Setup and Usage of BoTorch Models in Ax¶

Ax provides a set of flexible wrapper abstractions to mix-and-match BoTorch components like Model and AcquisitionFunction and combine them into a single Model object in Ax. The wrapper abstractions: Surrogate, Acquisition, and BoTorchModel – are located in ax/models/torch/botorch_modular directory and aim to encapsulate boilerplate code that interfaces between Ax and BoTorch. This functionality is in beta-release and still evolving.

This tutorial walks through setting up a custom combination of BoTorch components in Ax in following steps:

1. Quick-start example of BoTorchModel use
2. BoTorchModel = Surrogate + Acquisition (overview)
   1. Example with minimal options that uses the defaults
   2. Example showing all possible options
   3. Surrogate and Acquisition Q&A
3. I know which Botorch Model and AcquisitionFunction I'd like to combine in Ax. How do set this up?
   1. Making a Surrogate from BoTorch Model
   2. Using an arbitrary BoTorch AcquisitionFunction in Ax
4. Using Models.BOTORCH_MODULAR (convenience wrapper that enables storage and resumability)
5. Utilizing BoTorchModel in generation strategies (abstraction that allows to chain models together and use them in Ax Service API etc.)
   1. Specifying pending_observations to avoid the model re-suggesting points that are part of RUNNING or ABANDONED trials.
6. Customizing a Surrogate or Acquisition (for cases where existing subcomponent classes are not sufficient)

1. Quick-start example¶

Here we set up a BoTorchModel with SingleTaskGP with qLogNoisyExpectedImprovement, one of the most popular combinations in Ax:

Now we can use this model to generate candidates (gen), predict outcome at a point (predict), or evaluate acquisition function value at a given point (evaluate_acquisition_function).

Before you read the rest of this tutorial:

• Note that the concept of ‘model’ is Ax is somewhat a misnomer; we use 'model' to refer to an optimization setup capable of producing candidate points for optimization (and often capable of being fit to data, with exception for quasi-random generators). See Models documentation page for more information.
• Learn about ModelBridge in Ax, as users should rarely be interacting with a Model object directly (more about ModelBridge, a data transformation layer in Ax, here).

2. BoTorchModel = Surrogate + Acquisition¶

A BoTorchModel in Ax consists of two main subcomponents: a surrogate model and an acquisition function. A surrogate model is represented as an instance of Ax’s Surrogate class, which is a wrapper around BoTorch's Model class. The Surrogate is defined by a SurrogateSpec. The acquisition function is represented as an instance of Ax’s Acquisition class, a wrapper around BoTorch's AcquisitionFunction class.

2A. Example that uses defaults and requires no options¶

BoTorchModel does not always require surrogate and acquisition specification. If instantiated without one or both components specified, defaults are selected based on properties of experiment and data (see Appendix 2 for auto-selection logic).

2B. Example with all the options¶

Below are the full set of configurable settings of a BoTorchModel with their descriptions:

2C. Surrogate and Acquisition Q&A¶

Why is the surrogate argument expected to be an instance, but botorch_acqf_class –– a class? Because a BoTorch AcquisitionFunction object (and therefore its Ax wrapper, Acquisition) is ephemeral: it is constructed, immediately used, and destroyed during BoTorchModel.gen, so there is no reason to keep around an Acquisition instance. A Surrogate, on another hand, is kept in memory as long as its parent BoTorchModel is.

How to know when to use specify acquisition_class (and thereby a non-default Acquisition type) instead of just passing in botorch_acqf_class? In short, custom Acquisition subclasses are needed when a given AcquisitionFunction in BoTorch needs some non-standard subcomponents or inputs (e.g. a custom BoTorch MCAcquisitionObjective).

Please post any other questions you have to our dedicated issue on Github: https://github.com/facebook/Ax/issues/363. This functionality is in beta-release and your feedback will be of great help to us!

3. I know which Botorch Model and AcquisitionFunction I'd like to combine in Ax. How do set this up?¶

3a. Making a Surrogate from BoTorch Model:¶

Most models should work with base Surrogate in Ax, except for BoTorch ModelListGP. ModelListGP is a special case because its purpose is to combine multiple sub-models into a single Model in BoTorch. It is most commonly used for multi-objective and constrained optimization. Whether or not ModelListGP is used is determined automatically based on the Model class and the data being used via the ax.models.torch.botorch_modular.utils.use_model_list function.

If your Model is not a ModelListGP, the steps to set it up as a Surrogate are:

1. Implement a construct_inputs class method. The purpose of this method is to produce arguments to a particular model from a standardized set of inputs passed to BoTorch Model-s from Surrogate.construct in Ax. It should accept training data in form of a SupervisedDataset container and optionally other keyword arguments and produce a dictionary of arguments to __init__ of the Model. See SingleTaskMultiFidelityGP.construct_inputs for an example.
2. Pass any additional needed keyword arguments for the Model constructor (that cannot be constructed from the training data and other arguments to construct_inputs) via the model_options argument to ModelConfig in SurrogateSpec.

NOTE: if you run into a case where base Surrogate does not work with your BoTorch Model, please let us know in this Github issue: https://github.com/facebook/Ax/issues/363, so we can find the right solution and augment this tutorial.

3B. Using an arbitrary BoTorch AcquisitionFunction in Ax¶

Steps to set up any AcquisitionFunction in Ax are:

1. Define an input constructor function. The purpose of this method is to produce arguments to a acquisition function from a standardized set of inputs passed to BoTorch AcquisitionFunction-s from Acquisition.__init__ in Ax. For example, see construct_inputs_qEHVI, which creates a fairly complex set of arguments needed by qExpectedHypervolumeImprovement –– a popular multi-objective optimization acquisition function offered in Ax and BoTorch. For more examples, see this collection in BoTorch: botorch/acquisition/input_constructors.py
   1. Note that the new input constructor needs to be decorated with @acqf_input_constructor(AcquisitionFunctionClass) to register it.
2. Specify the BoTorch AcquisitionFunction class as botorch_acqf_class to BoTorchModel
3. (Optional) Pass any additional keyword arguments to acquisition function constructor or to the optimizer function via acquisition_options argument to BoTorchModel.

See section 2A for combining the resulting Surrogate instance and Acquisition type into a BoTorchModel. You can also leverage Models.BOTORCH_MODULAR for ease of use; more on it in section 4 below or in section 1 quick-start example.

4. Using Models.BOTORCH_MODULAR¶

To simplify the instantiation of an Ax ModelBridge and its undelying Model, Ax provides a Models registry enum. When calling entries of that enum (e.g. Models.BOTORCH_MODULAR(experiment, data)), the inputs are automatically distributed between a Model and a ModelBridge for a given setup. A call to a Model enum member yields a model bridge with an underlying model, ready for use to generate candidates.

Here we use Models.BOTORCH_MODULAR to set up a model with all-default subcomponents:

We can use the same Models.BOTORCH_MODULAR to set up a model for multi-objective optimization:

Furthermore, the quick-start example at the top of this tutorial shows how to specify surrogate and acquisition subcomponents to Models.BOTORCH_MODULAR.

5. Utilizing BoTorchModel in generation strategies¶

Generation strategy is a key concept in Ax, enabling use of Service API (a.k.a. AxClient) and many other higher-level abstractions. A GenerationStrategy allows to chain multiple models in Ax and thereby automate candidate generation. Refer to the "Generation Strategy" tutorial for more detail in generation strategies.

An example generation stategy with the modular BoTorchModel would look like this:

Set up an experiment and generate 10 trials in it, adding synthetic data to experiment after each one:

5a. Specifying pending_observations¶

Note that it's important to specify pending observations to the call to gen to avoid getting the same points re-suggested. Without pending_observations argument, Ax models are not aware of points that should be excluded from generation. Points are considered "pending" when they belong to STAGED, RUNNING, or ABANDONED trials (with the latter included so model does not re-suggest points that are considered "bad" and should not be re-suggested).

If the call to get_pending_observation_features becomes slow in your setup (since it performs data-fetching etc.), you can opt for get_pending_observation_features_based_on_trial_status (also from ax.modelbridge.modelbridge_utils), but note the limitations of that utility (detailed in its docstring).

Now we examine the experiment and observe the trials that were added to it and produced by the generation strategy:

6. Customizing a Surrogate or Acquisition¶

We expect the base Surrogate and Acquisition classes to work with most BoTorch components, but there could be a case where you would need to subclass one of aforementioned abstractions to handle a given BoTorch component. If you run into a case like this, feel free to open an issue on our Github issues page –– it would be very useful for us to know

One such example would be a need for a custom MCAcquisitionObjective or posterior transform. To subclass Acquisition accordingly, one would override the get_botorch_objective_and_transform method:

Then to use the new subclass in BoTorchModel, just specify acquisition_class argument along with botorch_acqf_class (to BoTorchModel directly or to Models.BOTORCH_MODULAR, which just passes the relevant arguments to BoTorchModel under the hood, as discussed in section 4):

To use a custom Surrogate subclass, pass the surrogate argument of that type:

Models.BOTORCH_MODULAR(
    experiment=experiment, 
    data=data,
    surrogate=CustomSurrogate(botorch_model_class=MyModelClass),
)

Appendix 1: Methods available on BoTorchModel¶

Note that usually all these methods are used through ModelBridge –– a convertion and transformation layer that adapts Ax abstractions to inputs required by the given model.

Core methods on BoTorchModel:

• fit selects a surrogate if needed and fits the surrogate model to data via Surrogate.fit,
• predict estimates metric values at a given point via Surrogate.predict,
• gen instantiates an acquisition function via Acquisition.__init__ and optimizes it to generate candidates.

Other methods on BoTorchModel:

• update updates surrogate model with training data and optionally reoptimizes model parameters via Surrogate.update,
• cross_validate re-fits the surrogate model to subset of training data and makes predictions for test data,
• evaluate_acquisition_function instantiates an acquisition function and evaluates it for a given point.

Appendix 2: Default surrogate models and acquisition functions¶

By default, the chosen surrogate model will be:

• if fidelity parameters are present in search space: SingleTaskMultiFidelityGP,
• if task parameters are present: a set of MultiTaskGP wrapped in a ModelListGP and each modeling one task,
• SingleTaskGP otherwise.

The chosen acquisition function will be:

• for multi-objective settings: qLogExpectedHypervolumeImprovement,
• for single-objective settings: qLogNoisyExpectedImprovement.

Appendix 3: Handling storage errors that arise from objects that don't have serialization logic in A¶

Attempting to store a generator run produced via Models.BOTORCH_MODULAR instance that included options without serization logic with will produce an error like: "Object <SomeAcquisitionOption object> passed to 'object_to_json' (of type <class SomeAcquisitionOption'>) is not registered with a corresponding encoder in ENCODER_REGISTRY."

The two options for handling this error are:

1. disabling storage of BoTorchModel's options by passing no_model_options_storage=True to Models.BOTORCH_MODULAR(...) call –– this will prevent model options from being stored on the generator run, so a generator run can be saved but cannot be used to restore the model that produced it,
2. specifying serialization logic for a given object that needs to occur among the Model or AcquisitionFunction options. Tutorial for this is in the works, but in the meantime you can post an issue on the Ax GitHub to get help with this.