private String formatNodeIds(Node[] nodes) {
        StringBuilder b = new StringBuilder("[");
        if (nodes != null) {
            for (int i = 0; i < nodes.length; i++) {
                b.append(nodes[i].idString());
                if (i < nodes.length - 1)
                    b.append(',');
            }
        }
        b.append("]");
        return b.toString();
    }
The subset of the replicas that are offline
clients/src/main/java/org/apache/kafka/common/PartitionInfo.java