What are aliases in elasticsearch for?

Question

I recently started working in a company that uses Elasticsearch. While most of its concepts are somewhat similar to relational databases and I am able to understand them, I

Answer 1

@arhak covered the topic pretty well. One use case that (at least) made me understand the value of indices was the need to remove out-of-date documents and more specifically when using time-based-indices.

For example, you need to keep the logs of an application for at least one year. You decide to use time-based-indices, meaning you save into indices with the following format: 2018-02-logs, 2018-03-logs etc.. In order to be able to search in every index you create the following alias:

POST /_aliases
{
 "actions": [{ 
     "add": {
          "alias": "current-logs", "indices": [ "2018-02-logs","2018-03-logs" ]
        }  
  }]
}

And query like:

GET /current-logs/_search

Another advantage is that you can delete the out-of-date values very easily:

POST /_aliases
{
  "actions": [

      { "remove": { "alias": "current-logs",  "index": "logs_2018-01" }}
  ]
}

and DELETE /logs_2018-01