集群：GET _cluster/health
索引：GET _cluster/health?pretty&level=indices 或 GET _cat/indices?v
分片：GET _cluster/health?pretty&level=shards 或 GET _cat/shards?v
恢复进展：GET _recovery?pretty 或 GET {索引名}/_recovery?pretty

随机查询分片未分配原因
GET _cluster/allocation/explain
指定分片查询未分配原因
GET _cluster/allocation/explain
{
  "index": "my-index-000001",
  "shard": 0,
  "primary": false
}

{
    "deciders":[
        {
            "decider":"disk_threshold",
            "decision":"NO",
            "explanation":"the node is above the high watermark cluster setting [cluster.routing.allocation.disk.watermark.high=90%], using more disk space than the maximum allowed [90.0%], actual free: [5.680906833439563%]"
        }
    ]
}

YES     : 满足条件   
NO      : 不满足条件(重点关注这个)    
THROTTLE: 限流    

public abstract class AllocationDecider {
    /**
     * Returns a {@link Decision} whether the given shard routing can be
     * re-balanced to the given allocation. The default is
     * {@link Decision#ALWAYS}.
     */
    public Decision canRebalance(ShardRouting shardRouting, RoutingAllocation allocation) {
        return Decision.ALWAYS;
    }

    /**
     * Returns a {@link Decision} whether the given shard routing can be
     * allocated on the given node. The default is {@link Decision#ALWAYS}.
     */
    public Decision canAllocate(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation) {
        return Decision.ALWAYS;
    }

    /**
     * Returns a {@link Decision} whether the given shard routing can be remain
     * on the given node. The default is {@link Decision#ALWAYS}.
     */
    public Decision canRemain(IndexMetadata indexMetadata, ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation) {
        return Decision.ALWAYS;
    }

场景1：
there are [%d] copies of this shard and [%d] values for attribute [%s] (%s from nodes in the cluster and %s) so there may be at most [%d] copies of this shard allocated to nodes with each value, but (including this copy) there 
 would be [%d] copies allocated to nodes with [node.attr.%s: %s]
场景2：
node does not contain the awareness attribute [%s]; required attributes cluster setting [%s=%s]

there are [2] copies of this shard and [2] values for attribute [set] ([100003] from nodes in the cluster and [100003, 100007] from forced awareness) so there may be at most [1] copies of this shard allocated to nodes with each value, but (including this copy) there would be [2] copies allocated to nodes with [node.attr.set: 100003]

场景1
the cluster has unassigned primary shards and cluster setting [cluster.routing.allocation.allow_rebalance] is set to [indices_primaries_active]
场景2
the cluster has inactive primary shards  and cluster setting [cluster.routing.allocation.allow_rebalance] is set to [indices_primaries_active]
场景3
the cluster has unassigned shards and cluster setting and cluster setting [cluster.routing.allocation.allow_rebalance] is set to [indices_all_active]
场景4
the cluster has inactive  shards  and cluster setting [cluster.routing.allocation.allow_rebalance] is set to [indices_all_active]

reached the limit of concurrently rebalancing shards [%d], cluster setting [%s=%d]

PUT _cluster/settings
{
  "persistent": {
    "cluster.routing.allocation.node_concurrent_incoming_recoveries": "4",
    "cluster.routing.allocation.node_concurrent_outgoing_recoveries": "4",
    "cluster.routing.allocation.node_concurrent_recoveries": "20",
    "cluster.routing.allocation.cluster_concurrent_rebalance": "20",
    "cluster.routing.allocation.node_initial_primaries_recoveries": "20",
    "indices.recovery.max_bytes_per_sec": "200mb"
  },
  "transient": {
    "cluster.routing.allocation.node_concurrent_incoming_recoveries": "4",
    "cluster.routing.allocation.node_concurrent_outgoing_recoveries": "4",
    "cluster.routing.allocation.node_concurrent_recoveries": "20",
    "cluster.routing.allocation.cluster_concurrent_rebalance": "20",
    "indices.recovery.max_bytes_per_sec": "200mb"
  }
}

canAllocate:
场景1：
the node is above the low watermark cluster setting [%s], having less than the minimum required [%s] free space, actual free: [%s], actual used: [%s]
解析：节点磁盘利用率超过集群low水位的设置值，会导致索引的副本分片无法分配，集群yellow

场景2：
the node is above the high watermark cluster setting [%s], having less than the minimum required [%s] free space, actual free: [%s], actual used: [%s]
解析：节点磁盘利用率超过集群high水位的设置值，会导致索引主分片无法分配，集群Red

场景3：						
allocating the shard to this node will bring the node above the high watermark cluster setting [%s] and cause it to have less than the minimum required [%s] of free space (free: [%s], used: [%s], estimated shard size: [%s])
解析：分片分配到目标节点会导致及诶单磁盘利用率超过高水位。禁止分配。

canRemain（非allocation，此处只列举，不分析）:
例4：
the shard cannot remain on this node because the node has fewer free bytes remaining than the total size of all incoming shards: free space [%s], relocating shards [%s]

例5：
the shard cannot remain on this node because it is above the high watermark cluster setting [%s] and there is less than the required [%s] free space on node, actual free: [%s], actual used: [%s]

例6：
the node has fewer free bytes remaining than the total size of all incoming shards: free space [%sB], relocating shards [%sB]

no allocations are allowed due to cluster setting [cluster.routing.allocation.enable=none]

PUT _cluster/settings
{
    "transient":{
        "cluster.routing.allocation.enable":"all"
    }
}

canAllocate:
non-new primary allocations are forbidden due to %s
replica allocations are forbidden due to %s
no rebalancing is allowed due to %s
以上三种场景也是因为不同配置导致的不同报错内容,会导致uassgin分片无法正常分配，影响集群健康值。

canRebalance:
no rebalancing is allowed due to %s
replica rebalancing is forbidden due to %s
primary rebalancing is forbidden due to %s
以上三种场景影响集群重新均衡。

索引3种场景
node does not match index setting [%s] filters [%s]
node does not match index setting [%s] filters [%s]
node matches index setting [%s] filters [%s]
集群3种场景
node does not match cluster setting [%s] filters [%s]
node does not cluster setting [%s] filters [%s]
node matches cluster setting [%s] filters [%s]

node does not match index setting [index.routing.allocation.require] filters [temperature:\\"warm\\",_id:\\"xxxxxxxxxxxx\\"]"

场景1：
shard has exceeded the maximum number of retries [%d] on failed allocation attempts - manually call [/_cluster/reroute?retry_failed=true] to retry, [%s]
场景2：
shard has exceeded the maximum number of retries [%d] on failed relocation attempts - manually call [/_cluster/reroute?retry_failed=true] to retry, [%s]

shard has exceeded the maximum number of retries [5] on failed allocation attempts - manually call [/_cluster/reroute?retry_failed=true] to retry, [unassigned_info[[reason=ALLOCATION_FAILED], at[2022-09-20T17:54:21.337Z], failed_attempts[5], failed_nodes[[xxxxxxx]], delayed=false, details[failed shard on node [xxxxxx]: failed to create shard, failure IOException[failed to obtain in-memory shard lock]; nested: ShardLockObtainFailedException[[xxxxxxxxx][2]: obtaining shard lock timed out after 5000ms, previous lock details: [shard creation] trying to lock for [shard creation]]; ], allocation_status[deciders_no]]]

PUT _cluster/reroute?retry_failed=true

场景1：
cannot allocate to node [%s] because a copy of this shard is already allocated to node [%s] with the same host address [%s] and [%s] is [true] which forbids more than one node on each host from holding a copy of this shard
场景2：
this shard is already allocated to this node []
场景3：
a copy of this shard is already allocated to this node []

例：调整成0副本
PUT mytest
{
  "settings":{
    "number_of_replicas":0
  }
}

场景1：
too many shards [%d] allocated to this node, cluster setting [%s=%d]
场景2：
too many shards [%d] allocated to this node for index [%s], index setting [%s=%d]

索引级别优先级高于集群级别：
PUT {索引名}/_settings
{
    "settings":{
        "index":{
            "routing":{
                "allocation":{
                    "total_shards_per_node":"2"
                }
            }
        }
    }
}

场景1：
reached the limit of ongoing initial primary recoveries [%d], cluster setting [%s=%d]

场景2：
reached the limit of incoming shard recoveries [%d], cluster setting [%s=%d] (can also be set via [%s])

场景3:
primary shard for this replica is not yet active

场景4：
reached the limit of outgoing shard recoveries [%d] on the node [%s] which holds the primary,cluster setting [%s=%d] (can also be set via [%s])

PUT _cluster/settings
{
  "persistent": {
    "cluster.routing.allocation.node_concurrent_incoming_recoveries": "4",
    "cluster.routing.allocation.node_concurrent_outgoing_recoveries": "4",
    "cluster.routing.allocation.node_concurrent_recoveries": "20",
    "cluster.routing.allocation.cluster_concurrent_rebalance": "20",
    "cluster.routing.allocation.node_initial_primaries_recoveries": "20",
    "indices.recovery.max_bytes_per_sec": "200mb"
  },
  "transient": {
    "cluster.routing.allocation.node_concurrent_incoming_recoveries": "4",
    "cluster.routing.allocation.node_concurrent_outgoing_recoveries": "4",
    "cluster.routing.allocation.node_concurrent_recoveries": "20",
    "cluster.routing.allocation.cluster_concurrent_rebalance": "20",
    "indices.recovery.max_bytes_per_sec": "200mb"
  }
}