Reasons for unassigned shard：
These are the possible reasons for a shard to be in a unassigned state:
1. INDEX_CREATED Unassigned as a result of an API creation of an index. 索引创建 由于API创建索引而未分配的
2. CLUSTER_RECOVERED Unassigned as a result of a full cluster recovery. 集群恢复 由于整个集群恢复而未分配
3. INDEX_REOPENED Unassigned as a result of opening a closed index. 索引重新打开
4. DANGLING_INDEX_IMPORTED Unassigned as a result of importing a dangling index. 导入危险的索引
5. NEW_INDEX_RESTORED Unassigned as a result of restoring into a new index. 重新恢复一个新索引
6. EXISTING_INDEX_RESTORED Unassigned as a result of restoring into a closed index. 重新恢复一个已关闭的索引
7. REPLICA_ADDED Unassigned as a result of explicit addition of a replica. 添加副本
8. ALLOCATION_FAILED Unassigned as a result of a failed allocation of the shard. 分配分片失败
9. NODE_LEFT Unassigned as a result of the node hosting it leaving the cluster. 集群中节点丢失
10. REROUTE_CANCELLED Unassigned as a result of explicit cancel reroute command. reroute命令取消
11. REINITIALIZED When a shard moves from started back to initializing, for example, with shadow replicas. 重新初始化
12. REALLOCATED_REPLICA A better replica location is identified and causes the existing replica allocation to be cancelled. 重新分配副本