public Zone zone() {
        return new Zone(SystemName.from(configserverConfig.system()),
                        Environment.from(configserverConfig.environment()),
                        RegionName.from(configserverConfig.region()));
    }
Gets the active Session for the given application id.

@return the active session, or null if there is no active session for the given application id.
configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java