错误如下： 

[2021-04-12T15:59:41,129][ERROR][o.e.x.s.a.e.ReservedRealm] [node-3] failed to retrieve password hash for reserved user [elastic]
org.elasticsearch.action.UnavailableShardsException: at least one primary shard for the index [.security-7] is unavailable
        at org.elasticsearch.xpack.security.support.SecurityIndexManager.getUnavailableReason(SecurityIndexManager.java:182) ~[x-pack-security-7.6.2.jar:7.6.2]
        at org.elasticsearch.xpack.security.authc.esnative.NativeUsersStore.getReservedUserInfo(NativeUsersStore.java:525) [x-pack-security-7.6.2.jar:7.6.2]
        at org.elasticsearch.xpack.security.authc.esnative.ReservedRealm.getUserInfo(ReservedRealm.java:212) [x-pack-security-7.6.2.jar:7.6.2]
        at org.elasticsearch.xpack.security.authc.esnative.ReservedRealm.doAuthenticate(ReservedRealm.java:93) [x-pack-security-7.6.2.jar:7.6.2]
        at org.elasticsearch.xpack.security.authc.support.CachingUsernamePasswordRealm.authenticateWithCache(CachingUsernamePasswordRealm.java:167) [x-pack-security-7.6.2.jar:7.6.2]
        at org.elasticsearch.xpack.security.authc.support.Cach

错误原因：本来我们es有时候就会丢失分片。通常情况下，我们通过重新路由能够把分片找回来。但是出现这样的错误，丢失的分片，恰好是是我们es用于管理用户的索引 index [.security-7] 。

解决方案：我是直接重启了一下集群，就恢复了。