ChirpStack Network Server forwards received uplink frames and acknowledgements to a so called Application Server component. You can either use ChirpStack Application Server (an Application Server provided by the ChirpStack open-source LoRaWAN® Network Server stack), or implement your own Application Server.
ChirpStack Network Server supports sending data for different devices to different Application Servers. See Routing Profile for more information.
ChirpStack Application Server
ChirpStack Application Server is an open-source implementation of a LoRaWAN Application Server.
Custom Application Server
It is also possible to implement a custom Application Server. The Application Server API has been defined as a gRPC service which allows you to easily generate stubs for various programming languages. See the gRPC site for more information about the gRPC framework and how to generate source-code using