An alternative application level protocol that is easily overlooked is CoAP (RFC 7252). A protocol designed for constrained devices (low memory or power, etc) that follows HTTP's REST interface, and also includes a observe (subscribe) method for watching sensor values.

It's a nice alternative to HTTP since it's a binary protocol, with little processing overhead. Supports UDP and works over 6LowPAN which MQTT and HTTP struggle with. Ideal for Bluetooth, ZigBee or Z-Wave