[Yandex Cloud documentation](../index.md) > [Yandex SpeechKit Hybrid](index.md) > Service architecture

# SpeechKit Hybrid architecture

SpeechKit Hybrid comprises the following components:

* **[Envoy](https://www.envoyproxy.io/docs)**: Routing server that receives incoming gRPC requests and determines the SpeechKit Hybrid components to handle them.
* **License server**: Envoy dynamic configuration server for accounting Yubikey licenses or providing data required to pay for resources.
* **STT server**: Speech recognition server.
* **TTS server**: Speech synthesis server.

## Relationship between SpeechKit Hybrid components {#components-interaction}

Component relationships depend on the licensing model selected by a partner.

### Yubikey {#yubikey}

The License server limits the maximum number of speech recognition sessions and maximum performance (requests per second) for speech synthesis. These parameters are read from a [Yubikey hardware authentication device](https://yubico.com). To increase their values, purchase additional Yubikey hardware authentication devices. The component relationships are shown on the diagram below:

![arch](../_assets/speechkit-hybrid-yubikey.svg)

### Cloud Billing {#billing}

No limit on the number of sessions and requests per second. Information about each service request is sent to Yandex Billing. Total service usage fees are specified in a payment invoice issued to a partner in their billing account. The component relationships are shown on the diagram below:

![arch](../_assets/speechkit-hybrid-billing-agent.svg)