To keep our design simple and clean, we would like our application to believe that we have a dedicated index per user—​or per forum in our example—​even if the reality is that we are using one big shared index. To do that, we need some way to hide the routing value and the filter on forum_id.

Index aliases allow us to do just that. When you associate an alias with an index, you can also specify a filter and routing values:

PUT /forums/_alias/baking
{
  "routing": "baking",
  "filter": {
    "term": {
      "forum_id": "baking"
    }
  }
}

Now, we can treat the baking alias as if it were its own index. Documents indexed into the baking alias automatically get the custom routing value applied:

PUT /baking/post/1 
{
  "forum_id": "baking", 
  "title":    "Easy recipe for ginger nuts",
  ...
}

Queries run against the baking alias are run just on the shard associated with the custom routing value, and the results are automatically filtered by the filter we specified:

GET /baking/post/_search
{
  "query": {
    "match": {
      "title": "ginger nuts"
    }
  }
}

Multiple aliases can be specified when searching across multiple forums:

GET /baking,recipes/post/_search 
{
  "query": {
    "match": {
      "title": "ginger nuts"
    }
  }
}