Below are some diagrams of OnDemand’s architecture:
Overview is a high level visual generated from Powerpoint
System context and Container context diagrams below follow the C4 model for software diagrams, are more technically detailed and are built using draw.io
Request flow diagram is a sequence diagram built using plantuml
Apache is the server front end, running as the Apache user, and accepting all requests from users and serves four primary functions
Starts Per-User NGINX processes (PUNs)
Reverse proxies each user to her PUN via Unix domain sockets
Reverse proxies to interactive apps running on compute nodes (RStudio, Jupyter, VNC desktop) via TCP sockets
The Per-User NGINX serves web apps in Ruby and NodeJS and is how users submit jobs and start interactive apps
Users use OnDemand to interact with their HPC resources through a web browser.
All the gray components are specific to a given site and outside the OnDemand system.
In the C4 nomenclature, ‘containers’ are one level below the system context. This is not to be confused with Linux containers via cgroups and namespaces (i.e. Docker or Singularity or OCI containers).
The Front-end proxy is the only component that is shared with all clients. The Front-end proxy will create Per User Nginx (PUN) processes (light blue boxes labeled “Per User Instance”).
Everything contained in the dotted line is a part of the OnDemand system (see blue box in System context diagram).
Everything outside of it in gray is site specific components.
The “Per User Instance” light blue boxes are replicated for every user accessing the system
This is the request flow through the OnDemand system. A user initiates a request through a browser and this illustrates how that request propagates through the system to a particular application (including the dashboard).