What is Machine Learning?

Machine learning is a subset of artificial intelligence that trains algorithms to learn from data, identify patterns and make predictions without being explicitly programmed. The algorithms iteratively learn from data to improve, gaining more accuracy over time. Machine learning is used for applications like fraud detection, image recognition, product recommendations, and more.

What is TensorFlow?

TensorFlow is an end-to-end open-source platform for machine learning developed by Google. It is one of the most popular frameworks for working with neural networks and other deep learning models. TensorFlow provides tools to build, train, and deploy ML models at scale.

Why Use TensorFlow for Machine Learning?

TensorFlow has become a popular choice for machine learning because of its flexibility, scalability, and wide range of tools and libraries. Key advantages of TensorFlow include:

• Expressive ecosystem for quickly building and training ML models
• State-of-the-art tools for computer vision, NLP, time series, and more
• Interoperability with frameworks like Keras and PyTorch
• Scales seamlessly to train models on CPU, GPU, or TPUs
• Robust production deployment options for serving predictions
• Supported by Google with a large open-source community

Overall, TensorFlow provides a unified framework to take models from concept to production deployment.

Getting Started with TensorFlow

Installing TensorFlow

The first step is to install TensorFlow. It supports Python, JavaScript, C++, Java, Go, Swift and more. For Python, use pip:

pip install tensorflow

This installs CPU support. For GPU support, install tensorflow-gpu instead. Verify the install:

import tensorflow as tf
print(tf.__version__)

For other languages, see TensorFlow install guides.

TensorFlow Architecture

Machine Learning TensorFlow uses a deferred execution model, meaning it first builds a graph representing the ML operations to perform. Then in a session, it runs the graph using optimized C++ code. This architecture offers flexibility and performance benefits.

Writing TensorFlow Code

Let’s look at a simple example for adding two constants:

import tensorflow as tf

# Define graph
a = tf.constant(5)
b = tf.constant(3)
c = tf.add(a, b)

# Create session to run graph
sess = tf.Session()

# Run op and fetch value 
print(sess.run(c))
# 8

This demonstrates the TensorFlow workflow:

1. Define operations in the graph, like tf.constant() and tf.add()
2. Initialize a tf.Session() to run the graph
3. Execute ops like c using sess.run() to fetch values

Now let’s look at how to work with tensors for data.

Core TensorFlow Concepts

Machine Learning Understanding key concepts like tensors, variables, and graphs unlocks TensorFlow’s full potential.

Tensors

Tensors are multi-dimensional arrays used to represent all data in TensorFlow. An operation takes tensors as input and produces tensors as output.

For example, two 2×3 tensor inputs:

import tensorflow as tf

tensor_one = tf.constant([[1, 2, 3], [4, 5, 6]]) 
tensor_two = tf.constant([[7, 8, 9], [10, 11, 12]])

TensorFlow supports different data types for tensors like float32, int32, string and more.

Variables

Variables allow us to add trainable parameters to a graph. They are initialized and can be modified during model training, like weights and biases. For example:

W = tf.Variable(tf.random.normal(shape=(2,3)))
b = tf.Variable(tf.random.normal(shape=(3,)))

The variables W and b can be optimized during model training.

Graphs

The TensorFlow graph contains a set of nodes representing ops and edges representing tensors flowing between them. All computation is represented by this graph.

We can visualize the computational graph using TensorBoard. This helps debug and optimize models.

Sessions

A session encapsulates the control and state of the TensorFlow runtime. When executing a graph, we need to create a tf.Session and call its run() method, passing in ops we want to compute.

For example:

sess = tf.Session()

c = tf.add(a, b)
print(sess.run(c))

This constructs the graph behind the scenes before running it.

Building Machine Learning Models with TensorFlow

TensorFlow provides high-level APIs like Keras and Estimators that make building ML models easy. Let’s walk through examples.

Linear Regression

Machine Learning Linear regression predicts a target variable by fitting a line to input data. We’ll use the Keras Sequential API:

import tensorflow as tf
from tensorflow import keras

model = keras.Sequential()
model.add(keras.layers.Dense(1, input_shape=(2,)))
model.compile(optimizer='SGD', loss='mse')

This defines a model with a single dense layer with 2 input units and 1 output unit. We compile it with the SGD optimizer and mean squared error loss.

To train:

import numpy as np

xs = np.array([1, 2, 3, 4, 5])
ys = np.array([1, 2, 3, 4, 5])

model.fit(xs, ys, epochs=500)

We can now make predictions on new data:

model.predict([6])
# array([5.982466], dtype=float32)

Logistic Regression

Machine Learning Logistic regression predicts categorical outcomes like spam/not-spam from input data. We instantiate a model:

model = keras.Sequential([
  keras.layers.Dense(1, input_shape=(4,), activation='sigmoid')  
])

model.compile(optimizer='rmsprop', loss='binary_crossentropy')

The sigmoid activation squashes outputs between 0-1 representing probabilities. Binary cross entropy is a common loss for binary classification.

We can train on labeled spam/not-spam email data:

import numpy as np

X = np.random.random((1000, 4))
y = np.random.randint(2, size=(1000, 1)) 

model.fit(X, y, epochs=5)

Now we can classify new emails:

model.predict(X_new)
# array([[0.2], [0.9]])

Neural Networks

Neural networks consist of layers of connected nodes inspired by biological neurons. Here’s an example 3 layer network:

from tensorflow import keras
from tensorflow.keras import layers

model = keras.Sequential()

model.add(layers.Dense(32, input_shape=(4,)))  
model.add(layers.Dense(16, activation='relu'))
model.add(layers.Dense(1, activation='sigmoid'))

This stacks dense layers sequentially, with ReLU activation on the hidden layer.

We compile and train it on data:

model.compile(optimizer='adam', loss='binary_crossentropy')
model.fit(X_train, y_train, epochs=5)

Deep neural networks have revolutionized fields like computer vision and NLP.

Convolutional Neural Networks

Convolutional neural networks (CNNs) are ideal for processing pixel data in images. They utilize convolutional layers that filter inputs for Machine Learning features.

Here’s a simple CNN for MNIST digit classification:

model = keras.Sequential()

model.add(layers.Conv2D(32, (3, 3), activation='relu', input_shape=(28, 28, 1)))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Flatten())  
model.add(layers.Dense(10, activation='softmax'))

It stacks convolutional, pooling, and dense layers to classify the handwritten digits 0-9.

We’d train it on labeled image data:

(X_train, y_train), (X_test, y_test) = keras.datasets.mnist.load_data()

model.fit(X_train, y_train, epochs=5)

Machine Learning CNNs have sparked breakthroughs in image recognition, video processing, and more.

Training Models in TensorFlow

Now Machine Learning that we can build models, let’s discuss how to train them effectively.

Feeding Data

We supply training data to TensorFlow using input functions that generate batches of data.

For example, to load MNIST data:

def load_data():
  (X_train, y_train), (X_test, y_test) = keras.datasets.mnist.load_data()
  return X_train, y_train

train_data = load_data()
train_batches = tf.data.Dataset.from_tensor_slices(train_data).shuffle(500).batch(32)

This returns a dataset we can iterate over in batches of 32 images and labels.

Optimizers

Optimizers like GradientDescentOptimizer, AdagradOptimizer, and AdamOptimizer in TensorFlow update model weights during training to minimize loss.

For example:

optimizer = tf.train.AdamOptimizer(learning_rate=0.001)

This initializes the Adam optimizer that adapts the learning rate.

Loss Functions

Loss functions like softmax_cross_entropy and mean_squared_error measure how far model predictions are from ground truth labels.

We use losses to optimize weights:

cross_entropy = tf.losses.softmax_cross_entropy(labels, logits)
train_op = optimizer.minimize(cross_entropy)

This minimizes cross-entropy loss to train a classification model.

Metrics and Evaluation

Metrics like accuracy tell us how well a model is performing during training and evaluation.

For example:

accuracy, update_accuracy = tf.metrics.accuracy(labels, predictions)
print('Validation accuracy:', update_accuracy.eval({...}))

This prints the accuracy on validation data. Monitoring metrics helps identify underfitting and overfitting.

Deploying TensorFlow Models

Once models are trained, TensorFlow provides options to deploy them for inference.

Serving Predictions

The TensorFlow Serving library allows serving predictions from trained models via REST and gRPC APIs. For example:

docker run -p 8501:8501 --name tfserving --mount type=bind,source=/models/myapp,target=/models/myapp -e MODEL_NAME=myapp -t tensorflow/serving

This exposes a REST endpoint to accept requests like:

curl http://localhost:8501/v1/models/myapp:predict -d '{"instances": [1.2, 2.4, 5.7]}'

And returns predictions from the served model.

Exporting Models

We can export trained Keras models to various formats for deployment:

model.save('my_model.h5') # Keras H5 format

tf.saved_model.save(model, 'my_model/') # SavedModel format

model.save('my_model.tflite') # TensorFlow Lite format

The SavedModel format is ideal for TensorFlow Serving.

TensorFlow Lite

TensorFlow Lite provides optimized models for mobile and embedded devices. We can convert Keras models:

converter = tf.lite.TFLiteConverter.from_keras_model(model)
tflite_model = converter.convert()
open('model.tflite', 'wb').write(tflite_model)

The TensorFlow Lite model can run on mobile devices using the TensorFlow Lite interpreter.

TensorFlow.js

TensorFlow.js allows deploying models to web browsers. For example:

<script src="https://cdn.jsdelivr.net/npm/@tensorflow/tfjs"></script>

<script>
  const model = tf.loadLayersModel('tfjs_model/model.json');

  model.predict(tensor).print(); 
</script>

This loads a pre-trained TensorFlow.js model to make predictions in the browser.

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Advanced Topics

Let’s discuss some advanced capabilities of TensorFlow.

Distributed Training

TensorFlow supports distributed training across multiple GPUs/TPUs using tf.distribute.Strategy APIs:

strategy = tf.distribute.MirroredStrategy()

with strategy.scope():
  model = build_and_compile_cnn_model()
  
  train_dataset, validation_dataset = get_datasets()  
  
  model.fit(train_dataset, epochs=10, validation_data=validation_dataset)

This wraps model building, compiling, and training in a distributed strategy for easy scaling.

TPUs

TensorFlow provides native acceleration on TPU hardware through Keras and Estimators:

resolver = tf.distribute.cluster_resolver.TPUClusterResolver(tpu='grpc://' + TPU_IP)
tf.config.experimental_connect_to_cluster(resolver)
tf.tpu.experimental.initialize_tpu_system(resolver)

strategy = tf.distribute.experimental.TPUStrategy(resolver)

with strategy.scope():
  model = build_and_compile_cnn_model()
  model.fit(...)

This connects to a TPU, instantiates a training strategy, and trains the model on TPUs.

Eager Execution

Eager execution evaluates ops immediately without building graphs. This enables the interactive use of Machine Learning TensorFlow:

tf.executing_eagerly() # Enables eager 

a = tf.constant(5)
b = tf.constant(3)

print(a + b) # Runs op and prints result

Eager execution makes debugging and prototyping models faster and easier. It’s enabled by default in TensorFlow 2.0.

TensorFlow 2.0

TensorFlow 2.0 provides major API updates like eager execution by default, Keras integration, tf.data for input pipelines, and tf.function for graph performance.

For example:

import tensorflow as tf

model = tf.keras.Sequential([...]) # tf.keras models

@tf.function
def train_step(X, y): # Graph performance
  with tf.GradientTape() as tape:
    predictions = model(X)
    loss = loss_fn(y, predictions)
  grads = tape.gradient(loss, model.trainable_variables)    
  optimizer.apply_gradients(zip(grads, model.trainable_variables))
  
for X, y in dataset:
  train_step(X, y) # Calls with eager+graph

TensorFlow 2.0 unifies the TensorFlow API for simplicity and ease of use.

Conclusion

TensorFlow provides a powerful platform for every step of the machine learning workflow – from flexible model building with Keras and Estimators, and distributed training with TPUs, to robust deployment for serving predictions. With continued innovations by Google, TensorFlow will keep growing as a ubiquitous ML platform.

Next Steps

To start building models with TensorFlow:

• Install TensorFlow and work through examples like linear regression
• Learn Keras APIs for defining models simply
• Understand core TensorFlow runtime concepts like graphs, sessions, and tensors
• Explore pretrained models like ResNet and BERT to tackle computer vision and NLP