The oldest warship still in active naval use

What is the oldest ship still in service and actively used by a navy?

A Brazilian river monitor is the suggested answer, but its name is not settled. A Russian floating dock is also mentioned, with doubts about its condition.

A Brazilian river monitor is suggested as the oldest ship still actively used by a navy, although its name is recalled uncertainly. A Russian floating dock is also raised, with uncertainty over its damaged condition. The exchange does not settle a precise identification or a verified comparison.