Elasticsearch and MongoDB data synchronization and distributed cluster construction (2)

A node will run an instance of ES, and a cluster will contain one or more nodes with the same cluster.name. These nodes work together to complete data sharing and load balancing. As nodes are added to or removed from the cluster, the cluster adjusts itself to evenly distribute data. One node in the cluster will be elected as the master node (Master Node), which is responsible for managing changes in the entire cluster, such as creating or deleting an index (Index), adding or deleting nodes to the cluster. Any node can become the master node. In our example there is only one node, so it assumes the function of the master node. ES distributes data across the cluster through sharding. Think of shards as containers for data. Documents are stored in shards, and shards are distributed among nodes in the cluster. As the cluster expands and becomes smaller, ES will automatically migrate shards between nodes to ensure that the cluster maintains a balance. A shard can be a primary shard (Primary Shard) or a replica shard (Replica Shard). Each document in the index belongs to a primary shard, so the number of primary shards determines the maximum amount of data your index can store. A replica shard is just a copy of the primary shard. Replicas are used to provide data redundancy, to protect data from loss in the event of hardware failure, and to handle read requests such as searching and retrieving documents. The number of primary shards is determined at the beginning of the index creation, while the number of replica shards can be changed at any time.