If a connection is mirrored to another server, it can be used to verify the integrity of and the behavior of upgraded systems. This helps figure out whether an upgraded system provides the same result as a reference system. By calculating a checksum for the resultset of the main connection and the duplicate connection, we know whether the results from the two servers are identical. Furthermore, by measuring the time it took for the query to complete changes in performance can be observed.
In 2.5 this can be easily implemented as a new router that behaves like the tee filter but instead of defining the service where the query is duplicated, the server where the result is returned is defined. Implementing it as a router also makes it much easier to calculate checksums and measure query durations.