Solr has been the preferred (and Sitecore-recommended) index provider for setting up Sitecore solutions, and is enabled by default in the Sitecore installation scripts.
Standalone Solr instances are the best option to consider for development and test environments, whereas SolrCloud is a more reliable and scalable option for UAT and production environments. Sitecore has started to provide full support on SolrCloud starting with XP 9.0 Update-2 and later.
With the increased popularity of setting up Sitecore PaaS, Microsoft Azure Search has also become popular. It’s one of the few options to configure search indexes for Sitecore as a service. However, it does come at an extra cost, and it has some limitations.
For solutions where the limitations introduced by Azure Search with Sitecore are not acceptable, or Sitecore instances hosted on other platforms than Azure, then Azure Search might not be an option.
One viable alternative for Sitecore index providers is available through SearchStax. SearchStax offers a robust solution by providing ‘Solr as a Service’ for Sitecore.
Find out more about Search Stax – an alternative to Solr Cloud.