Introduction

This book is about the design an internals of Mongoose OS, an Operating System for connected products on low power microcontrollers. If you are an embedded engineer who develop device firmware for IoT devices, this book is for you.

Here we share our vision and underlying rationale for the design decisions we made. The vast majority of these decisions were driven by our work for our customers, when we developed device firmware to bring their IoT products to the market.

We noticed the following:

  1. Each project was a start from scratch, more or less
  2. Only ~10-20% of the time was spent on the actual product-specific logic. The rest was a generic infrastructure - like configuration, OTA, etc

We refactored those generic pieces that take up to 90% of firmware development time, into a reusable set of components. We made it platform-independent - for example, toggling a GPIO code on Mongoose OS looks the same on all hardware platforms.

The result we called Mongoose OS.

Where does the name Mongoose come from? We are targeting IoT products, where networking is crucial. We use a mature and trusted Mongoose Networking Library as the networking core - that is the origin of the name. The networking library uses mg_ prefix for all API functions, and similarly Mongoose OS uses mgos_ prefix.

Our goal is to share our experience in the hope that it'll help other developers to save a great deal of time and effort, reusing a solid and reliable basis for their products.

Architecture

Components

Mongoose OS is a framework for building apps (firmwares) for low-power microcontrollers (uC), and consists of the following main components:

  • A mos tool. Provides device management and firmware building capabilities
  • A build toolchain. This is a docker image which contains hardware vendor's SDK together with mongoose-os sources. A mos build command builds a firmware (we call it an "app") by taking mos.yml file in the current directory and invoking a build docker image either remotely (mos build) or locally (mos build --local).
  • A collection of ready-to-use apps and libraries

Mongoose OS is based on the vendor's SDK, it extends the capabilities of the native SDK. For example, on ESP32 uC, Mongoose OS uses an ESP-IDF SDK, therefore it provides all capabilities that ESP-IDF provides, plus extra that come with Mongoose OS. If user code uses crossplatform API only, it can be built on all supported hardware platforms with no code changes:

If we zoom in the yellow "Mongoose OS" block, it is fragmented into several components as well. Some of them, like configuration, RPC, timers, networking API, etc, will be covered further down.

Source code structure

The main Mongoose OS lives at cesanta/mongoose-os on GitHub:

Boot process

The boot process is driven by a cross-platform mgos_init.c. In short, the subsystems are initialised in the following order:

Native SDK init, GPIO, configuration, WiFi, platform-specific init, libraries (they can define their initialisation order), user app init function mgos_app_init(), and at the end - all registered MGOS_HOOK_INIT_DONE hooks are invoked.

The initialisation function has the following prototype:

enum mgos_init_result mgos_XXX_init(void);

It returns MGOS_INIT_OK on success, or any other specific numeric code on error.

If any of those init functions returns an error, the firmware reboots immediately. This is done intentionally, in order to revert back to the previous firmware in case of failed OTA update.

File system

Mongoose OS implements Virtual File System layer, VFS. That means it can attach (mount) different storage types into a single file system tree. For example, a device can have an SPI flash storage and an SD card storage. For each storage type, a filesystem driver must be implemented. For example, it is possible to write a driver that implements a Dropbox or Google Drive storage type, and a device (e.g. ESP8266 module) can mount a Dropbox folder.

Mongoose OS provides a Filesystem RPC service that allows remote filesystem management - for example, you can edit files remotely.

The contents of the filesystem depends on the app and specific libraries that are used. For example, an mjs library which adds JavaScript support to the app, also adds a bunch of api_*.js files to the filesystem. Here is a typical layout:

conf0.json      - default app configuration, must NOT be edited manually
conf9.json      - user-specific overrides, changed by "mos config-set" command
index.html      - many apps define this file, which is served by a web server
ca.pem          - added by the ca-bundle library, contains ca root certs
init.js         - main file for the JavaScript-enabled apps
api_*.js        - JavaScript API files
api_*.jsc       - compiled JavaScript files

Main event loop

Mongoose OS contains Mongoose Networking Library as one of the core components. The networking library provides network protocol support, such as UDP, MQTT, etc. It consitutes the low level of Mongoose OS; it is non-blocking and event based, uses mg_ API prefix and expects the following usage pattern:

  • User creates listening connections by calling mg_bind() or variants
  • User creates outgoing connections by calling mg_connect() or variants
  • For all connections, listening or outgoing, user must define an event handler function
  • All connections are inserted in a linked list in a struct mg_mgr structure, which is an event manager
  • User makes an infinite event loop. On each iteration, Mongoose Networking library waits for IO on all connections. When it happens, an event handler is called for a appropriate connection

Mongoose OS does exactly that. It defines a "system" event manager instance, and runs a main event loop in a single task. That event loop dispatches events by calling event handlers. For example, mgos_gpio_set_button_handler() function sets up a button press event handler. When a hardware interrupt occurs, its handler queues the event, and the Mongoose OS task calls the user-defined button handler in its context.

For network connections, Mongoose OS defines wrappers for low-level mg_ functions. These wrappers use "system" event manager and provide the reconnection functionality for the outgoing connection.

For example, low-level mg_ API for MQTT protocol allows to create an MQTT client. If it disconnects for any reason, e.g. temporary WiFi connectivity loss, the connection closes. The mgos_ wrapper, however, would setup a reconnection timer with exponential backoff and re-establish the connection automatically. This is a valuable addon to the low-level mg_ API, therefore using mgos_ API is a good idea. Of course the low level mg_ API is also available.

You can get main event manager instance by calling mgos_get_mgr() function defined in mgos_mongoose.h.

Note that mgos_ API, as well as mg_ API, is cross-platform. A firmware written with that API only, is portable between supported architectures, as demonstrated by many example apps. However, the native SDK API is not in any way hidden and is fully available. For example, one could fire extra FreeRTOS tasks on platforms whose SDK use FreeRTOS. The price to pay is loss of portability.

Example footprint

Mongoose OS is highly modular - it is possible to include or exclude functionality depending on specific needs. That is implemented by the library mechanism, described later. In order to get a feeling about the resulting footprint, the table below shows measuremens done on TI CC3220SF platform for Mongoose OS 1.18 release, built with different options. RAM figures are measured after Mongoose OS is initialised, i.e. those numbers are what is available for the application code.

Name Code Size Free RAM Notes
minimal 113k 230k An example-no-libs-c app. Includes RTOS, TCP/UDP networking core, file system, configuration infrastructure, SNTP
minimal+mjs 160k 229k Minimal + mJS JavaScript engine
minimal+aws 133k 230k Minimal + AWS IoT support
minimal+gcp 159k 230k Minimal + Google IoT Core support
js-demo 304k 225k A default JS demo app. Includes HTTP, MQTT, WebSocket, mDNS networking, RPC subsystem, AWS IoT, Google IoT Core, JavaScript, I2C, SPI, PWM, ADC, and more - see js-demo-bundle library