/**
* @param part Partition.
* @param topVer Topology version.
* @return Primary node for given key.
*/
@Nullable
public GridNode primary(int part, long topVer) {
Collection<GridNode> nodes = nodes(part, topVer);
if (nodes.isEmpty()) return null;
return nodes.iterator().next();
}
/**
* @param part Partition.
* @param topVer Topology version.
* @return Backup nodes.
*/
public Collection<GridNode> backups(int part, long topVer) {
Collection<GridNode> nodes = nodes(part, topVer);
assert !F.isEmpty(nodes);
if (nodes.size() <= 1) return Collections.emptyList();
return F.view(nodes, F.notEqualTo(nodes.iterator().next()));
}