ElasticsearchDifference Between Indices and Types


Remarks

It's easy to see types like a table in an SQL database, where the index is the SQL database. However, that is not a good way to approach types.

All About Types

In fact, types are literally just a metadata field added to each document by Elasticsearch: _type. The examples above created two types: my_type and my_other_type. That means that each document associated with the types has an extra field automatically defined like "_type": "my_type"; this is indexed with the document, thus making it a searchable or filterable field, but it does not impact the raw document itself, so your application does not need to worry about it.

All types live in the same index, and therefore in the same collective shards of the index. Even at the disk level, they live in the same files. The only separation that creating a second type provides is a logical one. Every type, whether it's unique or not, needs to exist in the mappings and all of those mappings must exist in your cluster state. This eats up memory and, if each type is being updated dynamically, it eats up performance as the mappings change.

As such, it is a best practice to define only a single type unless you actually need other types. It is common to see scenarios where multiple types are desirable. For example, imagine you had a car index. It may be useful to you to break it down with multiple types:

  • bmw
  • chevy
  • honda
  • mazda
  • mercedes
  • nissan
  • rangerover
  • toyota
  • ...

This way you can search for all cars or limit it by manufacturer on demand. The difference between those two searches are as simple as:

GET /cars/_search

and

GET /cars/bmw/_search

What is not obvious to new users of Elasticsearch is that the second form is a specialization of the first form. It literally gets rewritten to:

GET /cars/_search
{
  "query": {
    "bool": {
      "filter": [
        {
          "term" : {
            "_type": "bmw"
          }
        }
      ]
    }
  }
}

It simply filters out any document that was not indexed with a _type field whose value was bmw. Since every document is indexed with its type as the _type field, this serves as a pretty simple filter. If an actual search had been provided in either example, then the filter would be added to the full search as appropriate.

As such, if the types are identical, it's much better to supply a single type (e.g., manufacturer in this example) and effectively ignore it. Then, within each document, explicitly supply a field called make or whatever name you prefer and manually filter on it whenever you want to limit to it. This will reduce the size of your mappings to 1/n where n is the number of separate types. It does add another field to each document, at the benefit of an otherwise simplified mapping.

In Elasticsearch 1.x and 2.x, such a field should be defined as

PUT /cars
{
  "manufacturer": { <1>
    "properties": {
      "make": { <2>
        "type": "string",
        "index": "not_analyzed"
      }
    }
  }
}
  1. The name is arbitrary.
  2. The name is arbitrary and it could match the type name if you wanted it too.

In Elasticsearch 5.x, the above will still work (it's deprecated), but the better way is to use:

PUT /cars
{
  "manufacturer": { <1>
    "properties": {
      "make": { <2>
        "type": "keyword"
      }
    }
  }
}
  1. The name is arbitrary.
  2. The name is arbitrary and it could match the type name if you wanted it too.

Types should be used sparingly within your indices because it bloats the index mappings, usually without much benefit. You must have at least one, but there is nothing that says you must have more than one.

Common Questions

  • What if I have two (or more) types that are mostly identical, but which have a few unique fields per type?

At the index level, there is no difference between one type being used with a few fields that are sparsely used and between multiple types that share a bunch of non-sparse fields with a few not shared (meaning the other type never even uses the field(s)).

Said differently: a sparsely used field is sparse across the index regardless of types. The sparsity does not benefit -- or really hurt -- the index just because it is defined in a separate type.

You should just combine these types and add a separate type field.

  • Why do separate types need to define fields in the exact same way?

Because each field is really only defined once at the Lucene level, regardless of how many types there are. The fact that types exist at all is a feature of Elasticsearch and it is only a logical separation.

  • Can I define separate types with the same field defined differently?

No. If you manage to find a way to do so in ES 2.x or later, then you should open up a bug report. As noted in the previous question, Lucene sees them all as a single field, so there is no way to make this work appropriately.

ES 1.x left this as an implicit requirement, which allowed users to create conditions where one shard's mappings in an index actually differed from another shard in the same index. This was effectively a race condition and it could lead to unexpected issues.

Exceptions to the Rule

  • Parent/child documents require separate types to be used within the same index.
    • The parent lives in one type.
    • The child lives in a separate type (but each child lives in the same shard as its parent).
  • Extremely niche use cases where creating tons of indices is undesirable and the impact of sparse fields is preferable to the alternative.
    • For example, the Elasticsearch monitoring plugin, Marvel (1.x and 2.x) or X-Pack Monitoring (5.x+), monitors Elasticsearch itself for changes in the cluster, nodes, indices, specific indices (the index level), and even shards. It could create 5+ indices each day to isolate those documents that have unique mappings or it could go against best practices to reduce cluster load by sharing an index (note: the number of defined mappings is effectively the same, but the number of created indices is reduced from n to 1).
    • This is an advanced scenario, but you must consider the shared field definitions across types!