nice copy of the data. You might wonder how this could ever lead to conflicts. Well, there can be conflicts.
Consider the following scenario: as you know, data is replicated with a very small delay. So, the XLOG ends up at the slave after it has been made on the master. This tiny delay can cause the scenario shown in the following diagram:
Let's assume that a slave starts to read a table. It is a long read operation. In the meantime, the master ...