recodex-wiki/Architecture.md

Overall Architecture

ReCodEx is designed to be very modular. WebApp + File Server are one instance of the application. They contain almost all logic of the app including user management and authentication, storing and versioning tasks, counting and assigning points to users etc. One instance of the app can be connected to one or more Workers and one Worker can be connected to more instances of the WebApp. Worker is connected with WebApp through messaging queue.

Worker

Worker's main role is securely compile, run and evaluate given submit against model solutions provided by author of each task. It is logicaly divided into three objects:

• Message Frontend communicates with WebApp using messaging queue ZeroMQ. It receives new submits, operates the evaluation through Work API and reports progress back.
• Worker Core can do all evaluating steps and is responsible for security of them. Sandbox Isolate is used.
• File Server Frontend ensures via File API access to files on File Server, where are stored testing inputs and corresponding outputs for each task and other required files. It's possible to upload files, too.

File Server

File Server stores data, which is better keep out of WebApp's database. 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
│           ├── bf216fa9274261628f4d952a103c6cfd1cbbc587
│           └── e6ae49bbfda4a8bb57aceeb64fb117990b226ca5
├── tasks
│   ├── a
│   │   ├── a014ed2abb56371bfaf2b4298a85d5dfb56509ed
│   │   └── a5edbd8b12e670ed1e3110d6c0524000cd4c3c7a
│   └── b
│       └── b1696358b8540923eb79b68f95c0f94c13a83fa7
└── temp
    └── 1795184136b8bdddabe50453cc2cc2d46f0f7c5e

• submits keep information about all files submited by users to ReCodEx. There are subdirectoris user_id and advanced_dot_net_1 which divides submits by users and class they took part of. This structure is easy to maintain for new and deleted users.
• tasks contains all files for all existing task in ReCodEx. To avoid too much 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 duplicity content. Worker also uses files by their hashes, which is great for local caching without worries about akctual version of given file. On the other hand, Database keep info about human readable names, so to users (teachers) are files presented in a friendly way.