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+# Improve Dependabot Configuration
+
+## Context
+
+The transitdata repositories currently have basic Dependabot configs (monthly schedule, no grouping, no cooldown, no Docker tracking). The goal is to reduce PR noise, gain supply-chain attack resilience via cooldown, and keep timely updates — applied consistently across all repos.
+
+Default branch is `main` across all repos. Dependabot targets the default branch by default, so no `target-branch` override is needed.
+
+## Approach: Alternative 2 Grouping + Cooldown + Multi-Ecosystem
+
+### Grouping Strategy
+
+For the primary package ecosystem (maven/gradle/npm/pip):
+- **`dev-all`** group: all update types for development dependencies → one PR
+- **`prod-minor-patch`** group: minor + patch for production dependencies → one PR
+- **Major production deps**: ungrouped → individual PRs (require changelog review)
+
+For infrastructure ecosystems (github-actions + docker):
+- **`hsldevcom-infra`** group (multi-ecosystem): all updates for HSLdevcom-originating docker and github-actions packages combined into a single PR, using `patterns` to match only `hsldevcom/*` / `HSLdevcom/*` packages
+- **Non-HSLdevcom packages**: grouped per-ecosystem for minor+patch, with majors ungrouped → individual PRs
+
+### Cooldown
+
+Applied to all ecosystems:
+```yaml
+cooldown:
+  default-days: 30
+  semver-major-days: 90
+  semver-minor-days: 60
+  semver-patch-days: 30
+```
+
+`default-days: 30` serves as fallback for packages that don't follow semver (e.g., some Docker images).
+
+**Security updates bypass cooldown** on the default branch. Per [GitHub docs](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#cooldown), cooldown does not delay security updates when PRs target the default branch.
+
+### Other Settings
+
+- `open-pull-requests-limit: 20` on all ecosystem entries
+- `schedule.interval: "daily"` — with auto-merge enabled, no extra delay is needed on top of cooldown
+
+## Template Configuration
+
+A single unified template is used for all repos. Dependabot ignores ecosystems that don't apply to a given repo, so one template everywhere simplifies centralized management.
+
+```yaml
+version: 2
+multi-ecosystem-groups:
+  hsldevcom-infra:
+    schedule:
+      interval: "daily"
+    updates:
+      - package-ecosystem: "docker"
+        directory: "/"
+        patterns: ["hsldevcom/*"]
+        multi-ecosystem-group: "hsldevcom-infra"
+      - package-ecosystem: "github-actions"
+        directory: "/"
+        patterns: ["HSLdevcom/*"]
+        multi-ecosystem-group: "hsldevcom-infra"
+updates:
+  - package-ecosystem: "maven"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 20
+    cooldown:
+      default-days: 30
+      semver-major-days: 90
+      semver-minor-days: 60
+      semver-patch-days: 30
+    groups:
+      dev-all:
+        dependency-type: "development"
+        update-types:
+          - "major"
+          - "minor"
+          - "patch"
+      prod-minor-patch:
+        dependency-type: "production"
+        update-types:
+          - "minor"
+          - "patch"
+  - package-ecosystem: "gradle"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 20
+    cooldown:
+      default-days: 30
+      semver-major-days: 90
+      semver-minor-days: 60
+      semver-patch-days: 30
+    groups:
+      dev-all:
+        dependency-type: "development"
+        update-types:
+          - "major"
+          - "minor"
+          - "patch"
+      prod-minor-patch:
+        dependency-type: "production"
+        update-types:
+          - "minor"
+          - "patch"
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 20
+    cooldown:
+      default-days: 30
+      semver-major-days: 90
+      semver-minor-days: 60
+      semver-patch-days: 30
+    groups:
+      dev-all:
+        dependency-type: "development"
+        update-types:
+          - "major"
+          - "minor"
+          - "patch"
+      prod-minor-patch:
+        dependency-type: "production"
+        update-types:
+          - "minor"
+          - "patch"
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 20
+    cooldown:
+      default-days: 30
+      semver-major-days: 90
+      semver-minor-days: 60
+      semver-patch-days: 30
+    groups:
+      dev-all:
+        dependency-type: "development"
+        update-types:
+          - "major"
+          - "minor"
+          - "patch"
+      prod-minor-patch:
+        dependency-type: "production"
+        update-types:
+          - "minor"
+          - "patch"
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 20
+    cooldown:
+      default-days: 30
+      semver-major-days: 90
+      semver-minor-days: 60
+      semver-patch-days: 30
+    ignore:
+      - dependency-name: "eclipse-temurin/*"
+        # Ignore non-LTS versions; extend as the release schedule evolves
+        versions: ["26", "27", "28", "30", "31", "32", "34", "35", "36"]
+    groups:
+      docker-minor-patch:
+        update-types:
+          - "minor"
+          - "patch"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 20
+    cooldown:
+      default-days: 30
+      semver-major-days: 90
+      semver-minor-days: 60
+      semver-patch-days: 30
+    groups:
+      actions-minor-patch:
+        update-types:
+          - "minor"
+          - "patch"
+```
+
+The `multi-ecosystem-groups` block groups HSLdevcom-originating docker and github-actions updates into a single PR, since new shared Dockerfile and shared workflow versions are often related. Non-HSLdevcom docker and github-actions updates are handled by their respective per-ecosystem groups (`docker-minor-patch`, `actions-minor-patch`) for minor+patch, with majors ungrouped as individual PRs.
+
+## Changes
+
+### Repos with existing dependabot.yml
+
+Overwrite with the unified template above. This replaces any existing grouping, schedule, and ecosystem configuration.
+
+### Repos without dependabot.yml
+
+Create a new `.github/dependabot.yml` using the unified template above.
+