feat: add Mill build system support with Thrift and Python API integration

Add Mill build configuration alongside existing sbt build:
- Mill package.mill files for all modules (api, aggregator, online, spark, flink, service)
- Thrift code generation for Java (api module)
- Python Thrift generation and wheel building commands
- CircleCI jobs for Mill compilation, tests, and Python API builds
- prepareScalaSources command to handle scala-2.12 version-specific sources

Mill commands:
  ./mill api.prepareScalaSources  # Copy scala-2.12 sources (one-time setup)
  ./mill <module>.compile          # Compile modules
  ./mill <module>.test             # Run tests
  ./mill generatePythonThrift      # Generate Python Thrift files
  ./mill buildPythonApi            # Build Python wheel

Test fixes:
- Migrate test modules from ScalaTest to JUnit4 (tests use JUnit annotations)
- Fix Mockito verification in FetcherCacheTest
- Fix MetadataExporterTest to use filesystem path instead of JAR resource
- Update Python build script to use venv

Both Mill and sbt build systems work simultaneously.

Author: Abby Whittier

.circleci/config.yml

@@ -15,6 +15,84 @@ executors:
       working_directory: /chronon
       docker:
         - image: houpy0829/chronon-ci:base--f87f50dc520f7a73894ae024eb78bd305d5b08e2
+    modern_ubuntu_executor:
+        resource_class: xlarge
+        working_directory: /chronon
+        docker:
+            - image: cimg/openjdk:11.0
+    modern_ubuntu_executor_xxlarge:
+        resource_class: 2xlarge
+        working_directory: /chronon
+        docker:
+            - image: cimg/openjdk:11.0
+
+commands:
+    install_build_dependencies:
+        description: "Install Thrift, Conda, SBT, and Mill on Ubuntu 22.04"
+        steps:
+            - run:
+                name: Install system dependencies
+                command: |
+                    sudo apt-get update
+                    sudo apt-get install -y \
+                        automake \
+                        bison \
+                        cmake \
+                        flex \
+                        g++ \
+                        git \
+                        libboost-dev \
+                        libboost-filesystem-dev \
+                        libboost-program-options-dev \
+                        libboost-system-dev \
+                        libboost-test-dev \
+                        libevent-dev \
+                        libssl-dev \
+                        libtool \
+                        make \
+                        pkg-config
+            - run:
+                name: Install Thrift 0.11.0 from source
+                command: |
+                    export THRIFT_VERSION=0.11.0
+                    curl -sSL "http://archive.apache.org/dist/thrift/$THRIFT_VERSION/thrift-$THRIFT_VERSION.tar.gz" -o thrift.tar.gz
+                    mkdir -p /tmp/thrift
+                    tar zxf thrift.tar.gz -C /tmp/thrift --strip-components=1
+                    rm thrift.tar.gz
+                    cd /tmp/thrift
+                    ./configure --without-python --without-cpp
+                    make
+                    sudo make install
+                    cd /
+                    rm -rf /tmp/thrift
+                    thrift --version
+            - run:
+                name: Install SBT
+                command: |
+                    echo "deb https://repo.scala-sbt.org/scalasbt/debian all main" | sudo tee /etc/apt/sources.list.d/sbt.list
+                    curl -sL "https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x2EE0EA64E40A89B84B2DF73499E82A75642AC823" | sudo apt-key add
+                    sudo apt-get update
+                    sudo apt-get install -y sbt
+            - run:
+                name: Install Miniconda
+                command: |
+                    wget -q https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh -O ~/miniconda.sh
+                    bash ~/miniconda.sh -b -p $HOME/miniconda
+                    echo 'export PATH=$HOME/miniconda/bin:$PATH' >> $BASH_ENV
+                    source $BASH_ENV
+                    conda create -y -n chronon_py python=3.7
+                    conda install -y -q -n chronon_py --no-deps virtualenv
+                    $HOME/miniconda/envs/chronon_py/bin/pip install \
+                        flake8==5.0.4 flake8-quotes==3.3.1 thrift==0.11.0 click==7.0 thrift_json==0.1.0 nose>=1.3.7
+                    $HOME/miniconda/envs/chronon_py/bin/pip install build
+            - run:
+                name: Install Mill with checksum verification
+                command: |
+                    curl -L https://github.com/com-lihaoyi/mill/releases/download/0.10.15/0.10.15 -o mill
+                    echo "d90132b1a4ebe4d55d2bc43b3f18b5d6e8e3d12d89a83f83ad2276867e127916  mill" | sha256sum -c -
+                    chmod +x mill
+                    sudo mv mill /usr/local/bin/mill
+                    mill --version
 
 jobs:
     "Pull Docker Image":
@@ -195,6 +273,64 @@ jobs:
             destination: spark_warehouse.tar.gz
             when: on_fail
 
+    "Mill -- Compile All":
+      executor: modern_ubuntu_executor
+      steps:
+        - checkout
+        - install_build_dependencies
+        - run:
+            name: Compile all modules with Mill
+            shell: /bin/bash -leuxo pipefail
+            command: |
+              source $BASH_ENV
+              conda activate chronon_py
+              # Prepare scala version-specific sources for Mill
+              mill api.prepareScalaSources
+              # Compile all modules
+              mill api.compile aggregator.compile online.compile spark.compile flink.compile
+
+    "Mill -- Tests":
+      executor: modern_ubuntu_executor_xxlarge
+      steps:
+        - checkout
+        - install_build_dependencies
+        - run:
+            name: Run all tests with Mill
+            shell: /bin/bash -leuxo pipefail
+            command: |
+              source $BASH_ENV
+              conda activate chronon_py
+              mill api.prepareScalaSources
+              mill api.test aggregator.test online.test spark.test flink.test
+        - run:
+            name: Compress spark-warehouse
+            command: |
+              cd /tmp/ && tar -czvf spark-warehouse.tar.gz chronon/spark-warehouse
+            when: on_fail
+        - store_artifacts:
+            path: /tmp/spark-warehouse.tar.gz
+            destination: spark_warehouse.tar.gz
+            when: on_fail
+
+    "Mill -- Python API Build":
+      executor: modern_ubuntu_executor
+      steps:
+        - checkout
+        - install_build_dependencies
+        - run:
+            name: Build Python API with Mill
+            shell: /bin/bash -leuxo pipefail
+            command: |
+              source $BASH_ENV
+              conda activate chronon_py
+              # Set project root for Mill commands
+              export CHRONON_ROOT=/chronon
+              # Generate Python Thrift and build wheel
+              mill generatePythonThrift
+              mill buildPythonApi
+        - store_artifacts:
+            path: /chronon/api/py/dist
+
 workflows:
     build_test_deploy:
         jobs:
@@ -221,5 +357,14 @@ workflows:
                   requires:
                       - "Pull Docker Image"
             - "Scala 13 -- Iceberg Table Utils Tests":
+                  requires:
+                      - "Pull Docker Image"
+            - "Mill -- Compile All":
+                  requires:
+                      - "Pull Docker Image"
+            - "Mill -- Tests":
+                  requires:
+                      - "Pull Docker Image"
+            - "Mill -- Python API Build":
                   requires:
                       - "Pull Docker Image"

aggregator/package.mill

@@ -0,0 +1,69 @@
+package build.aggregator
+
+import mill._
+import mill.javalib._
+import mill.javalib.publish._
+import mill.scalalib.SbtModule
+
+object `package` extends SbtModule with PublishModule {
+
+  def scalaVersion = "2.12.12"
+
+  def mvnDeps = Seq(
+    mvn"com.google.code.gson:gson:2.8.6",
+    mvn"com.yahoo.datasketches:sketches-core:0.13.4",
+    mvn"org.apache.commons:commons-lang3:3.12.0"
+  )
+
+  def moduleDeps = super.moduleDeps ++ Seq(build.api)
+
+  def compileMvnDeps = Seq(
+    mvn"org.apache.spark::spark-core:3.1.1",
+    mvn"org.apache.spark::spark-sql:3.1.1"
+  )
+
+  def pomSettings = PomSettings(
+    "Chronon is a feature engineering platform",
+    "ai.chronon",
+    "https://github.com/airbnb/chronon",
+    Seq(License(
+      "Apache 2",
+      "Apache 2",
+      "http://www.apache.org/licenses/LICENSE-2.0.txt",
+      false,
+      false,
+      "repo"
+    )),
+    VersionControl(
+      Some("https://github.com/airbnb/chronon"),
+      Some("scm:[email protected]:airbnb/chronon.git"),
+      None,
+      None
+    ),
+    Seq(Developer(
+      "nikhilsimha",
+      "Nikhil Simha",
+      "http://nikhilsimha.com",
+      None,
+      None
+    ))
+  )
+
+  def publishVersion = "awhittier-mill-0.0.110-SNAPSHOT"
+
+  object test extends SbtTests with TestModule.Junit4 {
+
+    def mvnDeps = Seq(
+      mvn"junit:junit:4.13.2",
+      mvn"com.novocode:junit-interface:0.11",
+      mvn"org.scalatest::scalatest:3.2.15",
+      mvn"org.apache.commons:commons-math3:3.6.1"
+    )
+
+    def moduleDeps = super.moduleDeps ++ Seq(build.api.test)
+
+    def testSandboxWorkingDir = false
+    def testParallelism = false
+
+  }
+}

api/package.mill

@@ -0,0 +1,105 @@
+package build.api
+
+import mill._
+import mill.javalib._
+import mill.javalib.publish._
+import mill.scalalib.SbtModule
+
+object `package` extends SbtModule with PublishModule {
+
+  def scalaVersion = "2.12.12"
+
+  def mvnDeps = Seq(
+    mvn"org.apache.thrift:libthrift:0.13.0",
+    mvn"org.scala-lang.modules::scala-collection-compat:2.6.0",
+    mvn"org.scala-lang:scala-reflect:2.12.12"
+  )
+
+  def compileMvnDeps = Seq(
+    mvn"org.apache.spark::spark-core:3.1.1",
+    mvn"org.apache.spark::spark-sql:3.1.1"
+  )
+
+  def pomSettings = PomSettings(
+    "Chronon is a feature engineering platform",
+    "ai.chronon",
+    "https://github.com/airbnb/chronon",
+    Seq(License(
+      "Apache 2",
+      "Apache 2",
+      "http://www.apache.org/licenses/LICENSE-2.0.txt",
+      false,
+      false,
+      "repo"
+    )),
+    VersionControl(
+      Some("https://github.com/airbnb/chronon"),
+      Some("scm:[email protected]:airbnb/chronon.git"),
+      None,
+      None
+    ),
+    Seq(Developer(
+      "nikhilsimha",
+      "Nikhil Simha",
+      "http://nikhilsimha.com",
+      None,
+      None
+    ))
+  )
+
+  def publishVersion = "awhittier-mill-0.0.110-SNAPSHOT"
+
+  /**
+   * Prepare Scala version-specific sources for Mill compilation.
+   *
+   * Mill's security model prevents reading from scala-2.12/ during Task execution,
+   * so we copy version-specific sources to the main scala/ directory before compilation.
+   *
+   * This is a one-time setup command - run it once per clean build:
+   *   ./mill api.prepareScalaSources
+   *
+   * SBT users don't need this - sbt automatically picks up scala-2.12/ directories.
+   */
+  def prepareScalaSources() = Task.Command {
+    val scala212Dir = millSourcePath0 / "src" / "main" / "scala-2.12"
+    val scalaDir = millSourcePath0 / "src" / "main" / "scala"
+
+    os.walk(scala212Dir)
+      .filter(_.ext == "scala")
+      .foreach { sourceFile =>
+        val relativePath = sourceFile.relativeTo(scala212Dir)
+        val targetFile = scalaDir / relativePath
+        os.makeDir.all(targetFile / os.up)
+        os.copy.over(sourceFile, targetFile, createFolders = true)
+        println(s"✓ Copied: ${sourceFile.last}")
+      }
+    println(s"✓ Scala 2.12 sources prepared for Mill compilation")
+    ()
+  }
+
+  // Generate Java sources from Thrift
+  def generatedSources = Task {
+    val thriftFile = millSourcePath0 / "thrift" / "api.thrift"
+    val outDir = Task.dest / "java"
+    os.remove.all(outDir)
+    os.makeDir.all(outDir)
+    os.proc("thrift", "--gen", "java", "-out", outDir, thriftFile)
+      .call(stdout = os.Inherit)
+
+    os.walk(outDir).filter(_.ext == "java").map(PathRef(_))
+  }
+
+  object test extends SbtTests with TestModule.Junit4 {
+
+    def mvnDeps = Seq(
+      mvn"com.novocode:junit-interface:0.11",
+      mvn"junit:junit:4.13.2",
+      mvn"org.scalatest::scalatest:3.2.15",
+      mvn"org.scalatestplus::mockito-3-4:3.2.10.0"
+    )
+
+    def testSandboxWorkingDir = false
+    def testParallelism = false
+
+  }
+}

api/py/python-api-build.sh

@@ -26,6 +26,23 @@ SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
 echo "Removing old distributions..."
 rm -f $SCRIPT_DIR/dist/*
 
+# Create and activate venv if it doesn't exist
+VENV_DIR="$SCRIPT_DIR/../../venv"
+if [[ ! -d "$VENV_DIR" ]]; then
+  echo "Creating Python virtual environment..."
+  python3 -m venv "$VENV_DIR"
+fi
+
+# Activate venv
+echo "Activating Python virtual environment..."
+source "$VENV_DIR/bin/activate"
+
+# Install build dependencies in venv if needed
+if ! python3 -c "import build" 2>/dev/null; then
+  echo "Installing Python build dependencies..."
+  pip install --quiet build twine
+fi
+
 # The default action is "build"
 if [[ -z "${ACTION}" ]] || [[ "${ACTION}" == "build" ]]; then
   PYPI_REPOSITORY="internal"