Using Bluetooth Wireless Sensors
Bluetooth wireless sensors are small, relatively inexpensive devices that can be retro-fitted into or onto existing systems. As they are wireless and battery powered, they are particularly suitable for placement in usually inaccessible places or locations without mains power. Batteries can last 5+ years depending on how often they need to report values.
Wireless sensors transmit standard Bluetooth LE advertising used by fitness trackers, healthcare devices and commercial equipment. The data within the advertising can be formatted as iBeacon with bits/bytes storing the sensed data or can have a proprietary format. Either way, any devices such as a gateway, smartphone or single board computers such as the Raspberry Pi can scan for beacons and see the data. Scanning doesn't connect to the beacon. It just sees the data that is being broadcast to everyone.
Example advertising from iGS01H Hall effect sensor
Additionally, some sensors also allow a connection from the scanning device, called a Bluetooth GATT connection. This allows the scanning device to view the current sensor values or arrange to be notified whenever the values change thus allowing a greater throughput of changes. However, a connection stops the advertising which means other devices can't see the sensor and connect.
Receiving devices such as smartphone apps and single board computers scan for beacons and connect via GATT using built in Bluetooth libraries. Separate sensor specific SDKs are not needed as the communication is standard Bluetooth LE. Subscriptions to server side platforms are also not needed.