recodex-wiki/Architecture.md

# Overall Architecture
![Overall Architecture](https://github.com/ReCodEx/GlobalWiki/blob/master/images/Overall_Architecture.png)

**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 Architecture](https://github.com/ReCodEx/GlobalWiki/blob/master/images/Worker_Architecture.png)

**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](http://zeromq.org/). 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](https://github.com/ioi/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.

### Internal Worker architecture
![Internal Worker architecture](https://github.com/ReCodEx/GlobalWiki/blob/master/images/Worker_Internal_Architecture.png)

## File Server
![File Server Infrastructure](https://github.com/ReCodEx/GlobalWiki/blob/master/images/File_Server.png)

**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 actual version number of given file. On the other hand, **Database** keeps info about human readable names, so to users (teachers) in **WebApp** are files presented in a friendly way.