Creating a search engine to search content across a polyglot website is not an easy task. In order to do that, you need to either:
- take a search query and match it across data repositories from different application processes, or
- have a process that will maintain an index of content from the different applications and search within that content.
Since Google had indexed most of the content in the Internet, Google Custom Search can be the easiest way for us to implement site-wide search. In addition to fast searching, Google Custom Search also comes with AdSense monetization.
In case you need it, this is how you can implement Google Custom Search as a WordPress page without using any plugin.