Currently, Horizon is dependent on a Stellar Core server. While we are working to reduce that dependence, at the moment it still needs access to both the SQL database and the HTTP API published by Stelar Core. See Run a Core Node to learn how to set up and administer a Stellar Core node.
Horizon is also dependent upon a postgres server, which it uses to store processed Core data for ease of use. Horizon requires postgres version >= 9.5.
In addition to the two prerequisites above, you may optionally install a redis server to be used for rate limiting requests.
Last updated Sep. 23, 2020