**ReCodEx** is designed to be very modular and configurable. One such configuration is sketched in the following picture. There are two separate frontend instances with distinct databases sharing common backend part. This configuration may be suitable for MFF UK -- basic programming course and KSP competition. Note, that connections between components are not fully accurate.
**Web app** is main part of whole project from user point of view. It provides nice user interface and it 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. Advanced users may connect to the API directly or may create custom frontends. **Broker** is essential part of whole architecture. 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 its results. **Monitor** resends evaluation progress messages to the **Web app** in order to be presented to users.
Detailed communication inside the ReCodEx project is captured in the following image and described in sections below. Red connections are through ZeroMQ sockets, blue are through WebSockets and green are through HTTP(S). All ZeroMQ messages are sent as multipart with one string (command, option) per part, with no empty frames (unles explicitly specified otherwise).
Broker acts as server when communicating with worker. Listening IP address and port are configurable, protocol family is TCP. Worker socket is of DEALER type, broker one is ROUTER type. Because of that, very first part of every (multipart) message from broker to worker must be target worker's socket identity (which is saved on its **init** command).
This protocol proved great robustness in real world testing. Thus whole backend is really reliable and can outlive short term issues with connection without problems. Also, increasing delay of ping messages does not flood the network when there are problems. We experienced no issues since we are using this protocol.
Worker is communicating with file server only from _execution thread_. Supported protocol is HTTP optionally with SSL encryption (**recommended**, you can get free trusted DV certificate from [Let's Encrypt](https://letsencrypt.org/) authority if you have not one yet). If supported by server and used version of libcurl, HTTP/2 standard is also available. File server should be set up to require basic HTTP authentication and worker is capable to send corresponding credentials with each request.
Worker is cabable of 2 things -- download file and upload file. Internally, worker is using libcurl C library with very similar setup. In both cases it can verify HTTPS certificate (on Linux against system cert list, on Windows against downloaded one from CURL website during installation), support basic HTTP authentication, offer HTTP/2 with fallback to HTTP/1.1 and fail on error (returned HTTP status code is >= 400). Worker have list of credentials to all available file servers in its config file.
File server has its own internal directory structure, where all the files are stored. It provides simple REST API to get them or create new ones. File server does not provide authentication or secured connection by itself, but it is supposed to run file server as WSGI script inside a web server (like Apache) with proper configuration. Relevant commands for communication with workers:
- **GET /submission_archives/\<id\>.\<ext\>** -- gets an archive with submitted source code and corresponding configuration of this job evaluation
- **GET /tasks/\<hash\>** -- gets a file, common usage is for input files or reference result files
- **PUT /results/\<id\>.\<ext\>** -- upload archive with evaluation results under specified name (should be same _id_ as name of submission archive). On successful upload returns JSON `{ "result": "OK" }` as body of returned page.
If not specified otherwise, `zip` format of archives is used. Symbol `/` in API description is root of file server's domain. If the domain is for example `fs.recodex.org` with SSL support, getting input file for one task could look as GET request to `https://fs.recodex.org/tasks/8b31e12787bdae1b5766ebb8534b0adc10a1c34c`.
### Broker - Monitor communication
Broker communicates with monitor also through ZeroMQ over TCP protocol. Type of
- **eval** -- evaluate a job. Requires at least 4 frames:
-`job_id` -- identifier of this job (in ASCII representation -- we avoid endianness issues and also support alphabetic ids)
-`header` -- additional header describing worker capabilities. Format must be `header_name=value`, every header shall be in a separate message frame. There is no maximum limit on number of headers. There may be also no headers at all.
- **ack** -- this is first message which is sent back to frontend right after eval command arrives, basically it means "Hi, I am all right and am capable of receiving job requests", after sending this broker will try to find acceptable worker for arrived request
- **accept** -- broker is capable of routing request to a worker
File server has a REST API for interaction with other parts of ReCodEx. Description of communication with workers is in [File server side](#file-server-side) section. On top of that, there are other commands for interaction with the API:
- **GET /results/\<id\>.\<ext\>** -- download archive with evaluated results of job _id_
- **POST /submissions/\<id\>** -- upload new submission with identifier _id_. Expects that the body of the POST request uses file paths as keys and the content of the files as values. On successful upload returns JSON `{ "archive_path": <archive_url>, "result_path": <result_url> }` in response body. From _archive_path_ the submission can be downloaded (by worker) and corresponding evaluation results should be uploaded to _result_path_.
- **POST /tasks** -- upload new files, which will be available by names equal to `sha1sum` of their content. There can be uploaded more files at once. On successful upload returns JSON `{ "result": "OK", "files": <file_list> }` in response body, where _file_list_ is dictionary of original file name as key and new URL with already hashed name as value.
Web API calls these fileserver endpoints with standard HTTP requests. There are no special commands involved. There is no communication in opposite direction.
Monitor interacts with web application through WebSocket connection. Monitor acts as server and browsers are connecting to it. IP address and port are configurable. When client connects to the monitor, it sends a message with string representation of channel id (which messages are interested in, usually id of evaluating job). There can be multiple listeners per channel, even (shortly) delayed connections will receive all messages from the very beginning.
- there is no WebSocket connection for listed channel (job id) -- message is dropped
- there is active WebSocket connection for listed channel -- message is parsed into JSON format (see below) and send as string to that established channel. Messages for active connections are queued, so no messages are discarded even on heavy workload.
Provided web application runs as javascript client inside user's browser. It communicates with REST API on the server through standard HTTP requests. Documentation of the main REST API is in separate [document](https://recodex.github.io/api/) due to its extensiveness. Results are returned as JSON payload, which is simply parsed in web application and presented to the users.
