recodex-wiki/Internal-architecture-(original).md

ReCodEx is designed to be very modular. In the following picture main components are arranged into one possible configuration. Note, that connections between components are not fully accurate.

Web app is main part of whole project for users. It provides nice user interface and is the only part, that interacts with outside world directly. Web API contains almost all logic of the app including user management and authentication, storing and versioning files (with help of File server), counting and assigning points to users etc. Broker is essential part of whole architecture and can be marked as single point of failure. It maintains list of available Workers, receives submissions from the Web API and routes them further and reports progress of evaluations back to the Web app. Worker securely runs each received job and evaluate it's results. Monitor resends evaluation progress messages to the Web app in order to be presented to users.

Almost whole communication goes through Broker and ZeroMQ messaging middleware. When Web app wants to execute submission then all datas are handed over to Worker through Broker, similar situation is with progress state which start in Worker goes through Broker then pass Monitor and end up in Web app (as WebSockets). Only part of communication, which does not include Broker, is communication with File server which is realized through HTTP commmunication. This communication can be initiated by Web API or by Worker, other services have no access to File server. Detailed view into communication is on separate page Communication.

Worker

Worker's main role is securely execute given submission and possibly evaluate results against model solutions provided by submitter. Worker is logicaly divided into two parts:

• Listener - listens and communicates with Broker through ZeroMQ. It receives new jobs, communicates with Evaluator part and sends back results or progress.
• Evaluator - gets jobs to evaluate from Listener part, evaluate them (possibly in sandbox) and get to know to other part that evaluation ended. This part also communicates with Fileserver, downloads needed files and uploads detailed results.

Worker after getting evaluation request has to:

• Download the archive containing submitted source files and configuration file
• Download any supplementary files based on the configuration file, such as test inputs or helper programs (This is done on demand, using a fetch command in the assignment configuration)
• Evaluate the submission accordingly to job configuration
• During evaluation progress states can be sent back to Broker
• Upload the results of the evaluation to the Fileserver
• Notify Broker that the evaluation finished

Internal Worker architecture

Picture below is overall internal architecture of worker which shows its defined classes with private variables and public functions. Vector version of this picture is available here.

File Server

File Server stores data, that should be kept outside of WebApp's database (both because storing files in a database is inefficient and because the workers need to access the files in the simplest possible way). It should meet following requirements:

• store files without duplicates
• keep consistent state with main database
• serve files to workers on demand
• allow versioning of tasks with revert back feature

To meet these requirements, Storage and Database must be set as bellow.

Storage

Storage is meant as disc space with some commonly used filesystem. We'll use ext4, but the other ones should work too. Storage file structure is:

.
├── submits
│   └── user_id
│       └── advanced_dot_net_1
│           └── submit_id
│               ├── eval.yml
│               └── source.cs
├── submit_archives
│   └── submit_id.tar.gz
├── tasks
│   ├── a
│   │   ├── a014ed2abb56371bfaf2b4298a85d5dfb56509ed
│   │   └── a5edbd8b12e670ed1e3110d6c0524000cd4c3c7a
│   └── b
│       └── b1696358b8540923eb79b68f95c0f94c13a83fa7
└── temp
    └── 1795184136b8bdddabe50453cc2cc2d46f0f7c5e

• submits keep information about all files submited by users to ReCodEx. There are subdirectories user_id and advanced_dot_net_1 which groups submits by users and courses the submits are for. This structure is easy to maintain for new and deleted users.
• submit_archives contains the student submissions in compressed archives so that they can be easily downloaded by workers.
• tasks contains supplementary files (such as test inputs or helper programs) for all existing task in ReCodEx. To avoid too many files in one directory, files are separated to subfolders by first character of their name.
• temp directory is dedicated to temporary storing outputs of programs on teachers' demand. This directory will be erased by cron job on daily basis.

Database

For user friendly access and modifying tasks following information should be stored in database:

• list of tasks with their newest version number
• for every task and version list of used files (their hashed names)
• for every hash name one human readable filename

Conclusion

Files are internally stored by their sha1sum hashes, so it's easy to implement versioning and get rid of files with duplicate content (multiple files can have the same content, which is only stored once). Worker also uses files by their hashes, which is great for local caching without worries about actual version number of given file. On the other hand, Database stores information about human readable names, so that the files are presented in a friendly way to users (teachers) in WebApp.