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hermes-protocol

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This repository contains the definition of the hermes protocol used by the Snips platform.

What is hermes?

hermes is a protocol used by all the components of the Snips platform to communicate with each other. For example an user app for the platform can use hermes to be notified that a new intent was detected. An other example could when the dialogue component asks the Automatic Speech Recognition (ASR) component to start capturing user speech, this will also be done through hermes.

Ontology

The various messages that can be sent using hermes are defined in what we call an ontology. It is an ensemble of rust structs definitions. Relevant parts of the ontology are available in the guest languages that hermes supports.

Sites

An important notion when using the Snips platform is the notion of "site". A site can be seen as an "interaction locus". It is throught a site than an user will interact with the platform. A site consists at least of a voice input (a microphone) and also probably of speakers and maybe other devices used to give information to the user (leds, screens...).

A site is identified in hermes by a site_id. When there is a single site, it is usually default.

Components and facades

The Snips platform consists of multiple components (for example the ASR, the dialogue, the TTS, the NLU...) and hermes has been designed around theses components.

Each component is represented in hermes by two "facades":

  • The "main" facade that is intended to by used by the one other components to communicate with the component.
  • The "backend" facade that is intended to be used only by the component as it represents the communication side that should be implemented by it.

Facades are retrieved through a ProtocolHandler more on that later.

Let's take the example of the detection of an hotword:

  1. The code that wants to react to an hotword first need to "subscribe" to the event:
  • It needs to use the main hotword facade, that can be obtained using the hotword method on the ProtocolHandler
  • It can then register a callback using the subscribeDetected or subscribeAllDetected methods on the facade (the first one provides and handy way to filter the hotword detected only on a specific site)
  1. The code that actually detects the hotword (aka snips-hotword) needs to publish the event when an hotword is detected:
  • It needs to use the backend hotword facade, that can be obtained using the hotwork_backend method on the ProtocolHandler
  • It can then publish the event using the publishDetected method on the facade
  1. The event is delivered to the callback registered a 1. and the code can react how it wants to the the hotword.

Dialogue

When writing an app for the Snips platform, the facade you'll want to use is the main dialogue facade. The dialogue component orchestrates the interaction with the user and your app should interact with the platform through it. You can find an overview of the available apis on the Snips documentation.

Communication layer and ProtocolHandlers

hermes has two implementations: one over MQTT and one using an inprocess bus. A standard installation on a RaspberryPi uses the MQTT version whereas the Android and iOs sdks use the inprocess one.

The rust crates hermes-mqtt and hermes-inprocess provide ProtocolHandlers implementation for the two communication layers. The guest language bindings for hermes wrap hermes-mqtt.

Quick description of the different dirs

  • hermes ontology and facades (ie protocol) definitions
  • hermes-ffi ffi bindings for ontology and facades
  • hermes-ffi-test echo lib that can be used to test guest language bindings
  • hermes-inprocess protocol implementation using an in-process bus (ripb) for communication
  • hermes-mqtt protocol implementation using MQTT for communication
  • hermes-mqtt-ffi lib exposing the MQTT impl to guest languages
  • hermes-test-suite test suite used to verify implementation correctness
  • platforms guest language bindings
    • c C header
    • hermes-kotlin jvm impl
    • hermes-python python impl
    • hermes-javascript js impl

License

Apache 2.0/MIT

Licensed under either of

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.