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Machine Learning Inference at Scale

March 10, 2020
Machine Learning Inference at Scale

Machine learning projects can be split into two phases:

  1. Training
  2. Inference

During the training phase, data science teams have to obtain, analyze and understand available data and generalize it into a mathematical model. The model uses the features of the sample data to reason about data it has never seen. Although it can be completely custom code, it is usually based on proven machine learning algorithms, such as Naïve Bayes, K Means, Linear Regression, Deep Learning, Random Forests or Decision Trees. The act of building the model from the sample (training) data is referred to as training.

The inference phase refers to using the model to predict an unknown property of the input data. This requires deploying the model into a production environment and operating it.

Operating Machine Learning

The most straightforward way to deploy the model is to wrap it in a REST web service and let other applications remotely invoke the inference service. Many machine learning frameworks provide such a service out-of-the-box to support simple deployments that don’t deal with much data.

What Hazelcast Jet adds to this story is a simple way to deploy the model so that it is automatically parallelized and scaled out across a cluster of machines.

Jet uses its parallel, distributed and resilient execution engine to turn the model into a high-performance inference service. To use all available CPU cores, Jet spins up multiple parallel instances of the model and spreads the inference requests among them. The Jet cluster is elastic; to scale with the workload, add or remove cluster members on the fly with no downtime.

Another trick is using a pipelined design instead of a request-reply pattern. It allows Jet to batch inference requests together and reduce fixed overheads of serving each request individually. This improves the overall throughput of the model significantly! The pipelined design requires a change in the client’s workflow. Instead of calling the inference service directly, it sends its inference request to an inbox. It may be implemented using a message broker such as JMS topic, Kafka or distributed topic of Hazelcast. Jet watches the inbox and groups multiple requests together to use the model service efficiently. It uses smart batching where the batch size changes with the data volume to keep the latency always low. The inference results are published to an outbox for callers to pick it up.

Models Supported

Python models

Python is the lingua franca of the data science world. There is a wide ecosystem of libraries and tools to build and train models in Python: TensorFlowKerasTheanoScikit-learn or PyTorch to name a few. Jet can host any Python model.

Upon model deployment, Jet’s JVM runtime launches Python processes and establishes bi-directional gRPC communication channels to stream inference requests through it. So, the model runs natively in a Python process that is completely managed by Jet. It can be tuned to spin multiple Python processes on each machine to make use of multicore processors.

Jet makes sure that the Python code is distributed to all machines that participate in the cluster. If you add another machine to a Jet cluster, it creates a directory on it and deploys the Python code there. Moreover, Jet can install all required Python libraries to prepare the runtime for your Python model.

Documentation: https://docs.hazelcast.org/docs/jet/latest/manual/#map-using-python

Code sample: https://github.com/hazelcast/hazelcast-jet/tree/master/examples/python

Java models

Java models are used for high-performance inference execution. Favourite Java model libraries include JPMMLTensorFlow for JavaMXNetXGBoost JVM Package and H2O.

Similarly to Python, the model is packaged as a Jet Job resource. The Job usually includes model inference code (the ML library) and a serialized model. Jet runs the Java models in-process with the cluster members so there is no need to start extra processes and there is no communication overhead (serialization, deserialization, networking). This makes Java model the best performing option. The inference job can be configured to use one model instance per JVM or multiple model instances.

Documentation: https://docs.hazelcast.org/docs/jet/latest/manual/#machine-learning-model-prediction

Code samples:

Remote services

We started this article by saying that using a model as an RPC service is simple but requires extra effort when scaling. Jet supports this pattern, too. The Jet Job can invoke a remote inference service. The model isn’t managed by Jet in this case, so the operational and performance advantages are gone. Jet still provides the convenience of smart batching, inbox/outbox connectors and many pipeline operators. Smart batching works only if the RPC service can operate on batches of input items.

Benefits of this setup

  • Isolating the model service and the data pipeline
  • Sharing the model among many Jet pipelines

Code samples:

Execution Mode Overview

Execution Mode
Java Model
Python Model
Remote Model
Model managed by Jet (tikot) (tikot) (chyba)
Model shared between Jobs (chyba) (chyba) (tikot)
Jet ↔ Model Communication Shared memory gRPC
(processes colocated)
Usually gRPC or REST
(processes on different machines)
Throughput (single node) 1M / sec 50k / sec Depends
Requirements Model runs in JVM Python runtime installed on all cluster machines Model service started prior to the Jet Job

Model available as a RPC service

Framework Integration Overview

Framework
Execution Mode
Code Sample
H2O Java Code Sample
TensorFlow for Java Java Code Sample
Custom Java Model Java Code Sample
PMML Java N/A, use JPMML Evaluator as a Custom Java Model
MXNet Java N/A, use MXNet Java Inference API as a Custom Java Model
XGBoost Java N/A, use XGBoost JVM Package as a Custom Java Model
KerasTheanoScikit-learn or PyTorch Python N/A, use the Custom Python Model
Custom Python Model Python Code Sample
Remote gRPC service Remote Code Sample
Remote TensorFlow service Remote Code Sample
About the Author

About the Author

Vladimir Schreiner

Vladimir Schreiner

Vladimir is a product manager with an engineering background and deep expertise in stream processing and real-time data pipelines. Ten years of building internal software platforms and development infrastructure have made him passionate about new technologies and finding ways to simplify data processing. Vladimir co-authored two white papers on the topic: Understanding Stream Processing: Fast Processing of Infinite and Big Data, and A Reference Guide to Stream Processing. His tutorial video on stream processing and real-time data pipelines discusses the building blocks of a stream processing pipeline and demonstrates how developers can write a full-blown streaming pipeline in less than a hundred lines of Java code for a variety of applications. Vladimir is also a lecturer with the Czechitas Foundation, whose mission is to inspire women and girls to explore the world of information technology. Czechitas Foundation teaches coding in various programming languages, software testing, and data analysis.

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