Software Architecture

RackHD provides a REST API for the automation using an underlying workflow engine (named the “monorail engine” after a popular Seattle coffee shop:

RackHD is also providing an implementation of the Redfish specification as an additional REST API to provide a common data model for representing bare metal hardware, and provides this as an aggregate for multiple back-end servers and systems.


The workflow engine operates with and coordinates with services to respond to protocols commonly used in hardware management. RackHD is structured with several independent processes, typically focused on specific function or protocol so that we can scaling or distribute them independently, using a pattern of Microservices.

RackHD communicates between these using message passing over AMQP and stores data in an included persistence store. MongoDB is the default, and configurable communications layers and persistence layers are in progress.



Major Components


This DHCP server provides IP addresses dynamically using the DHCP protocol. It is a critical component of a standard Preboot Execution Environment (PXE) process.


The DHCP protocol supports getting additional data specifically for the PXE process from a secondary service that also responds on the same network as the DHCP server. The DHCP proxy service provides that information, generated dynamically from the workflow engine.


TFTP is the common protocol used to initiate a PXE process. on-tftp is tied into the workflow engine to be able to dynamically provide responses based on the state of the workflow engine and to provide events to the workflow engine when servers request files via TFTP.


on-http provides both the REST interface to the workflow engine and data model APIs as well as a communication channel and potential proxy for hosting and serving files to support dynamic PXE responses. RackHD commonly uses iPXE as its initial bootloader, loading remaining files for PXE booting via HTTP and using that communications path as a mechanism to control what a remote server will do when rebooting.


on-syslog is a syslog receiver endpoint provideing annotated and structured logging from the hosts under management. It channels all syslog data sent to the host into the workflow engine.


on-taskgraph is the workflow engine, driving actions on remote systems and processing workflows for machines being managed. Additionally, the workflow engine provides the engine for polling and monitoring.

on-taskgraph also serves as the communication channel for the microkernel to support deep hardware interrogation, firmware updates, and other actions that can only be invoked directly on the hardware (not through an out of band management channel).