About the property “com.denodo.parser.connection.http.maxConnectionsPerHost”, it can be used for limiting the number of concurrent connections to a specific host.
Further, regarding the API throttle question, you could check the section [Limiting the Maximum number of Concurrent requests ](https://community.denodo.com/docs/html/browse/8.0/en/vdp/administration/server_configuration/limiting_the_number_of_concurrent_requests/limiting_the_number_of_concurrent_requests#limiting-the-number-of-concurrent-requests) from the Virtual DataPort Administration Guide which in general highlights about the requests processed in the Virtual DataPort server.
Regarding the last question, I understand that you are able to execute the independent views and when you perform the UNION operation by using those three independent views, you get this error. If this is the case, I would suggest you to enable **Cache** for the three independent views, maybe as an example, use full cache for the three independent views and load the data into the cache, and while performing the UNION operation the server fetches the data from the cache table and does not hit the API.
For more information, you could refer to the Knowledge Base Article [**Caching**](https://community.denodo.com/kb/en/view/document/Best%20Practices%20to%20Maximize%20Performance%20III:%20Caching?category=Best%20Practices#h.ycccegj3w8bn).
In case if you receive any other errors related to the API execution via Virtual DataPort, you can raise a support case on Denodo Support Site so that our support team can help you.
Hope this helps.!