public long requireActiveSessionOf(ApplicationId applicationId) {
        return activeSessionOf(applicationId)
                .orElseThrow(() -> new IllegalArgumentException("Application '" + applicationId + "' has no active session."));
    }
Return the active session id for a given application.

@param  applicationId an {@link ApplicationId}
@return session id of given application id.
@throws IllegalArgumentException if the application does not exist
configserver/src/main/java/com/yahoo/vespa/config/server/application/TenantApplications.java