diff --git a/.github/workflows/mad_modelDiff.yml b/.github/workflows/mad_modelDiff.yml
new file mode 100644
index 00000000000..a4b9169b44e
--- /dev/null
+++ b/.github/workflows/mad_modelDiff.yml
@@ -0,0 +1,103 @@
+name: Models as Data - Diff
+
+on:
+  workflow_dispatch:
+    inputs:
+      projects:
+        description: "The projects to generate models for"
+        required: true
+        default: '["netty/netty"]'
+  pull_request:
+    branches:
+      - main
+    paths:
+      - "java/ql/src/utils/model-generator/**/*.*"
+      - ".github/workflows/mad_modelDiff.yml"
+
+permissions:
+  contents: read
+
+jobs:
+  model-diff:
+    name: Model Difference
+    runs-on: ubuntu-latest
+    if: github.repository == 'github/codeql'
+    strategy:
+      matrix:
+        slug: ${{fromJson(github.event.inputs.projects || '["apache/commons-codec", "apache/commons-io", "apache/commons-beanutils", "apache/commons-logging", "apache/commons-fileupload", "apache/commons-lang", "apache/commons-validator", "apache/commons-csv", "apache/dubbo"]' )}}
+    steps:
+      - name: Clone github/codeql from PR
+        uses: actions/checkout@v2
+        if: github.event.pull_request
+        with:
+          path: codeql-pr
+      - name: Clone github/codeql from main
+        uses: actions/checkout@v2
+        with:
+          path: codeql-main
+          ref: main
+      - uses: ./codeql-main/.github/actions/fetch-codeql
+      - name: Download database
+        env:
+          SLUG: ${{ matrix.slug }}
+        run: |
+          set -x
+          mkdir lib-dbs
+          SHORTNAME=${SLUG//[^a-zA-Z0-9_]/}
+          projectId=`curl -s https://lgtm.com/api/v1.0/projects/g/${SLUG} | jq .id`
+          curl -L "https://lgtm.com/api/v1.0/snapshots/$projectId/java" -o "$SHORTNAME.zip"
+          unzip -q -d "${SHORTNAME}-db" "${SHORTNAME}.zip"
+          mkdir "lib-dbs/$SHORTNAME/"
+          mv "${SHORTNAME}-db/"$(ls -1 "${SHORTNAME}"-db)/* "lib-dbs/${SHORTNAME}/"
+      - name: Generate Models (PR and main)
+        run: |
+          set -x
+          mkdir tmp-models
+          MODELS=`pwd`/tmp-models
+          DATABASES=`pwd`/lib-dbs
+
+          analyzeDatabaseWithCheckout() {
+            QL_VARIANT=$1
+            DATABASE=$2
+            cd codeql-$QL_VARIANT
+            SHORTNAME=`basename $DATABASE`
+            python java/ql/src/utils/model-generator/GenerateFlowModel.py $DATABASE $MODELS/${SHORTNAME}.qll
+            mv $MODELS/${SHORTNAME}.qll $MODELS/${SHORTNAME}Generated_${QL_VARIANT}.qll
+            cd ..
+          }
+
+          for d in $DATABASES/*/ ; do
+            ls -1 "$d"
+
+            analyzeDatabaseWithCheckout "main" $d
+            if [[ "$GITHUB_EVENT_NAME" == "pull_request" ]]
+            then
+              analyzeDatabaseWithCheckout "pr" $d
+            fi
+          done
+      - name: Install diff2html
+        if: github.event.pull_request
+        run: |
+          npm install -g diff2html-cli
+      - name: Generate Model Diff
+        if: github.event.pull_request
+        run: |
+          set -x
+          MODELS=`pwd`/tmp-models
+          ls -1 tmp-models/
+          for m in $MODELS/*_main.qll ; do
+            t="${m/main/"pr"}"
+            basename=`basename $m`
+            name="diff_${basename/_main.qll/""}"
+            (diff -w -u $m $t | diff2html -i stdin -F $MODELS/$name.html) || true
+          done
+      - uses: actions/upload-artifact@v2
+        with:
+          name: models
+          path: tmp-models/*.qll
+          retention-days: 20
+      - uses: actions/upload-artifact@v2
+        with:
+          name: diffs
+          path: tmp-models/*.html
+          retention-days: 20