SLEP023: Callback API

index.rst

@@ -25,6 +25,7 @@
     slep012/proposal
     slep017/proposal
     slep019/proposal
+    slep023/proposal
 
 .. toctree::
     :maxdepth: 1

slep023/proposal.rst

@@ -0,0 +1,144 @@
+.. _slep_023:
+
+=====================
+SLEP023: Callback API
+=====================
+
+:Author: Jérémie du Boisberranger
+:Status: Draft
+:Type: Standards Track
+:Created: 2024-03-01
+
+Abstract
+--------
+
+This SLEP proposes an API to allow users to register callbacks to be called at
+specific points during the training of scikit-learn estimators.
+
+Motivation
+----------
+
+The current scikit-learn API provides a very limited way for users to inspect the steps
+of the training process (`fit`) of an estimator, even less a composition of several
+estimators and meta-estimators. Setting a verbosity level provides some information and
+only a few estimators expose a few public attributes containing some information
+accumulated during training (e.g. `HistGradientBoostingClassifier.train_score_`), and
+that's pretty much it.
+
+Some use cases that have been requested many times on the scikit-learn issue tracker
+which are not possible to achieve with the current API are for instance Progress bars,
+Structured logging, Monitoring, Snapshots, etc. It would also provide a generic and
+consistent API for early stopping, which is currently implemented differently in only a
+few estimators. For such widely requested features, scikit-learn would provide builtin
+callbacks, but users would also be able to implement their own callbacks for less common
+use cases.
+
+By providing a way to gather information during the training process, effectivley making
+scikit-learn implementation of machine learning algorithms more transparent, the
+callback API would also bring a lot of value for educational, testing and debugging
+purposes.
+
+Detailed description
+--------------------
+
+This section discusses the main challenges and the proposed solutions for the callback
+API to be generic and flexible enough despite the wide diversity of scikit-learn
+estimators and meta-estimators.
+
+Callbacks and hooks
+~~~~~~~~~~~~~~~~~~~
+
+This SLEP proposes to define a callback as an object containing methods called at
+specific points of `fit` (hooks). To keep the scope of this SLEP reasonable, only hooks
+called during `fit` are discussed here, but the API could be extended to other methods
+in the future.
+
+One hook is called at the beginning of `fit` (`on_fit_begin`) and one at the
+end (`on_fit_end`) to allow callback initialization and cleanup. A third hook is called
+at the end of each iteration (`on_fit_iter_end`), where an iteration is roughly defined
+as any loop step from the outermost loop in the `fit` method down to a loop over the
+full dataset or batch for online algorithms (e.g. `MiniBatchKMeans`).
+
+Due to the diversity of the estimators in scikit-learn, it's not realistic to define
+specific hooks for each of these kind of iteration, which would end up with estimator
+specific hooks, defeating the purpose of a generic API.
+
+Computation tree
+~~~~~~~~~~~~~~~~
+
+The computation steps of an estimator can be seen as a tree, where the root is the
+beginning of `fit` and nested loops are the different depth levels, each loop step
+being a node. The leaves are the steps of the innermost loop over the full dataset.
+Based on the above section, the `on_fit_iter_end` hook is thus called at each node
+of this tree.
+
+Callbacks need to act differently depending on the depth they are called at. This SLEP
+thus proposes that every estimator hold its computation tree as an attribute, such that
+callbacks can know at which node they are called and act accordingly. Because the tree
+depends on the value of the estimator's hyperparameters, this tree structure is created
+dinamically at the beginning of `fit`. When estimators are nested, the trees are merged,
+the root of the inner estimator being a leaf of the outer estimator.
+
+Performance
+~~~~~~~~~~~
+
+It's inevitable that callbacks will have a performance cost, especially when called
+within cython nogil code. The most important thing is to make sure is that when no
+callbacks are registered the performance is not affected (not acquiring the GIL for
+instance).
+
+In addition, some information given to the `on_fit_iter_end` hook might be expensive to
+compute but is not needed by all callbacks. This SLEP proposes that callbacks must
+explicitly request the information which is expensive to compute, and that the estimator
+gives this information to the hook in a lazy way.
+
+Callback propagation
+~~~~~~~~~~~~~~~~~~~~
+
+When composing estimators and meta-estimators, some callbacks are intuitively expected
+to be applied to all of them. For instance progress bars should be able to report the
+progress of all parts of such compositions. This SLEP proposes that some callbacks have
+the property of being automatically propagated to inner estimators, such that they only
+need to be registered on the outermost estimator.
+
+Parallelism
+~~~~~~~~~~~
+
+Many scikit-learn estimators use multiprocessing or multithreading which can make the
+design of callbacks more complex, because callbacks don't share their state between
+processes. The file system or `multiprocessing.Manager` objects  for instance should be
+used to overcome this issue.
+
+Implementation
+--------------
+
+An implementation of this SLEP is being developed in :pr:`27663`, which targets the
+`callbacks` feature branch. In this PR, only the base infrastructure and one builtin
+callback (progress bars) are implemented. Other callbacks and documention will be
+added incrementally in follow-up PRs.
+
+Discussion
+----------
+
+The goal of this SLEP is to provide a common solution for a wide range of use cases
+discussed in many long-standing issues and PRs, going back to :issue:`78`. Here are
+some related issues and PRs: :issue:`4863`, :issue:`7574`, :issue:`8433`,
+:issue:`8994`, :issue:`9136`, :issue:`10489`, :issue:`10973`, :issue:`12325`,
+:issue:`14338`, :issue:`14531`, :issue:`16118`, :issue:`18507`, :issue:`18748`,
+:issue:`18773`, :issue:`20127`, :issue:`20668`, :issue:`23156`, :issue:`24524`,
+:issue:`25187`, :issue:`26395`, :issue:`26494`, :issue:`26532`, :pr:`1171`, :pr:`3817`,
+:pr:` 8317`, :pr:`16925`, :pr:`22000`.
+
+References and Footnotes
+------------------------
+
+.. [1] Each SLEP must either be explicitly labeled as placed in the public
+   domain (see this SLEP as an example) or licensed under the `Open Publication
+   License`_.
+
+.. _Open Publication License: https://www.opencontent.org/openpub/
+
+Copyright
+---------
+
+This document has been placed in the public domain. [1]_