[[shared-index]] === Shared Index We can use a large shared index for the many smaller ((("scaling", "shared index")))((("indices", "shared")))forums by indexing the forum identifier in a field and using it as a filter: [source,json] ------------------------------ PUT /forums { "settings": { "number_of_shards": 10 <1> }, "mappings": { "post": { "properties": { "forum_id": { <2>   "type": "string", "index": "not_analyzed" } } } } } PUT /forums/post/1 { "forum_id": "baking", <2> "title": "Easy recipe for ginger nuts", ... } ------------------------------ <1> Create an index large enough to hold thousands of smaller forums. <2> Each post must include a `forum_id` to identify which forum it belongs to. We can use the `forum_id` as a filter to search within a single forum. The filter will exclude most of the documents in the index (those from other forums), and filter caching will ensure that responses are fast: [source,json] ------------------------------ GET /forums/post/_search { "query": { "filtered": { "query": { "match": { "title": "ginger nuts" } }, "filter": { "term": { <1> "forum_id": { "baking" } } } } } } ------------------------------ <1> The `term` filter is cached by default. This approach works, but we can do better. ((("shards", "routing a document to"))) The posts from a single forum would fit easily onto one shard, but currently they are scattered across all ten shards in the index. This means that every search request has to be forwarded to a primary or replica of all ten shards. What would be ideal is to ensure that all the posts from a single forum are stored on the same shard. In <<routing-value>>, we explained((("routing a document to a shard"))) that a document is allocated to a particular shard by using this formula: shard = hash(routing) % number_of_primary_shards The `routing` value defaults to the document's `_id`, but we can override that and provide our own custom routing value, such as `forum_id`. All documents with the same `routing` value will be stored on the same shard: [source,json] ------------------------------ PUT /forums/post/1?routing=baking <1> { "forum_id": "baking", <1> "title": "Easy recipe for ginger nuts", ... } ------------------------------ <1> Using `forum_id` as the routing value ensures that all posts from the same forum are stored on the same shard. When we search for posts in a particular forum, we can pass the same `routing` value to ensure that the search request is run on only the single shard that holds our documents: [source,json] ------------------------------ GET /forums/post/_search?routing=baking <1> { "query": { "filtered": { "query": { "match": { "title": "ginger nuts" } }, "filter": { "term": { <2> "forum_id": { "baking" } } } } } } ------------------------------ <1> The query is run on only the shard that corresponds to this `routing` value. <2> We still need the filter, as a single shard can hold posts from many forums. Multiple forums can be queried by passing a comma-separated list of `routing` values, and including each `forum_id` in a `terms` filter: [source,json] ------------------------------ GET /forums/post/_search?routing=baking,cooking,recipes { "query": { "filtered": { "query": { "match": { "title": "ginger nuts" } }, "filter": { "terms": { "forum_id": { [ "baking", "cooking", "recipes" ] } } } } } } ------------------------------ While this approach is technically efficient, it looks a bit clumsy because of the need to specify `routing` values and `terms` filters on every query or indexing request. Index aliases to the rescue!