There are certain tasks that only the master can perform, such as creating a new index or moving shards around the cluster. Since a cluster can have only one master, only one node can ever process cluster-level metadata changes. For 99.9999% of the time, this is never a problem. The queue of metadata changes remains essentially zero.새로운 index를 생성하거나, cluster에서 shard의 이동처럼, master만이 할 수 있는 특정 작업이..