a set of overlapping N-grams that are most likely to give the desired search results in each search engine. Another challenge faced in the implementation of federated search engines is scalability. It is difficult to maintain the performance, the response speed, of a federated search engine as it combines more and more information sources together. One implementation of federated search that has begun to address this issue is WorldWideScience, hosted by the U.S. Department of Energy's Office of Scientific and Technical Information. WorldWideScience is composed of more than 40 information sources, several of which are federated search portals themselves. One such portal is Science.gov which itself federates more than 30 information sources representing most of the R&D output of the U.S. Federal government. Science.gov returns its highest ranked results to WorldWideScience, which then merges and ranks these results with the search returned by the other information sources that comprise WorldWideScience. This approach of cascaded federated search enables large number of information sources to be searched via a single query. Another application Sesam running in both Norway and Sweden has been built on top of an open sourced platform specialised for federated search solutions. Sesat, an acronym for Sesam Search Application Toolkit, is a platform that provides much of the framework and functionality required for handling parallel and pipelined searches and displaying them elegantly in a user interface, allowing engineers to focus on the index/database configuration tuning. To personalize vertical orders in federated search, LinkedIn search engine exploits the searcher's profile and recent activities to infer his or her intent, such as hiring, job seeking and content consuming, then uses the intent, along with many other signals, to rank vertical order that personally relevant to the individual searcher. SWIRL Search is an open source federated search engine, released under the Apache 2.0 license. It includes pre-built connectors to popular open source search engines, and re-ranks results using cosine vector similarity. Challenges When federated search is performed against secure data sources, the users' credentials must be passed on to each underlying search engine, so that appropriate security is maintained. If the user has different login credentials for different systems, there must be a means to map their login ID to each search engine's security domain. Another challenge is mapping results list navigators into a common form. Suppose 3 real-estate sites are searched, each provides a list of hyperlinked city names to click