Mess up internal search and you’ll frustrate your employees. But mess up external search and you’ll alienate your customers.

No wonder that e-commerce company execs like Jeff Zwelling of YLighting bear down hard on this problem: Zwelling changed his Web site’s search engine three times in the past four years, unhappy with the search results that his company’s site was giving customers — or rather wasn’t giving them. Nothing changed until the fourth try in late 2006.

Graeme McCracken, the COO of RB Search, a subsidiary of Reed Business charged with making the publisher’s content available through the consolidated site, faced the same frustration three years ago. His search engine didn’t give readers a complete, accurate picture of his company’s many magazines and newsletters.

Mired in the problems of external search, both companies found that the Google approach — the one most commonly tried first — doesn’t always keep customers happy.

E-commerce and media businesses have similar needs for external search: guided navigation and contextual search to help users quickly narrow down their desired results using categories, user profiles and other metadata. Even database-driven e-commerce sites must go beyond database content to handle vague searches like “red lamp,” says Zwelling, YLighting’s president.

External (keyword) search must help customers get to the same result as using the site’s navigation, says Chris Cummings, CIO of online retailer eToys Direct.

By contrast, internal search focuses on discovering data “hidden” in documents, databases, and so forth. Google follows the internal search approach: Users typically want anything that answers their query, not a specific, repeatable result.

E-commerce vendors and content publishers have come to these realizations early, says Tony Byrne, founder of the research firm CMS Watch, because the success of search relates directly to sales of goods and advertising. But other businesses can use search to im