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My application supports listing in a very wide variety of categories. Is there a way to get recommendations for all categories of a site at once?
Yes that is possible. Here's how you could go about it:
Sample request and response for GetCategorySpecifics
<?xml version="1.0" encoding="utf-8"?>
<?xml version="1.0" encoding="UTF-8" ?>
If you call GetCategoryFeatures, you may detect that custom Item Specifics have been newly enabled in certain categories since you last downloaded recommendations. In this case, if the file IDs returned by GetCategorySpecifics haven't changed, you can call GetCategorySpecifics directly to retrieve any new recommendations for those specific categories.