Say you have an application using PG asynchronous streaming replication to some hot standbys, to distribute the read load. The application itself is a typical web application consisting of multiple servers, serving a number of sessions (perhaps belonging to different users), and the workload is OLTP-ish, with each session continually issuing a bunch of transactions. To guarantee session timeline consistency for clients of the application, you want to make sure that they can read data that's at least as new as anything they've read/written previously, never traveling back in time.
With asynchronous replication, after seeing a new version of the data from one standby, you may see an older version from a subsequent query to another standby. The question: what are some ways to provide this form of consistency in the context of PG asynchronous replication?
Is the standard/recommended approach to use a sequence representing the global database version? Here, the application is responsible for incrementing this from update transactions. In read transactions, check that the sequence value is >= the session's highest-seen-value, and raise the latter if necessary.
After an UPDATE, your application can cache the info from 'pg_current_xlog_location()' result on the primary and then compare that with the result of 'pg_last_xlog_receive_location()' on the standby to see if it is seeing fresh enough data.