System Architecture
Description of the different modules and how they communicate with each other.
The common library provides core functions for all DroneBridge modules. This shared code base implements the DroneBridge raw protocol and therefor handles the setup of the required sockets based on user input. The library provides convenience functions for sending & receiving data using the raw protocol, can encrypt payloads (AES + EAX, Python only for now), and defines structures to be used for inter-process communication between major modules.
DroneBridge common library concept
All DroneBridge applications are started by the
start_db.service. Most of the startup scripts are located inside the startup folder. Like any service it can be started, stopped & re-started. During stopping or restarting there might still be some bugs. 
Startup Sequence of DroneBridge for Raspberry Pi
Following Block Diagram shows how the different DroneBridge modules communicate with each other on the network & long range link. Some modules that do not have complex communication paths are not displayed (SysLogServer, OSD). Same goes for communication via shared memory.
In this case a GCS like DroneBridge for Android or QGroundControl is connected via TCP to the DroneBridge GND side. A UDP data stream (video) is sent via unicast to
192.168.2.1. A different destination IP can be registered by sending a UDP packet of any size and content to the video module on UDP port 5000.
Ground Control Station (GCS) connected to DroneBridge GND station via network
In this case of a connected android phone to the DB GND station, the USB Bridge module auto-connects to all relevant modules and forwards the messages to the phone using the Android Accessory Protocol. At this point only the DroneBridge for Android app supports this mode.
DroneBridge for Android connected via USB