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An ONNX (Open Neural Network eXchange) API, Code Generator and Backend for Typeful, Numerically Generic, Functional Deep Learning in Scala
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Latest commit b896357 Feb 16, 2020

Build status Latest version

Getting Started

Add this to the build.sbt in your project:

libraryDependencies += "com.github.EmergentOrder" %% "onnx-scala-backends" % "0.2.0"

As of v0.1.0, artifacts are published to Sonatype OSS / Maven Central. For the latest, build and publish locally from master.

Full ONNX model inference quick start

First, download the model file for SqueezeNet.

Using the console, from this project root:

project backendsJVM

or from your project:

sbt console

Run SqueezeNet image classification inference on an "image" composed entirely of pixel value 42:

import java.nio.file.{Files, Paths}
import org.emergentorder.onnx.{Tensor, TensorFactory}
import org.emergentorder.onnx.backends.NGraphOperatorBackendFull
import org.emergentorder.onnx.backends.NGraphModelBackend

val squeezenetBytes = Files.readAllBytes(Paths.get("squeezenet1.1.onnx"))

val squeezenet = new NGraphModelBackend(squeezenetBytes)

val imageTens = TensorFactory.getTensor(Array.fill(1*3*224*224){42f},Array(1,3,224,224))

Note that ONNX Tensor content is in row-major order.

val out: Tensor[Float] = squeezenet.fullModel((Some(imageTens), None, None, None, None, None, None, None, None))
// out: (Array[Float], Array[Int], org.emergentorder.onnx.package.Axes) = (
//   Array(
//     0.8230884F,
//     2.3508213F,
//     5.1132765F,
//     4.885488F,
//     5.4786053F,
// ...

// res0: Array[Int] = Array(1, 1000)

// res1: Int = 418

Referring to the ImageNet 1000 class labels, we see that the predicted class is "ballpoint pen".

In terms of performance, based on some simple benchmarks (see below) comparing it against ONNX Runtime, SqueezeNet inference in ONNX-Scala is at least on par, and up to 10x faster(!) depending on the setting.

Operator-level (Fine-grained) API and generated programs

You can call individual operators:

val onnx = new NGraphOperatorBackendFull()
// onnx: NGraphOperatorBackendFull = org.emergentorder.onnx.backends.NGraphOperatorBackendFull@513b862a

onnx.Sqrt6("sqrt", Some(imageTens))
// res2: (Array[Float], Array[Int], org.emergentorder.onnx.package.Axes) = (
//   Array(
//     6.4807405F,
//     6.4807405F,
// ...

val longTens = TensorFactory.getTensor(Array.fill(1*3*224*224){-42l},Array(1,3,224,224))
// longTens: (Array[Long], Array[Int], org.emergentorder.onnx.package.Axes) = (
//   Array(
//     -42L,
//     -42L,
// ...

onnx.Abs6("abs", Some(longTens))
// res3: (Array[Long], Array[Int], org.emergentorder.onnx.package.Axes) = (
//   Array(
//     42L,
//     42L,
// ...

Sqrt will fail to compile because it's not defined for Long:

onnx.Sqrt6("sqrt", Some(longTens))
// error: Cannot prove that org.emergentorder.onnx.!![Long] <:< org.emergentorder.onnx.Float16 => Nothing with Float => Nothing with Double => Nothing with org.emergentorder.onnx.UNil => Nothing => Nothing.
// onnx.Sqrt6("sqrt", Some(longTens))
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

And similarly you can call generated programs composed of these operators (details on how to generate from onnx file follow):

import org.emergentorder.onnx.Squeezenet1dot1
val generatedSqueezenet = new Squeezenet1dot1(squeezenetBytes)
// generatedSqueezenet: Squeezenet1dot1 = org.emergentorder.onnx.Squeezenet1dot1@259ad91f
val result = generatedSqueezenet.program(imageTens)
// result: List[(Array[Float], Array[Int], org.emergentorder.onnx.package.Axes)] = List(
//   (
//     Array(
//       0.8230884F,
//       2.3508213F,
//       5.1132765F,
//       4.885488F,
//       5.4786053F,
// ...

// res5: Array[Int] = Array(1, 1000, 1, 1)

// res6: Int = 418

And you can freely combine the two:

onnx.Softmax1("softmax", None, Some(result(0))) 
// res7: (Array[Float], Array[Int], org.emergentorder.onnx.package.Axes) = (
//   Array(
//     2.251989E-9F,
//     1.0376532E-8F,
//     1.6435071E-7F,
//     1.3087133E-7F,
//     2.3682735E-7F,
// ...

Note the type-safety (the full model version shown above fails at runtime):

// error: type mismatch;
//  found   : org.emergentorder.onnx.Tensor[Long]
//     (which expands to)  (Array[Long], Array[Int], org.emergentorder.onnx.Axes)
//  required: org.emergentorder.onnx.Tensor[Float]
//     (which expands to)  (Array[Float], Array[Int], org.emergentorder.onnx.Axes)
// generatedSqueezenet.program(longTens)
//                             ^^^^^^^^

Take note however, the generated version runs ~10x slower on this example and up to ~100x slower averaged over many iterations.

Also note that in real use backends should be closed to prevent native memory leaks.

Project Overview


A complete, versioned, numerically generic, type-safe / typeful API to ONNX(Open Neural Network eXchange, an open format to represent deep learning and classical machine learning models), derived from the Protobuf definitions and the operator schemas (defined in C++) via the JavaCPP Preset for ONNX. We also generate implementations for each operator in terms of core methods to be implemented by the backend.

This API is expressed via traits, with version-named methods. For example, Abs, the absolute value operator (defined here for operator set 6):

import scala.{specialized => sp}
import spire.math.UByte
import spire.math.UShort
import spire.math.UInt
import spire.math.ULong
import spire.math.Numeric
import spire.implicits._
import scala.reflect.ClassTag
import org.emergentorder.onnx._

trait Abs extends Operator {
  def Abs6[@sp T: Numeric: ClassTag](name: String, X: Option[Tensor[T]])(
        implicit evT: Contains[
    ): (Tensor[T]) //Implementation omitted

A few more examples of the type constraints in action:

val stringTens = TensorFactory.getTensor(Array.fill(3*224*224){"test"},Array(3,224,224))
onnx.Abs6("abs", Some(stringTens))
// error: could not find implicit value for evidence parameter of type spire.math.Numeric[String]
// onnx.Abs6("abs", Some(stringTens))
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
val aBigInt = new BigInt(new java.math.BigInteger("5"))
val bigIntTens = TensorFactory.getTensor(Array.fill(3*224*224){aBigInt},Array(3,224,224))
onnx.Abs6("abs", Some(bigIntTens))
// error: Cannot prove that org.emergentorder.onnx.!![scala.math.BigInt] <:< org.emergentorder.onnx.Float16 => Nothing with Float => Nothing with Double => Nothing with spire.math.UByte => Nothing with spire.math.UShort => Nothing with spire.math.UInt => Nothing with spire.math.ULong => Nothing with Byte => Nothing with Short => Nothing with Int => Nothing with Long => Nothing with org.emergentorder.onnx.UNil => Nothing => Nothing.
// onnx.Abs6("abs", Some(bigIntTens))
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

B) Program Generator

Capable of translating ONNX model Protobuf (.onnx) files into Scala programs written in terms of this API.
For example, an "absolute value network":

Depending on the size of the ONNX model, you may need to add

export SBT_OPTS="-XX:+CMSClassUnloadingEnabled -Xmx16G -Xss8M -XX:MaxMetaspaceSize=1024M"

to your ~/.bashrc file or equivalent, or you will encounter errors.

Move the model file to programGenerator/.jvm/src/main/resources/absnet.onnx). More models can be found in the ONNX Model Zoo.

sbt "project programGeneratorJVM" "run absnet.onnx"

The resulting generated program appears as programGenerator/src/gen/scala/Absnet.scala:

import org.emergentorder.onnx.backends._

class Absnet(byteArray: Array[Byte]) {
  val Abs: org.emergentorder.onnx.Abs = new NGraphOperatorBackendFull()
  def program(inputDatax: Tensor[Float]): List[Tensor[Float]]  = 
    for {
      nodex <- List(inputDatax)
      nodey <- List(Abs.Abs6("y" ,X = Some(nodex)))
    } yield (nodey)

and you can run sbt compile to confirm that the generated code compiles.

C) Backend

Currently. at the operator level, a single partial backend implementation of ONNX, accessible from the JVM, is available.

This backend is based on nGraph, via nGraph JavaCPP Preset.

Supported ONNX input and output tensor data types:

  • Byte
  • Short
  • Int
  • Long
  • Float
  • Double

Supported ONNX ops:

  • All those supported by nGraph, currently 100 of 153 total. The rest are in the API, but will error if called.

ONNX Runtime, which supports all ONNX ops, is the next targeted backend.

You can also pass entire models to nGraph (see Execution Modes below).

All together, these should enable model inspection and modification, extra compile-time assurances, mixing/matching of backend operator implementations and integration into JVM-based production systems, for a start.


A few simple benchmarks, run on my laptop, using full model inference:

Over 1000 iterations of cold start (using a new backend/session each time) SqueezeNet inference, ONNX-Scala took ~0.04696 seconds on average vs ~0.05948 seconds for ONNX Runtime 1.0 (Python API), > 20% faster. If we reuse the backend/session, the gap is larger still.

Over 10000 sequential runs with the same backend/session, it took ~0.0068 seconds on average, vs ~ 0.0701 seconds for ONNX Runtime, over 10x faster.

Finally, after modifying the model to use fixed batch size of 1000 (and learning along the way that nGraph does not currently support dynamic batch sizes!), it took ~0.0077 seconds on average vs ~0.0321 seconds for ONNX Runtime, over 4x faster.

The resulting output values also match ONNX Runtime.

Example execution

The most extensive working example at the moment is zio/src/main/scala/NCFZIO.scala, an implementation of Neural Collaborative Filtering, although you currently need to provide your own model file to load params from at zio/.jvm/src/main/resources/NCF.onnx, as well item and user id maps at zio/.jvm/src/main/resources/itemIds.csv and zio/.jvm/src/main/resources/userIds.csv.

This example provides full model execution via the fullNCF method.

To run it, use:

sbt "project zioJVM" "run"`

Build / Publish

You'll need sbt.

To build and publish locally:

sbt publishLocal


sbt +publishLocal

to build against all of Scala 2.11, 2.12, 2.13 and Dotty/3.0, where possible.

Program Execution

There are 3 execution modes:

Full model / Black-box execution

In this mode we load the provided ONNX model file and pass it as-is to the underlying ONNX backend. The model is represented as a Scala function with appropriately typed inputs and outputs. Although we also generate the ONNX-Scala version of the program for type-checking and readability reasons, in the live code path the model internals are invisible to Scala. This is the most performant execution mode, and is recommended in cases where static, pre-defined graphs are sufficient.

Fine-grained mode

In this mode, each ONNX operation is executed on the underyling backend individually. As a result, you can write your own models from scratch in Scala using ONNX-Scala operations, injecting parameters from outside sources as need be. You can also generate an ONNX-Scala program from an ONNX model file and then customize it as you see fit.

Type-checking is done at the operation level on inputs and outputs for data types, with support for type-checking over axis labels and tensor shapes coming. This allows for dynamic graph structure, in which the execution itself defines the graph, similar to PyTorch and Tensorflow Eager. The trade-off made for this flexibility is that the underlying ONNX backend can no longer optimize the full graph, and the boundary-crossing at each operation results in additional overhead.

Project Details

Automatic differentiation to enable training is under consideration (ONNX does not provide facilities for training).

Balancing the interests of minimal imposition of dependencies with purely functional programming, ONNX-Scala comes in two flavors: Vanilla and ZIO-infused.

The ONNX-Scala core API is cross-built against Scala JVM (for Scala 2.11, 2.12, 2.13 and Dotty/3.0) , Scala.js / JavaScript (for Scala 2.11, 2.12 and 2.13) and Scala Native (for Scala 2.11). The Scala Native build will fail unless you apply this PR.

Currently at ONNX 1.6.0.

Type-safe Tensors (Experimental, Scala 2.13 only)

Featuring type-checked axis labels (Dim), which along with literal types (new in Scala 2.13) for dimension sizes allow for axes-typed/shape-typed (Axes) tensors (TypesafeTensor). Using ONNX docs for dimension and type denotation as a reference, and inspired by Nexus, Neurocat and Named Tensors.

import org.emergentorder.onnx._

trait DataBatch extends Dim
trait DataChannel extends Dim
trait DataFeature extends Dim

val imageAxes = new Tuple3OfDim(3, new DataChannel{}, 224, new DataFeature{},224, new DataFeature{})
type ImageAxes = imageAxes.type
type ImageTensor = TypesafeTensor[Float, ImageAxes]

val typesafeTens: ImageTensor = TensorFactory.getTypesafeTensor(Array.fill(3*224*224){42f},imageAxes) 
onnx.Sqrt6[Float, ImageAxes]("sqrt", Some(typesafeTens))
onnx.Sqrt6("sqrt", Some(typesafeTens))

val textAxes = (new Vec(100, new DataFeature{}))
type TextAxes = textAxes.type
type TextTensor = TypesafeTensor[Float, TextAxes]
//Fails at runtime, as designed
val wrongSizeDataTens: ImageTensor = TensorFactory.getTypesafeTensor(Array.fill(3*224*225){42f},imageAxes)
// java.lang.IllegalArgumentException: requirement failed
// 	at scala.Predef$.require(Predef.scala:327)
// 	at org.emergentorder.onnx.package$TensorFactory$.getTypesafeTensor(ONNX213.scala:109)
// 	at repl.Session$App$$anonfun$46.apply$mcV$sp(
// 	at repl.Session$App$$anonfun$46.apply(
// 	at repl.Session$App$$anonfun$46.apply(
//The rest fail to compile, as designed

val wordShouldBeImageTens: TextTensor = TensorFactory.getTypesafeTensor(Array.fill(3*224*224){42f},imageAxes)
// error: type mismatch;
//  found   : repl.Session.App.imageAxes.type (with underlying type org.emergentorder.onnx.Tuple3OfDim[3,repl.Session.App.DataChannel,224,repl.Session.App.DataFeature,224,repl.Session.App.DataFeature])
//  required: repl.Session.App.TextAxes
//     (which expands to)  repl.Session.App.textAxes.type
// val wordShouldBeImageTens: TextTensor = TensorFactory.getTypesafeTensor(Array.fill(3*224*224){42f},imageAxes)
//                                                                                                    ^^^^^^^^^
onnx.Sqrt6[Float, TextAxes]("sqrt", Some(typesafeTens))
// error: type mismatch;
//  found   : repl.Session.App.ImageTensor
//     (which expands to)  (Array[Float], Array[Int], repl.Session.App.imageAxes.type)
//  required: org.emergentorder.onnx.TypesafeTensor[Float,repl.Session.App.TextAxes]
//     (which expands to)  (Array[Float], Array[Int], repl.Session.App.textAxes.type)
// onnx.Sqrt6[Float, TextAxes]("sqrt", Some(typesafeTens))
//                                          ^^^^^^^^^^^^
val wrongSizedImageAxes = (new Tuple3OfDim(15, new DataChannel{}, 224, new DataFeature{}, 224, new DataFeature{}))
type WrongSizedImageAxes = wrongSizedImageAxes.type
onnx.Sqrt6[Float, WrongSizedImageAxes]("sqrt", Some(typesafeTens))
// error: type mismatch;
//  found   : repl.Session.App.ImageTensor
//     (which expands to)  (Array[Float], Array[Int], repl.Session.App.imageAxes.type)
//  required: org.emergentorder.onnx.TypesafeTensor[Float,repl.Session.App.WrongSizedImageAxes]
//     (which expands to)  (Array[Float], Array[Int], repl.Session.App.wrongSizedImageAxes.type)
// onnx.Sqrt6[Float, WrongSizedImageAxes]("sqrt", Some(typesafeTens))
//                                                     ^^^^^^^^^^^^
val wrongDimTypeAxes = (new Tuple3OfDim(3, new DataBatch{}, 224, new DataFeature{}, 224, new DataFeature{}))
type WrongDimTypeAxes = wrongDimTypeAxes.type
onnx.Sqrt6[Float, WrongDimTypeAxes]("sqrt", Some(typesafeTens))
// error: type mismatch;
//  found   : repl.Session.App.ImageTensor
//     (which expands to)  (Array[Float], Array[Int], repl.Session.App.imageAxes.type)
//  required: org.emergentorder.onnx.TypesafeTensor[Float,repl.Session.App.WrongDimTypeAxes]
//     (which expands to)  (Array[Float], Array[Int], repl.Session.App.wrongDimTypeAxes.type)
// onnx.Sqrt6[Float, WrongDimTypeAxes]("sqrt", Some(typesafeTens))
//                                                  ^^^^^^^^^^^^

Built With


  • ONNX via JavaCPP Preset for ONNX 1.6.0 - Open Neural Network Exchange / The missing bridge between Java and native C++ libraries (For access to Protobuf definitions and operator schemas)

  • Spire - Typelevel project enabling generic numeric programming (For support for unsigned ints, complex numbers, the Numeric type class and type specialization to avoid boxing overhead)

Optional - Dotty Variant

  • Dotty - A next-generation compiler that will become Scala 3 (For native union types, formerly used here to express ONNX type constraints, but currently using cross-version source compatibile union types instead)

Optional - ZIO Variant

  • ZIO - A type-safe, composable library for asynchronous and concurrent programming in Scala

Program Generator

  • Scalameta - Library to read, analyze, transform and generate Scala programs (For a runtime parse pass of generated programs)




  • Neurocat - From neural networks to the Category of composable supervised learning algorithms in Scala with compile-time matrix checking based on singleton-types

  • Nexus - Experimental typesafe tensors & deep learning in Scala

  • Lantern - Machine learning framework prototype in Scala. The design of Lantern is built on two important and well-studied programming language concepts, delimited continuations (for automatic differentiation) and multi-stage programming (staging for short).

  • DeepLearning.scala - A simple library for creating complex neural networks

  • Deeplearning4j / Scalnet / ND4S - ScalNet is a wrapper around Deeplearning4J emulating a Keras like API for deep learning.


  • Backprop - Heterogeneous automatic differentiation ("backpropagation") in Haskell

  • Grenade - Grenade is a composable, dependently typed, practical, and fast recurrent neural network library for concise and precise specifications of complex networks in Haskell.

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