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+# TFX Tuner Component
+
+| Status        | Proposed                                                  |
+| :------------ | :-------------------------------------------------------- |
+| **Author(s)** | Jiayi Zhao (jyzhao@google.com), Amy Wu (wuamy@google.com) |
+| **Sponsor**   | Zhitao Li (zhitaoli@google.com), Tom O'Malley (omalleyt@google.com), Matthieu Monsch (mtth@google.com), Makoto Uchida (muchida@google.com), Goutham Bhat (goutham@google.com) |
+| **Updated**   | 2020-04-20                                                |
+
+## Objective
+
+### Goal
+
+*   A new Tuner component in TFX for automated hyper-parameter tuning, which is
+    based on abstractions from
+    [KerasTuner library](https://github.com/keras-team/keras-tuner), in order to
+    reuse abstractions and algorithms from latter.
+
+### Non Goal
+
+*   Natively support multi-worker tuning by the system. As TFX doesn't have
+    ability to manage multi-worker clusters, running multiple trials in parallel
+    (parallel tuning) and running each trial in distributed env (distributed
+    training) are not supported natively. Parallel tuning may instead be
+    realized by a particular implementation of TFX Tuner (custom Executor),
+    e.g., in Google Cloud environment.
+*   Implementation of custom tuner for
+    [KerasTuner library](https://github.com/keras-team/keras-tuner) is out of
+    scope of this design discussion, e.g., a built-in EstimatorTuner support.
+    However, user project can still implement a tuner that inherits from
+    [`kerastuner.BaseTuner`](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/base_tuner.py)
+    and provide it to the proposed TFX Tuner component.
+
+## Background and Motivation
+
+A hyperparameter is a parameter whose value is used to control the learning
+process of a model or the model itself (e.g., layers and number of nodes). By
+contrast, the values of other parameters (typically node weights) are learned.
+
+Hyperparameter optimization is a critical part of many machine learning
+pipelines. Thus we propose a new TFX component, with the given search space
+which specifies the hyperparameter configuration (name, type, range etc.). TFX
+will optimize the hyperparameters based on the tuning algorithm.
+
+## User Benefit
+
+This document proposes a built-in TFX Tuner component, which works seamlessly
+with Trainer and other TFX components. As the Tuner component will utilize the
+[KerasTuner library](https://github.com/keras-team/keras-tuner), all supported
+tuning methods will be available to TFX, including custom implementation of
+KerasTuner.
+
+## Design Proposal
+
+TFX Tuner component will be built with the
+[KerasTuner library](https://github.com/keras-team/keras-tuner). In the
+following sections, we will first briefly go over the KerasTuner library and
+several concepts in hyperparameter optimization. Then we will focus on our Tuner
+component interface and how we utilize the KerasTuner library. After that, we
+will discuss parallel tuning and our plan on Google Cloud integration.
+
+### KerasTuner Library
+
+The following graph shows a typical workflow of hyperparameter tuning under the
+KerasTuner framework:
+
+<div style="text-align:center"><img src='20200420-tfx-tuner-component/workflow.png', width='600'></div>
+
+Given the user provided model which accepts a hyperparameter container, tuner
+can search optimization through trials created by the tuning algortihm. For each
+trial, values within search spaces will be assigned to hyperparameter
+containers, and the user model will be trained with these hyperparameter values
+and evaluated based on the objective provided to the tuner. The evaluation
+results will be reported back to tuner and the tuning algorithm will decide the
+hyperparameter values for the next trial. After reaching certain conditions,
+e.g., max trials, the tuner will stop iteration and return the optimal
+hyperparameters.
+
+KerasTuner library provides above tuning functionality, here are some
+abstractions in KerasTuner:
+
+*   [`HyperParameters`](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/hyperparameters.py):
+    Hyperparameter container for both search space, and current values.
+*   [`Oracle`](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/oracle.py):
+    Implementation of a hyperparameter tuning algorithm, e.g., random search,
+    including state management of the algorithm’s progress.
+*   [`Trial`](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/trial.py):
+    Provided by the Oracle, contains information about Hyperparameter values for
+    the current iteration.
+*   [`BaseTuner`](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/base_tuner.py):
+    a base tuner interface for above tuning workflow, responsible for the
+    iteration of trial execution:
+    *   Generates Trial using Oracle.
+    *   Trains user model with the HyperParameters in the current Trial.
+    *   Evaluates metrics and reports back to Oracle for next Trial.
+*   [`Tuner`](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/tuner.py):
+    An implementation of BaseTuner, for Keras model tuning.
+
+Note: Other than the Tuner, abstractions defined by `HyperParameters`, `Oracle`,
+`Trial` and `BaseTuner` are not restricted to Keras models, although the library
+is called KerasTuner.
+
+For more details and code examples, please refer to
+[here](https://github.com/keras-team/keras-tuner).
+
+### Component Interface
+
+Tuner component takes raw or transformed examples as input, along with schema or
+transform_graph for the feature specification, and outputs the hyperparameter
+tuning results, below shows the specification of Tuner component:
+
+```python
+class TunerSpec(ComponentSpec):
+  """ComponentSpec for TFX Tuner Component."""
+
+  PARAMETERS = {
+    # Specify a python module file which contains a UDF `tuner_fn`.
+    'module_file': ExecutionParameter(type=(str, Text), optional=True),
+    # Specify the steps for the training stage of each trial’s execution.
+    'train_args': ExecutionParameter(type=trainer_pb2.TrainArgs),
+    'eval_args': ExecutionParameter(type=trainer_pb2.EvalArgs),
+  }
+
+  INPUTS = {
+    'examples': ChannelParameter(type=standard_artifacts.Examples),
+    'schema': ChannelParameter(
+            type=standard_artifacts.Schema, optional=True),
+    'transform_graph':
+        ChannelParameter(
+            type=standard_artifacts.TransformGraph, optional=True),
+  }
+
+  OUTPUTS = {
+    'best_hyperparameters':
+        ChannelParameter(type=standard_artifacts.HyperParameters),
+  }
+```
+
+Trainer has an optional hyperparameters input; tuning result can be fed into it
+so that Trainer can utilize best hyperparameters to construct the model. Below
+shows an example about how tuner and trainer are chained in the pipeline:
+
+```python
+# TrainerSpec:
+  INPUTS = {
+    ...
+    'hyperparameters':
+        ChannelParameter(
+            type=standard_artifacts.HyperParameters, optional=True),
+  }
+
+# Pipeline DSL Example:
+  tuner = Tuner(
+      examples=example_gen.outputs['examples'],
+      schema=schema_gen.outputs['schema'],
+      module_file=module_file,
+      train_args=trainer_pb2.TrainArgs(num_steps=1000),
+      eval_args=trainer_pb2.EvalArgs(num_steps=500))
+
+  trainer = Trainer(
+      module_file=module_file,
+      examples=example_gen.outputs['examples'],
+      schema=schema_gen.outputs['schema'],
+      hyperparameters=tuner.outputs['best_hyperparameters'],
+      train_args=trainer_pb2.TrainArgs(num_steps=10000),
+      eval_args=trainer_pb2.EvalArgs(num_steps=5000))
+```
+
+For Trainer, users need to define model code and training logic
+([Generic Trainer](https://github.com/tensorflow/tfx/blob/r0.21.2/docs/guide/trainer.md#generic-trainer))
+in the module_file. For Tuner, in addition to model code, users also need to
+define hyperparameters, search space and a tuning algorithm in the module_file.
+A `tuner_fn` with the following signature is required for Tuner:
+
+```python
+from kerastuner.engine import base_tuner
+import tensorflow as tf
+from tfx.components.trainer.executor import TrainerFnArgs
+
+# Current TrainerFnArgs will be renamed to FnArgs as a util class.
+FnArgs = TrainerFnArgs
+TunerFnResult = NamedTuple('TunerFnResult',
+                          [('tuner', base_tuner.BaseTuner),
+                           ('fit_kwargs', Dict[Text, Any])])
+
+def tuner_fn(fn_args: FnArgs) -> TunerFnResult:
+  """Build the tuner using the KerasTuner API.
+
+  Args:
+    fn_args: Holds args as name/value pairs.
+      working_dir: working dir for tuning. Automatically set by Executor.
+      train_files: List of file paths containing training tf.Example data.
+      eval_files: List of file paths containing eval tf.Example data.
+      train_steps: number of train steps.
+      eval_steps: number of eval steps.
+      schema: optional schema file of the input data.
+      transform_graph: optional transform graph produced by TFT.
+
+  Returns:
+    A namedtuple contains the following:
+      - tuner: A BaseTuner that will be used for tuning.
+      - fit_kwargs: Args to pass to tuner’s run_trial function for fitting the
+                    model , e.g., the training and validation dataset. Required
+                    args depend on the above tuner’s implementation.
+  """
+```
+
+The TunerFnResult returned by the above tuner_fn contains an instance that
+implements the
+[`BaseTuner`](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/base_tuner.py)
+interface, that’s the contract required by Tuner for tuning. The model code,
+hyperparameters, search space and tuning algorithm are hidden under the
+BaseTuner abstraction so the Tuner itself is generic and agnostic to the model
+framework and tuning logic. Below shows an example module file with Keras model:
+
+```python
+import kerastuner
+import tensorflow as tf
+...
+
+def _input_fn(file_pattern: Text, ...) -> tf.data.Dataset:
+  ...
+
+# Model code for Trainer and Tuner.
+def _build_keras_model(hp: kerastuner.HyperParameters) -> tf.keras.Model:
+  ...
+  for _ in range(hp.get('num_layers')):
+    ...
+  ...
+  model = tf.keras.Model(...)
+  model.compile(
+     optimizer=tf.keras.optimizers.Adam(hp.get('learning_rate')),
+     loss='sparse_categorical_crossentropy',
+     metrics=[tf.keras.metrics.Accuracy()])
+  return model
+
+# This will be called by TFX Tuner.
+def tuner_fn(fn_args: FnArgs) -> TunerFnResult:
+  hp = kerastuner.HyperParameters()
+  # Defines search space.
+  hp.Choice('learning_rate', [1e-1, 1e-3])
+  hp.Int('num_layers', 1, 5)
+
+  # RandomSearch is a subclass of Keras model Tuner.
+  tuner = kerastuner.RandomSearch(
+      _build_keras_model,
+      max_trials=5,
+      hyperparameters=hp,
+      allow_new_entries=False,
+      objective='val_accuracy',
+      directory=fn_args.working_dir,
+      project_name='test')
+
+  train_dataset=_input_fn(fn_args.train_files, ...)
+  eval_dataset=_input_fn(fn_args.eval_files, ...)
+
+  return TunerFnResult(
+      tuner=tuner,
+      fit_kwargs={'x': train_dataset,
+                  'validation_data': eval_dataset,
+                  'steps_per_epoch': fn_args.train_steps,
+                  'validation_steps': fn_args.eval_steps})
+
+# This will be called by TFX Generic Trainer.
+def run_fn(fn_args: FnArgs) -> None:
+  hp = kerastuner.HyperParameters.from_config(fn_args.hyperparameters)
+  model = _build_keras_model(hp)
+  model.fit(...)
+  model.save(...)
+```
+
+In Tuner’s executor, `tuner_fn` will be called with information resolved from
+component inputs, then we call the `search` function of the returned tuner with
+`fit_kwargs` to launch trials for tuning, and finally emit the best trial’s
+hyperparameters:
+
+```python
+# Executor of Tuner Component:
+class Executor(base_executor.BaseExecutor):
+
+ def Do(self,
+        input_dict: Dict[Text, List[types.Artifact]],
+        output_dict: Dict[Text, List[types.Artifact]],
+        exec_properties: Dict[Text, Any]) -> None:
+  ...
+  tuner_spec = tuner_fn(self._create_fn_args(input_dict, exec_properties))
+  tuner_spec.tuner.search(**tuner_spec.fit_kwargs)
+  # Output file contains json format string of hyperparameters.get_config().
+  self._emit_best_hyperparameters(
+    output_dict, tuner_spec.tuner.get_best_hyperparameters()[0])
+```
+
+### Parallel Tuning
+
+In parallel tuning, multiple trials are executed in parallel. In this section,
+we will discuss how distribution works for KerasTuner library and the status of
+TFX.
+
+In the `search` function of tuner, trials will be run in sequence instead of in
+parallel. To support parallel tuning, we need to launch multiple tuners (the
+tuner here refers to the one in KerasTuner library, not TFX Tuner component),
+and have an optimization service for managing the state of the tuning algorithm,
+with which oracle of each tuner communicates, and retrieves the trials for each
+tuner.
+
+<div style="text-align:center"><img src='20200420-tfx-tuner-component/parallel_tuning.png', width='600'></div>
+
+The above graph shows a parallel tuning of three tuners. Each tuner runs as a
+different worker, and it retrieves trials from its own oracle, which talks to
+optimization service. Trials of different tuners can run in parallel but trials
+within the same tuner will still execute in sequence. When launching tuners, the
+same identifier will be assigned to each oracle, thus the optimization service
+knows they are in the same tuning job group and will assign hyperparameter
+values for their trials based on the algorithm.
+
+The number of parallel tuners can be passed to component by the `TuneArgs` as
+shown below:
+
+```python
+# Args specific to tuning.
+message TuneArgs {
+  # Number of trials to run in parallel.
+  # Each trial will be trained and evaluated by separate worker jobs.
+  int32 num_parallel_trials = 1;
+}
+
+class TunerSpec(ComponentSpec):
+
+  PARAMETERS = {
+    ...
+    'tune_args': ExecutionParameter(type=tuner_pb2.TuneArgs),
+  }
+```
+
+The KerasTuner library allows users to config
+[`tf.distribute.Strategy`](https://www.tensorflow.org/tutorials/distribute/kerass)
+if they are using
+[`kerastuner.Tuner`](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/tuner.py)
+class (or subclasses of it). In above parallel tuning, each trial (each model
+training) is executed in a single worker, as such only single machine strategy
+is allowed. To support multi-worker distributed training, we need to be able to
+execute the trial (training) on different workers.
+
+At the time of writing, KerasTuner library can be used for parallel tuning with
+single machine `tf.distribute.Strategy`, e.g.,
+[`MirroredStrategy`](https://www.tensorflow.org/api_docs/python/tf/distribute/MirroredStrategy)
+, multi-worker strategy (distributed training for trial) support is on the
+roadmap (note that cluster managing is not part of the library).
+
+At the time of writing, TFX doesn’t have the ability to manage the multi-worker
+cluster and the centralized optimization service, so parallel tuning or
+distributed training is not supported natively in TFX (local or on-prem), but in
+the next section, we will discuss the integration for Google Cloud. Similar
+parallel tuning support can be built for other execution environments.
+
+### Google Cloud Integration
+
+In this section, we discuss the Tuner component with
+[Google Cloud AI Platform](https://cloud.google.com/ai-platform) (CAIP),
+specifically, an implementation of KerasTuner Oracle that talks to the
+[AI Platform Optimizer](https://cloud.google.com/ai-platform/optimizer/docs/overview)
+as the centralized optimization service, and a custom Tuner executor
+implementation that makes use of the Cloud Optimizer-based Oracle (symbol names
+are subject to change).
+
+As mentioned above in the parallel tuning section, KerasTuner uses a centralized
+optimization service that manages states of a tuning study and trials. In
+addition to that, we will create a `CloudOracle` as a client to the AI Platform
+Optimizer service, and a `CloudTuner` which inherits from Keras
+[Tuner](https://github.com/keras-team/keras-tuner/blob/1.0.0/kerastuner/engine/tuner.py).
+In the module file, users create the `tuner_fn` with `CloudTuner`, and then
+users configure the TFX Tuner component to use the a custom Tuner executor
+(`CloudExecutor`), which launches multiple `CloudTuner`s on a Google Cloud AI
+Platform Training job with possibly multiple worker machines running various
+trials concurrently. Below shows the workflow for in process tuning and Cloud
+tuning.
+
+<div style="text-align:center"><img src='20200420-tfx-tuner-component/cloud.png', width='600'></div>
+
+## Future work
+
+*   Native support for multi-worker parallel tuning.
+*   Custom Tuner (inherits from BaseTuner) examples, e.g., for Estimator support
+    or Keras custom training loop support.
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