.. _model_persistence:

=================
Model persistence
=================

After training a scikit-learn model, it is desirable to have a way to persist
the model for future use without having to retrain. Based on your use-case,
there are a few different ways to persist a scikit-learn model, and here we
help you decide which one suits you best. In order to make a decision, you need
to answer the following questions:

1. Do you need the Python object after persistence, or do you only need to
   persist in order to serve the model and get predictions out of it?

If you only need to serve the model and no further investigation on the Python
object itself is required, then :ref:`ONNX <onnx_persistence>` might be the
best fit for you. Note that not all models are supported by ONNX.

In case ONNX is not suitable for your use-case, the next question is:

2. Do you absolutely trust the source of the model, or are there any security
   concerns regarding where the persisted model comes from?

If you have security concerns, then you should consider using :ref:`skops.io
<skops_persistence>` which gives you back the Python object, but unlike
`pickle` based persistence solutions, loading the persisted model doesn't
automatically allow arbitrary code execution. Note that this requires manual
investigation of the persisted file, which :mod:`skops.io` allows you to do.

The other solutions assume you absolutely trust the source of the file to be
loaded, as they are all susceptible to arbitrary code execution upon loading
the persisted file since they all use the pickle protocol under the hood.

3. Do you care about the performance of loading the model, and sharing it
   between processes where a memory mapped object on disk is beneficial?

If yes, then you can consider using :ref:`joblib <pickle_persistence>`. If this
is not a major concern for you, then you can use the built-in :mod:`pickle`
module.

4. Did you try :mod:`pickle` or :mod:`joblib` and found that the model cannot
   be persisted? It can happen for instance when you have user defined
   functions in your model.

If yes, then you can use `cloudpickle`_ which can serialize certain objects
which cannot be serialized by :mod:`pickle` or :mod:`joblib`.


Workflow Overview
-----------------

In a typical workflow, the first step is to train the model using scikit-learn
and scikit-learn compatible libraries. Note that support for scikit-learn and
third party estimators varies across the different persistence methods.

Train and Persist the Model
...........................

Creating an appropriate model depends on your use-case. As an example, here we
train a :class:`sklearn.ensemble.HistGradientBoostingClassifier` on the iris
dataset::

  >>> from sklearn import ensemble
  >>> from sklearn import datasets
  >>> clf = ensemble.HistGradientBoostingClassifier()
  >>> X, y = datasets.load_iris(return_X_y=True)
  >>> clf.fit(X, y)
  HistGradientBoostingClassifier()

Once the model is trained, you can persist it using your desired method, and
then you can load the model in a separate environment and get predictions from
it given input data. Here there are two major paths depending on how you
persist and plan to serve the model:

- :ref:`ONNX <onnx_persistence>`: You need an `ONNX` runtime and an environment
  with appropriate dependencies installed to load the model and use the runtime
  to get predictions. This environment can be minimal and does not necessarily
  even require Python to be installed to load the model and compute
  predictions. Also note that `onnxruntime` typically requires much less RAM
  than Python to to compute predictions from small models.

- :mod:`skops.io`, :mod:`pickle`, :mod:`joblib`, `cloudpickle`_: You need a
  Python environment with the appropriate dependencies installed to load the
  model and get predictions from it. This environment should have the same
  **packages** and the same **versions** as the environment where the model was
  trained. Note that none of these methods support loading a model trained with
  a different version of scikit-learn, and possibly different versions of other
  dependencies such as `numpy` and `scipy`. Another concern would be running
  the persisted model on a different hardware, and in most cases you should be
  able to load your persisted model on a different hardware.


.. _onnx_persistence:

ONNX
----

`ONNX`, or `Open Neural Network Exchange <https://onnx.ai/>`__ format is best
suitable in use-cases where one needs to persist the model and then use the
persisted artifact to get predictions without the need to load the Python
object itself. It is also useful in cases where the serving environment needs
to be lean and minimal, since the `ONNX` runtime does not require `python`.

`ONNX` is a binary serialization of the model. It has been developed to improve
the usability of the interoperable representation of data models. It aims to
facilitate the conversion of the data models between different machine learning
frameworks, and to improve their portability on different computing
architectures. More details are available from the `ONNX tutorial
<https://onnx.ai/get-started.html>`__. To convert scikit-learn model to `ONNX`
`sklearn-onnx <http://onnx.ai/sklearn-onnx/>`__ has been developed. However,
not all scikit-learn models are supported, and it is limited to the core
scikit-learn and does not support most third party estimators. One can write a
custom converter for third party or custom estimators, but the documentation to
do that is sparse and it might be challenging to do so.

.. dropdown:: Using ONNX

  To convert the model to `ONNX` format, you need to give the converter some
  information about the input as well, about which you can read more `here
  <http://onnx.ai/sklearn-onnx/index.html>`__::

      from skl2onnx import to_onnx
      onx = to_onnx(clf, X[:1].astype(numpy.float32), target_opset=12)
      with open("filename.onnx", "wb") as f:
          f.write(onx.SerializeToString())

  You can load the model in Python and use the `ONNX` runtime to get
  predictions::

      from onnxruntime import InferenceSession
      with open("filename.onnx", "rb") as f:
          onx = f.read()
      sess = InferenceSession(onx, providers=["CPUExecutionProvider"])
      pred_ort = sess.run(None, {"X": X_test.astype(numpy.float32)})[0]

.. _skops_persistence:

`skops.io`
----------

:mod:`skops.io` avoids using :mod:`pickle` and only loads files which have types
and references to functions which are trusted either by default or by the user.
Therefore it provides a more secure format than :mod:`pickle`, :mod:`joblib`,
and `cloudpickle`_.


.. dropdown:: Using skops

  The API is very similar to :mod:`pickle`, and you can persist your models as
  explained in the `documentation
  <https://skops.readthedocs.io/en/stable/persistence.html>`__ using
  :func:`skops.io.dump` and :func:`skops.io.dumps`::

      import skops.io as sio
      obj = sio.dump(clf, "filename.skops")

  And you can load them back using :func:`skops.io.load` and
  :func:`skops.io.loads`. However, you need to specify the types which are
  trusted by you. You can get existing unknown types in a dumped object / file
  using :func:`skops.io.get_untrusted_types`, and after checking its contents,
  pass it to the load function::

      unknown_types = sio.get_untrusted_types(file="filename.skops")
      # investigate the contents of unknown_types, and only load if you trust
      # everything you see.
      clf = sio.load("filename.skops", trusted=unknown_types)

  Please report issues and feature requests related to this format on the `skops
  issue tracker <https://github.com/skops-dev/skops/issues>`__.


.. _pickle_persistence:

`pickle`, `joblib`, and `cloudpickle`
-------------------------------------

These three modules / packages, use the `pickle` protocol under the hood, but
come with slight variations:

- :mod:`pickle` is a module from the Python Standard Library. It can serialize
  and  deserialize any Python object, including custom Python classes and
  objects.
- :mod:`joblib` is more efficient than `pickle` when working with large machine
  learning models or large numpy arrays.
- `cloudpickle`_ can serialize certain objects which cannot be serialized by
  :mod:`pickle` or :mod:`joblib`, such as user defined functions and lambda
  functions. This can happen for instance, when using a
  :class:`~sklearn.preprocessing.FunctionTransformer` and using a custom
  function to transform the data.

.. dropdown:: Using `pickle`, `joblib`, or `cloudpickle`

  Depending on your use-case, you can choose one of these three methods to
  persist and load your scikit-learn model, and they all follow the same API::

      # Here you can replace pickle with joblib or cloudpickle
      from pickle import dump
      with open("filename.pkl", "wb") as f:
          dump(clf, f, protocol=5)

  Using `protocol=5` is recommended to reduce memory usage and make it faster to
  store and load any large NumPy array stored as a fitted attribute in the model.
  You can alternatively pass `protocol=pickle.HIGHEST_PROTOCOL` which is
  equivalent to `protocol=5` in Python 3.8 and later (at the time of writing).

  And later when needed, you can load the same object from the persisted file::

      # Here you can replace pickle with joblib or cloudpickle
      from pickle import load
      with open("filename.pkl", "rb") as f:
          clf = load(f)

.. _persistence_limitations:

Security & Maintainability Limitations
--------------------------------------

:mod:`pickle` (and :mod:`joblib` and :mod:`clouldpickle` by extension), has
many documented security vulnerabilities by design and should only be used if
the artifact, i.e. the pickle-file, is coming from a trusted and verified
source. You should never load a pickle file from an untrusted source, similarly
to how you should never execute code from an untrusted source.

Also note that arbitrary computations can be represented using the `ONNX`
format, and it is therefore recommended to serve models using `ONNX` in a
sandboxed environment to safeguard against computational and memory exploits.

Also note that there are no supported ways to load a model trained with a
different version of scikit-learn. While using :mod:`skops.io`, :mod:`joblib`,
:mod:`pickle`, or `cloudpickle`_, models saved using one version of
scikit-learn might load in other versions, however, this is entirely
unsupported and inadvisable. It should also be kept in mind that operations
performed on such data could give different and unexpected results, or even
crash your Python process.

In order to rebuild a similar model with future versions of scikit-learn,
additional metadata should be saved along the pickled model:

* The training data, e.g. a reference to an immutable snapshot
* The Python source code used to generate the model
* The versions of scikit-learn and its dependencies
* The cross validation score obtained on the training data

This should make it possible to check that the cross-validation score is in the
same range as before.

Aside for a few exceptions, persisted models should be portable across
operating systems and hardware architectures assuming the same versions of
dependencies and Python are used. If you encounter an estimator that is not
portable, please open an issue on GitHub. Persisted models are often deployed
in production using containers like Docker, in order to freeze the environment
and dependencies.

If you want to know more about these issues, please refer to these talks:

- `Adrin Jalali: Let's exploit pickle, and skops to the rescue! | PyData
  Amsterdam 2023 <https://www.youtube.com/watch?v=9w_H5OSTO9A>`__.
- `Alex Gaynor: Pickles are for Delis, not Software - PyCon 2014
  <https://pyvideo.org/video/2566/pickles-are-for-delis-not-software>`__.


.. _serving_environment:

Replicating the training environment in production
..................................................

If the versions of the dependencies used may differ from training to
production, it may result in unexpected behaviour and errors while using the
trained model. To prevent such situations it is recommended to use the same
dependencies and versions in both the training and production environment.
These transitive dependencies can be pinned with the help of package management
tools like `pip`, `mamba`, `conda`, `poetry`, `conda-lock`, `pixi`, etc.

It is not always possible to load an model trained with older versions of the
scikit-learn library and its dependencies in an updated software environment.
Instead, you might need to retrain the model with the new versions of the all
the libraries. So when training a model, it is important to record the training
recipe (e.g. a Python script) and training set information, and metadata about
all the dependencies to be able to automatically reconstruct the same training
environment for the updated software.

.. dropdown:: InconsistentVersionWarning

  When an estimator is loaded with a scikit-learn version that is inconsistent
  with the version the estimator was pickled with, a
  :class:`~sklearn.exceptions.InconsistentVersionWarning` is raised. This warning
  can be caught to obtain the original version the estimator was pickled with::

    from sklearn.exceptions import InconsistentVersionWarning
    warnings.simplefilter("error", InconsistentVersionWarning)

    try:
        with open("model_from_prevision_version.pickle", "rb") as f:
            est = pickle.load(f)
    except InconsistentVersionWarning as w:
        print(w.original_sklearn_version)


Serving the model artifact
..........................

The last step after training a scikit-learn model is serving the model.
Once the trained model is successfully loaded, it can be served to manage
different prediction requests. This can involve deploying the model as a
web service using containerization, or other model deployment strategies,
according to the specifications.


Summarizing the key points
--------------------------

Based on the different approaches for model persistence, the key points for
each approach can be summarized as follows:

* `ONNX`: It provides a uniform format for persisting any machine learning or
  deep learning model (other than scikit-learn) and is useful for model
  inference (predictions). It can however, result in compatibility issues with
  different frameworks.
* :mod:`skops.io`: Trained scikit-learn models can be easily shared and put
  into production using :mod:`skops.io`. It is more secure compared to
  alternate approaches based on :mod:`pickle` because it does not load
  arbitrary code unless explicitly asked for by the user. Such code needs to be
  packaged and importable in the target Python environment.
* :mod:`joblib`: Efficient memory mapping techniques make it faster when using
  the same persisted model in multiple Python processes when using
  `mmap_mode="r"`. It also gives easy shortcuts to compress and decompress the
  persisted object without the need for extra code. However, it may trigger the
  execution of malicious code when loading a model from an untrusted source as
  any other pickle-based persistence mechanism.
* :mod:`pickle`: It is native to Python and most Python objects can be
  serialized and deserialized using :mod:`pickle`, including custom Python
  classes and functions as long as they are defined in a package that can be
  imported in the target environment. While :mod:`pickle` can be used to easily
  save and load scikit-learn models, it may trigger the execution of malicious
  code while loading a model from an untrusted source. :mod:`pickle` can also
  be very efficient memorywise if the model was persisted with `protocol=5` but
  it does not support memory mapping.
* `cloudpickle`_: It has comparable loading efficiency as :mod:`pickle` and
  :mod:`joblib` (without memory mapping), but offers additional flexibility to
  serialize custom Python code such as lambda expressions and interactively
  defined functions and classes. It might be a last resort to persist pipelines
  with custom Python components such as a
  :class:`sklearn.preprocessing.FunctionTransformer` that wraps a function
  defined in the training script itself or more generally outside of any
  importable Python package. Note that `cloudpickle`_ offers no forward
  compatibility guarantees and you might need the same version of
  `cloudpickle`_ to load the persisted model along with the same version of all
  the libraries used to define the model. As the other pickle-based persistence
  mechanisms, it may trigger the execution of malicious code while loading
  a model from an untrusted source.

.. _cloudpickle: https://github.com/cloudpipe/cloudpickle
