There are a few concepts that you should be familiar with before starting to work with the Hydrosphere platform.

Resource definitions

Resource definitions describe Models, Applications, and Deployment Configurations in the YAML format. You can learn more about them in the How to write resource definitions section.


Models & Model Versions

A Model is a machine learning model or a processing function that consumes provided inputs and produces predictions or transformations.

Within the Hydrosphere platform, we break down a model into its versions. Each Model version represents a single Docker image containing all the artifacts that you have uploaded to the platform. Consequently, Model is a group of Model versions with the same name.

Model vs Model Version Difference


A Runtime is a Docker image with the predefined gRPC interface which loads and serves your model.

Place of Runtimes in the Architecture

We have implemented a few runtimes, which you can use in your own projects.


Servable is a deployed instance of a Model version combined with a Runtime. It exposes a gRPC endpoint that can be used to send requests.

Users should not use Servables as-is, since they are designed to be building blocks, rather than inference endpoints. Hydrosphere provides a better alternative to deploy a Model version — Application.


An Application is a pipeline of one or more stages, each consisting of one or multiple Model Versions. Data sent to an application stage is shadowed to all of its model versions. The output of a stage is picked randomly with respect to weights.

Example of a multi-staged output with an A/B test on the second stage

When a user creates an Application, the Manager service automatically deploys appropriate Servables. The Application handles monitoring of your models and can perform A/B traffic splits.

Each Application has publicly available HTTP and gRPC endpoints that you can send requests to.

Deployment Configurations

A Deployment Configuration is a collection of Kubernetes settings that you can set for your Servables and Model Versions used inside of Application stages.

Deployment Configuration covers:

  • Horizontal Pod Autoscaler specs

  • Container Specs

    • Resource requirements: limits and requests

  • Pod Specs

    • Node Selectors

    • Affinity

    • Tolerations

  • Deployment Specs

    • Replicas count

Model's Signature

A Model's Signature is a specification of your model computation which identifies the name of a function with its inputs and outputs, including their names, shapes, and data types.

Example of a signature defined in a YAML file:

name: predict
shape: [-1, 2]
type: double
profile: numerical
shape: scalar
type: int
profile: categorical


A Field is a basic element of a Model's signature. It has a name, shape, data type, and profile.

Example of a model's signature field defined in a YAML file:

shape: [-1, 2]
type: double
profile: numerical

Field`s Profile

A Profile is a special tag that tells how Hydrosphere should interpret the field's data.

There are multiple available tags: Numerical, Categorical, Image, Text, Audio, Video, etc.



Data coming through deployed Model Versions can be monitored with metrics.

Metric is a Model Version that takes a combination of inputs & outputs from another monitored Model Version, receives every request and response from the monitored model, produces a single value, and compares it with a threshold to determine whether this request was healthy or not.

Every request is evaluated against all metrics assigned to the model.

Auto OD Metric is an automatically generated Outlier Detection metric. More details are described here.


A check is a boolean condition associated with a field of a Model Version signature which shows for every request whether the field value is acceptable or not.

For example, Min/Max checks ensure that a field value is in an acceptable range which is inferred from training data values.