Which node to connect to

Hello, this is a dum question. But I have two nodes on my cluster (One is master). And I always use the master IP (or URL) to fetch for Info, for example in my Grafana data source I use the Node1 URL to Connect.
Is always this way? I need always to connect to the master node in order to fetch for data? Or I can use any node to fetch for data (for example in Grafana, Elastalert or any other systems).

Does this practice doesn't achieve availability ? Since I'm always using one node to fetch for data

The node you connect to in order to run searches is coordinator node/role which every node is capable of doing.

A 2 nodes cluster is to be avoided as it means there is no high-availability (better to have 3 master-data nodes - or 2 master-data and a voting-only node) and there is also no advantage of dedicated master node if you only have 2 nodes...
It's not clear in your question what are the roles (GET _cat/nodes?v) but assuming 2 scenarios :

  1. If it's one master-dedicated and a data node, you should use the data nodes as coordinator. In this scenario if the data-node fails, as there is no replica shard assigned with only one data node the cluster and all indices will turn red (no shard/index available for search nor write), if the master fails the cluster turns red because it lacks an elected master.
  2. If you have one master-data and one data node, you would normally connect to either/both nodes as coordinator node with a note in this case that assuming indices have 1 replica, cluster will turn yellow (search and write still available) when the data-only node fails, while obviously a failure of the master-data means the cluster becomes unavailable because it has no elected master (you should better have 2 master-data and add a low-specs voting-only node for quorum, the voting-only node would not be used as coordinator node in that case)


Now it's a little bit more clear. I thought that you can only have one master node. So my best scenario is to have 3 master-data nodes (what about ingest nodes?). And If I want to retrieve Info I can use any node

Yes, by default nodes have all roles assigned, ingest (for running ingest pipelines if you use that) and coordinator (for collecting result of multiple searches) roles

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