/**
     * Returns a list of currently active instances based on non-expired heartbeat documents.
     *
     * @return the list of active instances.
     */
    public List<InstanceInfo> getActiveInstances() {
        final List<InstanceInfo> instances = new ArrayList<>();
        final long now = System.currentTimeMillis();
        final String query = toJson(Map.of( //

        final CoordinatorHelper helper = new CoordinatorHelper() {
            @Override
            public List<InstanceInfo> getActiveInstances() {
                final InstanceInfo info = new InstanceInfo();
                info.instanceId = "node1@host1";
                return List.of(info);
            }
        };
        try {