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Layer Structure: meta-medtech

This document describes the organization of the meta-medtech custom Yocto layer, explains recipe conventions, and provides a guide for adding new service recipes.

Table of Contents

Layer Overview

meta-medtech is the top-level custom Yocto layer for MedTech Device OS. It contains:

  • The final image definition (core-image-medtech)
  • All medtech-specific service recipes
  • Support recipes for third-party packages (mosquitto, Python packages, TFLite)
  • Custom image class (medtech-image.bbclass)

The layer sits at priority 6, above the default OpenEmbedded layers (priority 5), so its .bbappend files and overrides take precedence.

Directory Structure

yocto/meta-medtech/
│
├── conf/
│   └── layer.conf                      # Layer registration and metadata
│
├── classes/
│   └── medtech-image.bbclass           # Shared image policy (no debug pkgs, etc.)
│
├── recipes-core/
│   └── medtech-system/
│       └── medtech-system.bb           # Base OS configuration and release files
│
├── recipes-image/
│   └── core-image-medtech/
│       └── core-image-medtech.bb       # Final image definition
│
├── recipes-qt/
│   └── (Qt6 configuration appends)
│
├── recipes-services/
│   ├── medtech-vitals-publisher/
│   │   └── medtech-vitals-publisher_1.0.bb
│   ├── medtech-edge-analytics/
│   │   └── medtech-edge-analytics_1.0.bb
│   └── medtech-clinician-ui/
│       └── medtech-clinician-ui_1.0.bb
│
└── recipes-support/
    ├── mosquitto/
    │   └── mosquitto_%.bbappend        # Mosquitto configuration append
    ├── python3-paho-mqtt/
    │   └── python3-paho-mqtt_1.6.1.bb  # Python MQTT client library
    └── tensorflow-lite/
        └── tensorflow-lite_2.14.0.bb   # TFLite for sepsis detection

Layer Configuration

The conf/layer.conf file registers the layer with BitBake. Key settings:

Setting Value Purpose
BBFILE_PRIORITY 6 Higher than OE layers (5); our appends win
LAYERDEPENDS core, openembedded-layer, networking-layer, qt6-layer Required layers
LAYERSERIES_COMPAT kirkstone langdale scarthgap Yocto versions supported

See the file itself (yocto/meta-medtech/conf/layer.conf) for detailed comments on each setting.

Recipe Categories

recipes-core/

Base system configuration. medtech-system.bb installs /etc/medtech-release with build version and timestamp, and any core OS tweaks.

recipes-image/

Image definition. core-image-medtech.bb specifies IMAGE_INSTALL (the full package list) and inherits from medtech-image.bbclass for shared policies.

recipes-services/

The medtech application services. Each service has:

  • A Python or compiled source tarball (or inline files)
  • A systemd service file
  • Proper RDEPENDS on runtime libraries

recipes-support/

Third-party packages not available upstream or requiring customization:

  • mosquitto_%.bbappend — enables specific Mosquitto build features
  • python3-paho-mqtt_1.6.1.bb — MQTT client for vitals publisher
  • tensorflow-lite_2.14.0.bb — TFLite for edge analytics

classes/

medtech-image.bbclass provides shared image policies applied to all medtech images, such as disabling debug packages and setting default features.

Recipe Naming Conventions

<package-name>_<version>.bb

Examples:

  • medtech-vitals-publisher_1.0.bb — version 1.0 of the vitals publisher
  • python3-paho-mqtt_1.6.1.bb — Python paho-mqtt version 1.6.1
  • tensorflow-lite_2.14.0.bb — TFLite version 2.14.0

Appends use % as a wildcard for the version:

  • mosquitto_%.bbappend — applies to any version of mosquitto

Adding a New Service Recipe

This example adds a hypothetical medtech-alarm-manager service.

Step 1: Create the recipe directory

mkdir -p yocto/meta-medtech/recipes-services/medtech-alarm-manager

Step 2: Create the recipe file

cat > yocto/meta-medtech/recipes-services/medtech-alarm-manager/medtech-alarm-manager_1.0.bb
DESCRIPTION = "MedTech alarm manager — monitors vitals thresholds and triggers alerts"
LICENSE = "CLOSED"
LIC_FILES_CHKSUM = ""

# Source: local files in this recipe directory
SRC_URI = "file://alarm-manager.py \
           file://medtech-alarm-manager.service \
          "

# Runtime dependencies
RDEPENDS:${PN} = "python3 python3-paho-mqtt"

# Install the service
do_install() {
    install -d ${D}${bindir}
    install -m 0755 ${WORKDIR}/alarm-manager.py ${D}${bindir}/alarm-manager

    install -d ${D}${systemd_system_unitdir}
    install -m 0644 ${WORKDIR}/medtech-alarm-manager.service \
        ${D}${systemd_system_unitdir}/medtech-alarm-manager.service
}

# Enable the systemd service on boot
inherit systemd
SYSTEMD_SERVICE:${PN} = "medtech-alarm-manager.service"
SYSTEMD_AUTO_ENABLE = "enable"

Step 3: Add source files

yocto/meta-medtech/recipes-services/medtech-alarm-manager/
├── medtech-alarm-manager_1.0.bb
├── alarm-manager.py
└── medtech-alarm-manager.service

Step 4: Add to the image

Edit yocto/meta-medtech/recipes-image/core-image-medtech/core-image-medtech.bb:

IMAGE_INSTALL:append = " medtech-alarm-manager"

Step 5: Build and test

# Build only this recipe first
bitbake medtech-alarm-manager

# Then rebuild the full image
bitbake core-image-medtech

Systemd Service File Pattern

All medtech services use systemd. Follow this pattern for consistency:

[Unit]
Description=MedTech Alarm Manager
After=medtech-vitals-publisher.service
Requires=mosquitto.service

[Service]
Type=simple
ExecStart=/usr/bin/alarm-manager
Restart=on-failure
RestartSec=5s
StandardOutput=journal
StandardError=journal

[Install]
WantedBy=multi-user.target

Key points:

  • Use After= for ordering (not strict dependency)
  • Use Requires= only for hard dependencies (the service won't start if the dependency fails)
  • Use Restart=on-failure for resilience
  • Log to journal with StandardOutput=journal

Versioning Strategy

Recipe versions follow semantic versioning (MAJOR.MINOR):

Version When to bump
1.0 Initial release
1.1 Bug fixes, minor feature additions
2.0 Breaking API changes or major rewrites

When bumping a version, rename the .bb file:

mv medtech-vitals-publisher_1.0.bb medtech-vitals-publisher_1.1.bb

BitBake will automatically use the latest version by default. To pin to a specific version in the image:

PREFERRED_VERSION_medtech-vitals-publisher = "1.0"

Layer Dependencies

Why each dependency is needed:

Layer Required for
core (Poky/OE-Core) Base Linux, kernel, busybox, systemd, Python3
openembedded-layer (meta-oe) mosquitto, additional Python packages
networking-layer (meta-networking) Network tools, socket libraries
qt6-layer (meta-qt6) Qt6 framework for medtech-clinician-ui

These are declared in LAYERDEPENDS_meta-medtech in conf/layer.conf.