Improve performance by enabling language routing on search requests

In Find 13, we introduced language routing on indexing to improve indexing performance for indexes setup with multiple languages.

With the release of Find 13.4.9, we further introduced language routing on query time. We recommend moving up to 13.5.7 due to other bug fixes. This feature is also available in .NET flavours: 14.2.4 and 15.0.3

This feature enables passing the query only to the specific language index, rather than having all language indexes process the query even though most of them would only respond with 0 hits as we're usually filtering on language properties in the index.

Since 13.5.1 this feature is disabled by default.

To enable this feature, there are a few steps to follow:

1. Ensure that you explicitly or implicitly set the language for your query.
   You typically achieve this by using one of the following approaches:
   

   .Search<type>(language) or .UnifiedSearch(language)

   Alternatively, you can rely on the extensions provided by Episerver.Find.CMS and use
   

   .UnifiedSearch() or .UnifiedSearchFor()

   which will automatically set the preferred culture if you don't specify one. Be careful not to use the Episerver.Find extensions, as they do not resolve language.
   
   
2. You can verify that everything is working correctly by checking the analyzer set in the JSON for the request against the _search. 
   It should say analyzer:[language]. If you are using .UsingSynonyms() it should say analyzer: synonym_[language_code].
   
   
3. If you are using CMS 11, you can do this in the web.config file by adding the following:
   

   <appSettings>
       <add key="episerver:EnableLanguageRoutingInSearchRequest" value="true"/>

   If you are using CMS 12, you can do this in the appsettings.json file by adding the following:
   

   "EPiServer": {
       "Find": {
         "EnableLanguageRoutingInSearchRequest":true
       }
   }

Note that if you have custom InField extension that makes it possible to query fields for multiple language fields, language routing query time cannot be used.

Our tests have shown a decrease in response times of around 25% for queries with language routing enabled. 
However, this improvement may vary depending on your implementation, the number of languages enabled on the index, and the data indexed.

Find versions that support language routing on search requests
https://nuget.optimizely.com/package/?id=EPiServer.Find&v=15.0.3
https://nuget.optimizely.com/package/?id=EPiServer.Find&v=14.2.4
https://nuget.optimizely.com/package/?id=EPiServer.Find&v=13.5.7