recodex-wiki/Rewritten-docs.md

# Introduction

Generally, there are many different ways and opinions on how to teach people something new. However, most people agree that a hands-on experience is one of the best ways to make the human brain remember a new skill. Learning must be entertaining and interactive, with fast and frequent feedback. Some kinds of knowledge are more suitable for this practical type of learning than others, and fortunately, programming is one of them.

University education system is one of the areas where this knowledge can be applied. In computer programming, there are several requirements such as the code being syntactically correct, efficient and easy to read, maintain and extend. Correctness and efficiency can be tested automatically to help teachers save time for their research, but reviewing bad design, bad coding habits and logical mistakes is really hard to automate and requires manpower.

Checking programs written by students takes a lot of time and requires a lot of mechanical, repetitive work. The first idea of an automatic evaluation system comes from Stanford University profesors in 1965. They implemented a system which evaluated code in Algol submitted on punch cards. In following years, many similar products were written.

There are two basic ways of automatically evaluating code -- statically (check the code without running it; safe, but not very precise) or dynamically (run the code on testing inputs with checking the outputs against reference ones; needs sandboxing, but provides good real world experience).

This project focuses on the machine-controlled part of source code evaluation. First, the problems of the software used at our university previously were discussed and similar projects at other educational institutions were examined. With acquired knowledge from such projects in production, we set up goals for the new evaluation system, designed the architecture and implemented a fully operational solution. The system is now ready for production testing at our university.

Assignment

The major goal of this project is to create a grading application that will be used for programming classes at the Faculty of Mathematics and Physics of the Charles University in Prague. However, the application should be designed in a modular fashion so that it can be easily extended or modified to make other ways of using it possible.

The project has a great starting point -- there is an old grading system currently used at the university (CodEx), so its flaws and weaknesses can be addressed. Furthermore, many teachers are willing to use and test the new system. Following requirements were collected both from our personal experience with CodEx and from teachers' requests.

### Basic grading system requirements:

These are the features which are necessary for any system for evaluation of programming coding assignments used in any university programming course:

• students can use an intuitive user interface for interaction with the system, mainly for viewing assigned exercises, uploading their own solutions to the assignments, and viewing the results of the solutions after an automatic evaluation is finished
• teachers can create exercises including textual description, sample inputs and correct reference outputs (for example "sum all numbers from given file and write the result to the standard output")
• teachers can assigning an existing exercise to their class with some specific properties set (deadlines, etc.)
• teachers can specify their scale of points which will be awarted to the students depending on the correctness of his/her solution (expressed in percentage points)
• teachers can view all of the solutions their students submitted and also the results of the evaluations and they can override the automatically assigned points to the solutions manually
• teachers can see the statistics of their classes and individual students of these claseese
• administrators can depend on a safe environment in which the students' solutions will be executed
• administrators can manage users with support of roles (at least two -- student and supervisor)

CodEx satisfies all these requirements and a few more that originate from the way courses are organized at our university -- for example, users have roles (student, supervisor and administrator) that determine their capabilities in the system and students are divided into groups that correspond to lab groups.

However, further requirements arose during the ten year long lifetime of the old system. There are not many ways to improve it from the perspective of a student, but a lot of feature requests came from administrators and supervisors. The ideas were mostly gathered from meetings with faculty staff involved with the current system.

### Requested features for the new system:

• logging in through a university authentication system (e.g. LDAP)
• support for multiple programming environments at once to avoid unacceptable workload for administrator (maintain separate installations for many courses) and high hardware occupation
• localization (both UI and exercises)
• Markdown support for exercise texts
• tagging exercises and search by tags
• comments, comments, comments (exercises, tests, solutions, ...)
• edit student solution and privately resubmit it
• resubmit solution with saving all results (including temporary ones)
• mark one student's solution as accepted (used for grading this assignment)
• web and command-line submission tool
• SIS (university information system) integration for fetching personal user data
• plagiarism detection
• advanced low-level evaluation flow configuration with high-level abstraction layer for ordinary configuration cases
• use of modern technologies with state-of-the-art compilers

The survey shows that the system is used in many different ways, but the core functionality is the same for all of them. When the system is ready it is likely that there will be new ideas of how to use the system and thus the system must be designed to be easily extendable, so everyone can develop their own feature. This also means that widely used programming languages and techniques should be used, so users can quickly understand the code and make changes.

To find out the current state in the field of automatic grading systems we did a short survey at universities, programming contests, and other available tools.

Related work

This is not a complete list of available evaluators, but only a few projects which are used these days and can be an inspiration for our project. Each project from the list has a brief description and some key features mentioned.

CodEx

Currently used grading solution at the Faculty of Mathematics and Physics of the Charles University in Prague which was implemented in 2006 by a group of students. It is called CodEx -- The Code Examiner and it has been used with some improvements since then. The original plan was to use the system only for basic programming courses, but there was a demand for adapting it for many different subjects.

CodEx is based on dynamic analysis. It features a web-based interface, where supervisors can assign exercises to their students and the students have a time window to submit their solutions. Each solution is compiled and run in sandbox (MO-Eval). The metrics which are checked are: corectness of the output, time and memory limits. It supports programs written in C, C++, C#, Java, Pascal, Python and Haskell.

Current system is old, but robust. There were no major security incidents during its production usage. However, from today's perspective there are several drawbacks. The main ones are:

• web interface -- The web interface is simple and fully functional. But rapid development in web technologies opens new horizons of how web interface can be made.
• web API -- CodEx offers a very limited XML API based on outdated technologies that is not sufficient for users who would like to create custom interfaces such as a command line tool or mobile application.
• sandboxing -- MO-Eval sandbox is based on principle of monitoring system calls and blocking the bad ones. This can be easily done for single-threaded applications, but proves difficult with multi-threaded ones. In present day, parallelism is a very important area of computing, so there is requirement to test multi-threaded applications too.
• instances -- Different ways of CodEx usage scenarios requires separate instances (Programming I and II, Java, C#, etc.). This configuration is not user friendly (students have to register in each instance separately) and burdens administrators with unnecessary work. CodEx architecture does not allow sharing hardware between instances, which results in an inefficient use of hardware for evaluation.
• task extensibility -- There is a need to test and evaluate complicated programs for classes such as Parallel programming or Compiler principles, which have a more difficult evaluation chain than simple compilation/execution/evaluation provided by CodEx.

Progtest

Progtest is private project of FIT ČVUT in Prague. As far as we know it is used for C/C++, Bash programming and knowledge-based quizzes. There are several bonus points and penalties and also a few hints what is failing in the submitted solution. It is very strict on source code quality, for example -pedantic option of GCC, Valgrind for memory leaks or array boundaries checks via mudflap library.

Codility

Codility is a web based solution primary targeted to company recruiters. It is a commercial product available as a SaaS and it supports 16 programming languages. The UI of Codility is opensource, the rest of source code is not available. One interesting feature is 'task timeline' -- captured progress of writing code for each user.

CMS

CMS is an opensource distributed system for running and organizing programming contests. It is written in Python and contains several modules. CMS supports C/C++, Pascal, Python, PHP, and Java programming languages. PostgreSQL is a single point of failure, all modules heavily depend on the database connection. Task evaluation can be only a three step pipeline -- compilation, execution, evaluation. Execution is performed in Isolate, sandbox written by the consultant of our project, Mgr. Martin Mareš, Ph.D.

MOE

MOE is a grading system written in Shell scripts, C and Python. It does not provide a default GUI interface, all actions have to be performed from command line. The system does not evaluate submissions in real time, results are computed in batch mode after exercise deadline, using Isolate for sandboxing. Parts of MOE are used in other systems like CodEx or CMS, but the system is generally obsolete.

Kattis

Kattis is another SaaS solution. It provides a clean and functional web UI, but the rest of the application is too simple. A nice feature is the usage of a standardized format for exercises. Kattis is primarily used by programming contest organizators, company recruiters and also some universities.

ReCodEx goals

None of the existing systems we came across is capable of all the required features of the new system. There is no grading system which is designed to support a complicated evaluation pipeline, so this part is an unexplored field and has to be designed with caution. Also, no project is modern and extensible so it could be used as a base for ReCodEx. After considering all these facts, it was clear that a new system has to be written from scratch. This implies, that only a subset of all the features will be implemented in the first version, the other in the following ones.

Gathered features are categorized based on priorities for the whole system. The highest priority has main functionality similar to current CodEx. It is a base line to be useful in production environment, but a new design allows to easily develop further. On top of that, most of ideas from faculty staff belongs to second priority bucket, which will be implemented as part of the project. Most advanced tasks from this category are advanced low-level evaluation configuration format, using modern tools, connecting to a university systems and merging separate system instances into single one. Other tasks are scheduled for next releases after successful project defense. Namely, these are high-level exercise evaluation configuration with user-friendly interface for common exercise types, SIS integration (when some API will be available from their side) and command-line submit tool. Plagiarism detection is not likely to be part of any release in near future unless someone other makes the engine. The detection problem is too hard to be solved as part of this project.

We named the project as ReCodEx -- ReCodEx Code Examiner. The name should point to the old CodEx, but also reflect the new approach to solve issues. Re as part of the name means redesigned, rewritten, renewed, or restarted.

At this point there is a clear idea how the new system will be used and what are the major enhancements for future releases. With this in mind, the overall architecture can be sketched. From the previous research, we set up several goals, which the new system should have. They mostly reflect drawbacks of the current version of CodEx and some reasonable wishes of university users. Most notable features are following:

• modern HTML5 web frontend written in JavaScript using a suitable framework
• REST API implemented in PHP, communicating with database, evaluation backend and a file server
• evaluation backend implemented as a distributed system on top of a message queue framework (ZeroMQ) with master-worker architecture
• worker with basic support of the Windows environment (without sandbox, no general purpose suitable tool available yet)
• evaluation procedure configured in a YAML file, compound of small tasks connected into an arbitrary oriented acyclic graph

Intended usage

The whole system is intended to help both teachers (supervisors) and students. To achieve this, it is crucial to keep in mind typical usage scenarios of the system and try to make these typical tasks as simple as possible.

The system has a database of users. Each user has a role assigned, which correspond to his/her privileges. User can be logged in via email and password or using the university system. There are groups of users, which corresponds to the lectured courses. Groups can be hierarchically ordered to reflect additional metadata such as the academic year. For example, a reasonable group hierarchy can look like this:

Summer term 2016
|-- Language C# and .NET platform
|   |-- Labs Monday 10:30
|   `-- Labs Thursday 9:00
|-- Programming I
|   |-- Labs Monday 14:00
 ...

In this example, students are members of the leaf groups, the higher level groups are just for keeping the related groups together. The hierarchy tree can be modified and altered to fit specific needs of the university or any other organization, even the flat structure (i.e., no hierarchy) is possible.

One user can be part of multiple groups and also one group can of course have multiple users. Each user in a group has also a specific role for the given group. Priviledged user (supervisor) can assign a new exercise in his/her group, change assignment details, view results of other users and manually change them. Normal user (student) can join a group, get list of assigned exercises, view assignment detail, submit his/her solution and view the results of the evaluation.

Database of exercises (algorithmic problems) is another part of the project. Each exercise consists of a text in multiple language variants, an evaluation configuration and a set of inputs and reference outputs. Exercises are created by instructed priviledged users. Assigning an exercise to a group means to choose one of the available exercises and specifying additional properties. An assignment has a deadline (optionally a second deadline), a maximum amount of points, a configuration for calculating the final score, a maximum number of submissions, and a list of supported runtime environemnts (e.g., programming languages) including specific time and memory limits for the sandboxed tasks.

Exercise evaluation chain

The most important part of the system is the evaluation of the solutions submitted by the users for their assigned exercises. Concepts of consecutive steps from source code of solution to results is described on architecture with two layer -- presentation (frontend) and executive (backend).

First thing users have to do is to submit their solutions to frontend which provides interface to upload files and then submit them. It checks the assignment invariants (deadlines, count of submissions, ...) and stores submitted files. The runtime environment is automatically detected based on input files and suitable exercise configuration variant is chosen (one exercise can have multiple variants, for example C and Java languages). Matching exercise configuration is then send to backend alongside solution source files.

Backend can have multiple engines to allow processing more jobs in parallel and a loadbalancer, which tracks states of incoming jobs and performs scheduling of them. The decission is made based on capabilities of each engine and also job requirements. When a match is found, the job is held until the particular engine is jobless and can receive an evaluation request.

Job processing itself stars with obtaining source files and job configuration. The configuration is parsed into small tasks with simple piece of work. Evaluation itself goes in direction of tasks ordering. It is crucial to keep executive computer secure and stable, so isolated sandboxed environment is used when dealing with unknown source code. When the execution is finished, results are uploaded back to frontend.

The frontend is immediately notified about finished job. The outcomes are parsed and results of important tasks (comparing actual and expected results) saved into storage. Also, points are calculated depending on solution correctness and assignment configuration. Data presented back to users includes overview which part succeeded and which failed (optionally with reason like "memory limit exceeded") and amount of awarded points.

Analysis

Solution concepts analysis

@todo: what problems were solved on abstract and high levels, how they can be solved and what was the final solution

• which problems are they? ... these ones below:
• what type of users there should be, why they are needed
• explain why there is exercise and assignment division, what means what and how they are used
• explain instances why they are usefull what they solve and also discuss licences concept
• groups, they can be public and private and why is that, what it solves, explain amd discuss treshold and other group features
• extended execution pipeline (not just compilation/execution/evaluation) and why it is needed
• progress state, how it can be done and displayed to user, why random messages
• how to display generally all outputs of executed programs to user (supervisor, student), what students can or cannot see and why
• judges, discuss what they possibly can do and what it can be used for (returning for instance 2 numbers instead of 1 and why we return just one)
• discuss points assigned to solution, why are there bonus points, explain minimal point threshold
• discuss several ways how points can be assigned to solution, propose basic systems but also general systems which can use outputs from judges or other executed programs, there is need for variables or other concept, explain why
• and many many more general concepts which can be discussed and solved... please append more of them if something comes to your mind... thanks

Structure of the project

The ReCodEx project is divided into two logical parts – the Backend and the Frontend – which interact which each other and which cover the whole area of code examination. Both of these logical parts are independent of each other in the sense of being installed on separate machines at different locations and that one of the parts can be replaced with a different implementation and as long as the communication protocols are preserved, the system will continue working as expected.

Backend is the part which is responsible solely for the process of evaluation a solution of an exercise. Each evaluation of a solution is referred to as a job. For each job, the system expects a configuration document of the job, supplementary files for the exercise (e.g., test inputs, expected outputs, predefined header files), and the solution of the exercise (typically source codes created by a student). There might be some specific requirements for the job, such as a specific runtime environment, specific version of a compiler or the job must be evaluated on a processor with a specific number of cores. The backend infrastructure decides whether it will accept a job or decline it based on the specified requirements. In case it accepts the job, it will be placed in a queue and it will be processed as soon as possible. The backend publishes the progress of processing of the queued jobs and the results of the evaluations can be queried after the job processing is finished. The backend produces a log of the evaluation and scores the solution based on the job configuration document.

Frontend on the other hand is responsible for the communication with the users and provides them a convenient access to the Backend infrastructure. The Frontend manages user accounts and gathers them into units called groups. There is a database of exercises which can be assigned to the groups and the users of these groups can submit their solutions for these assignments. The Frontend will initiate evaluation of these solutions by the Backend and it will store the results afterwards. The results will be visible to authorized users and the results will be awarded with points according to the score given by the Backend in the evaluation process. The supervisors of the groups can edit the parameters of the assignments, review the solutions and the evaluations in detail and award the solutions with bonus points (both positive and negative) and discuss about the solution with the author of the solution. Some of the users can be entitled to create new exercises and extend the database of exercises which can be assigned to the groups later on.

The Frontend developed as part of this project was created with the needs of the Faculty of Mathematics and Physics of the Charles university in Prague in mind. The users are the students and their teachers, groups correspond to the different courses, the teachers are the supervisors of these groups. We believe that this model is applicable to the needs of other universities, schools, and IT companies, which can use the same system for their needs. It is also possible to develop their own frontend with their own user management system for their specific needs and use the possibilities of the Backend without any changes, as was mentioned in the previous paragraphs.

In the latter parts of the documentation, both of the Backend and Frontend parts will be introduced separately and covered in more detail. The communication protocol between these two logical parts will be described as well.

@todo: move "General backend implementation" here

@todo: move "General frontend implementation" here

Evaluation unit executed on backend

@todo: describe possibilities of "piece of work" which can backend execute, how they can look like, describe our job and its tasks

@todo: why is there division to internal and external tasks and why it is needed

@todo: division to initiation, execution and evaluation why and what for

@todo: in what order should tasks be executed, how to sort them

@todo: how to solve problem with specific worker environment, mention internal job variables

Implementation analysis

When developing a project like ReCodEx there has to be some discussion over implementation details and how to solve some particular problems properly. This discussion is a never ending story which is done through the whole development process. Some of the most important implementation problems or interesting observations will be discussed in this chapter.

General backend implementation

There are numerous ways how to divide some sort of system into separated services, from one single component to many and many single-purpose components. Having only one big service is not feasible, not scalable enough and mainly it would be one big blob of code which somehow works and is very complex, so this is not the way. The quite opposite, having a lot of single-purpose components is also somehow impractical. It is scalable by default and all services would have quite simple code but on the other hand communication requirements for such solution would be insane. So there has to be chosen approach which is somehow in the middle, that means services have to communicate in manner which will not bring network down, code basis should be reasonable and the whole system has to be scalable enough. With this being said there can be discussion over particular division for ReCodEx system.

From the scalable point of view there are two necessary components, the one which will execute jobs and component which will distribute jobs to the instances of the first one. This ensures scalability in manner of parallel execution of numerous jobs which is exactly what is needed. Implementation of these services are called 'broker' and 'worker', first one handles distribution, latter execution. These components should be enough to fulfil all above said, but for the sake of simplicity and better communication gateways with frontend two other components were added, 'fileserver' and 'monitor'. Fileserver is simple component whose purpose is to store files which are exchanged between frontend and backend. Monitor is also quite simple service which is able to serve job progress state from worker to web application. These two additional services are on the edge of frontend and backend (like gateways) but logically they are more connected with backend, so it is considered they belong there.

@todo: what type of communication within backend could be used, mention some frameworks, queue managers, protocols, which was considered

Fileserver

@todo: fileserver and why is separated

@todo: mention hashing on fileserver and why this approach was chosen

@todo: what can be stored on fileserver

@todo: how can jobs be stored on fileserver, mainly mention that it is nonsence to store inputs and outputs within job archive

Broker

@todo: assigning of jobs to workers, which are possible algorithms, queues, which one was chosen

@todo: how can jobs be sent over zeromq, mainly mention that files can be transported, but it is not feasible

@todo: making action and reaction over zeromq more general and easily extensible, mention reactor and why is needed and what it solves

Worker

Worker is component which is supposed to execute incoming jobs from broker. As such worker should work and support wide range of different infrastructures and maybe even platforms/operating systems. Support of at least two main operating systems is desirable and should be implemented. Worker as a service does not have to be much complicated, but a bit of complex behaviour is needed. Mentioned complexity is almost exclusively concerned about robust communication with broker which has to be regularly checked. Ping mechanism is usually used for this in all kind of projects. This means that worker should be able to send ping messages even during execution. So worker has to be divided into two separate parts, the one which will handle communication with broker and the another which will execute jobs. The easiest solution is to have these parts in separate threads which somehow tightly communicates with each other. For inner process commucation there can be used numerous technologies, from shared memory to condition variables or some kind of in-process messages. Already used library ZeroMQ is possible to provide in-process messages working on the same principles as network communication which is quite handy and solves problems with threads synchronization and such.

At this point we have worker with two internal parts listening one and execution one. Implementation of first one is quite straighforward and clear. So lets discuss what should be happening in execution subsystem. Jobs as work units can quite vary and do completely different things, that means configuration and worker has to be prepared for this kind of generality. Configuration and its solution was already discussed above, implementation in worker is then quite straightforward. Worker has internal structures to which loads and which stores metadata given in configuration. Whole job is mapped to job metadata structure and tasks are mapped to either external ones or internal ones (internal commands has to be defined within worker), both are different whether they are executed in sandbox or as internal worker commands.

@todo: task types: initiation, execution, evaluation and inner... mainly describe what is happening if inner task fails

@todo: maybe describe folders within execution and what they can be used for?

After successful arrival of job, worker has to prepare new execution environment, then solution archive has to be downloaded from fileserver and extracted. Job configuration is located within these files and loaded into internal structures and executed. After that results are uploaded back to fileserver. These steps are the basic ones which are really necessary for whole execution and have to be executed in this precise order.

Interesting problem is with supplementary files (inputs, sample outputs). There are two approaches which can be observed. Supplementary files can be downloaded either on the start of the execution or during execution. If the files are downloaded at the beginning execution does not really started at this point and if there are problems with network worker find it right away and can abort execution without executing single task. Slight problems can arise if some of the files needs to have same name (e.g. solution assumes that input is input.txt), in this scenario downloaded files cannot be renamed at the beginning but during execution which is somehow impractical and not easily observed. Second solution of this problem when files are downloaded on the fly has quite opposite problem, if there are problems with network worker will find it during execution when for instance almost whole execution is done, this is also not ideal solution if we care about burnt hardware resources. On the other hand using this approach users have quite advanced control of execution flow and know what files exactly are available during execution which is from users perspective probably more appealing then the first solution. Based on that downloading of supplementary files using 'fetch' tasks during execution was chosen and implemented.

Caching mechanism

As described in fileserver section stored supplementary files have special filenames which reflects hashes of their content. As such there are no duplicates stored in fileserver. Worker can use feature too and caches these files for some while and saves precious bandwith. This means there has to be system which can download file, store it in cache and after some time of inactivity delete it. Because there can be multiple worker instances on some particular server it is not efficient to have this system in every worker on its own. So it is feasible to have this feature somehow shared among all workers on the same machine. Solution may be again having separate service connected through network with workers which would provide such functionality but this would mean component with another communication for the purpose where it is not exactly needed. But mainly it would be single-failure component if it would stop working it is quite problem. So there was chosen another solution which assumes worker has access to specified cache folder, to this folder worker can download supplementary files and copy them from here. This means every worker has the \possibility to maintain downloads to cache, but what is worker not able to properly do is deletion of unused files after some time. For that single-purpose component is introduced which is called 'cleaner'. It is simple script executed within cron which is able to delete files which were unused for some time. Together with worker fetching feature cleaner completes machine specific caching system.

Cleaner as mentioned is simple script which is executed regularly as cron job. If there is caching system like it was introduced in paragraph above there are little possibilities how cleaner should be implemented. On various filesystems there is usually support for two particular timestamps, last access time and last modification time. Files in cache are once downloaded and then just copied, this means that last modification time is set only once on creation of file and last access time should be set every time on copy. This imply last access time is what is needed here. But last modification time is widely used by operating systems, on the other hand last access time is not by default. More on this subject can be found here. For proper cleaner functionality filesystem which is used by worker for caching has to have last access time for files enabled.

Having cleaner as separated component and caching itself handled in worker is kind of blury and is not clearly observable that it works without any race conditions. The goal here is not to have system without races but to have system which can recover from them. Implementation of caching system is based upon atomic operations of underlying filesystem. Follows description of one possible robust implementation. First start with worker implementation:

• worker discovers fetch task which should download supplementary file
• worker takes name of file and tries to copy it from cache folder to its working folder
  • if successful then last access time should be rewritten (by filesystem itself) and whole operation is done
  • if not successful then file has to be downloaded
    • file is downloaded from fileserver to working folder
    • downloaded file is then copied to cache

Previous implementation is only within worker, cleaner can anytime intervene and delete files. Implementation in cleaner follows:

• cleaner on its start stores current reference timestamp which will be used for comparision and load configuration values of caching folder and maximal file age
• there is a loop going through all files and even directories in specified cache folder
  • last access time of file or folder is detected
  • last access time is subtracted from reference timestamp into difference
  • difference is compared against specified maximal file age, if difference is greater, file or folder is deleted

Previous description implies that there is gap between detection of last access time and deleting file within cleaner. In the gap there can be worker which will access file and the file is anyway deleted but this is fine, file is deleted but worker has it copied. Another problem can be with two workers downloading the same file, but this is also not a problem file is firstly downloaded to working folder and after that copied to cache. And even if something else unexpectedly fails and because of that fetch task will fail during execution even that should be fine. Because fetch tasks should have 'inner' task type which implies that fail in this task will stop all execution and job will be reassigned to another worker. It should be like the last salvation in case everything else goes wrong.

Sandboxing

@todo: sandboxing, what possibilites are out there (Linux, Windows), what are general and really needed features, mention isolate, what are isolate features

Monitor

Users want to view real time evaluation progress of their solution. It can be easily done with established double-sided connection stream, but it is hard to achive with web technologies. HTTP protocol works differently on separate requests basis with no longterm connection. However, there is widely used technology to solve this problem, WebSocket protocol.

Working with WebSocket protocol from the backend is possible, but not ideal from design point of view. Backend should be hidden from public internet to minimize surface for possible attacks. With this in mind, there are two possible options:

• send progress messages through API
• make separate component for progress messages

Each of the two possibilities has some pros and cons. The first one is good beacuse there is no additional component and API is already publicly visible. On the other side, working with WebSocket protocol from PHP is not much pleasant (but it is possible) and embedding this functionality into API is not extendable. The second approach is better for future changing the protocol or implementing extensions like caching of messages. Also, the progress feature is considered only optional, because there may be clients for which this feature is useless. Major drawback of separate component is another part, which needs to be publicly exposed.

We decided to make a separate component, mainly because it is smaller component with only one role, better maintainability and optional demands for progress callback.

There are several possibilities how to write the component. Notably, considered options were already used languages C++, PHP, JavaScript and Python. At the end, the Python language was chosen for its simplicity, great support for all used technologies and also there are free Python developers in out team. Then, responsibility of this component is determined. Concept of message flow is on following picture.

The message channel inputing the monitor uses ZeroMQ as main message framework used by backend. This decission keeps rest of backend avare of used communication protocol and related libraries. Output channel is WebSocket as a protocol for sending messages to web browsers. In Python, there are several WebSocket libraries. The most popular one is websockets in cooperation with asyncio. This combination is easy to use and well documented, so it is used in monitor component too. For ZeroMQ, there is zmq library with binding to framework core in C++.

Incomming messages are cached for short period of time. Early testing shows, that backend can start sending progress messages sooner than client connects to the monitor. To solve this, messages for each job are hold 5 minutes after reception of last message. The client gets all already received messages at time of connection with no message loss.

General frontend implementation

The frontend is the part of the whole system, which is directly available to the user. It must hold the whole state of the system - the login credentials, users' data, hierarchy of groups, available exercises, exercises assignments, users' solutions, and much more. User must be able to access and modify this state through some kind of an user interface (e.g., a web application, mobile application, or a command-line tool). We want to implement a web application as part of this project, but we want to make it possible for other developers to contribute to the platform and to create their specific clients for their specific needs.

We decided to split the frontend in two logical parts: a server side and a client side. The server side is responsible for managing the state and the client side gives instructions to the server side based on the inputs from the user. This decoupling gives us the ability to create multiple client side tools which may address different needs of the users.

The first thing we need to address is the communication protocol of this client-server architecture. There are several options:

• TCP sockets -- TCP sockets give a reliable means of a full-duplex communication. All major operating systems support this protocol and there are libraries which simplify the implementation. On the other side, it is not possible to initiate a TCP socket from a web browser.
• WebSockets -- The WebSocket standard is built on top of TCP. It enables a web browser to connect to a server over a TCP socket. WebSockets are implemented in recent versions of all modern web browsers and there are libraries for several programming languages like Python or JavaScript (running in Node.js). Encryption of the communication over a WebSocket is supported as a standard.
• HTTP protocol -- The HTTP protocol is a state-less protocol implemented on top of the TCP protocol. The communication between the client and server consists of a requests sent by the client and reponses to these requests sent back by the sever. The client can send as many requests as needed and it may ignore the responses from the server, but the server must respond only to the requests of the client and it cannot initiate communication on its own. End-to-end encryption can be achieved easily using SSL (HTTPS).

We chose the HTTP(S) protocol because of the simple implementation in all sorts of operating systems and runtime environments on both the client and the server side.

The API of the server should expose basic CRUD (Create, Read, Update, Delete) operations. There are some options on what kind of messages to send over the HTTP:

• SOAP -- a protocol for exchanging XML messages. It is very robust and complex.
• REST -- is a stateless architecture style, not a protocol or a technology. It relies on HTTP (but not necessarily) and its method verbs (e.g., GET, POST, PUT, DELETE). It can fully implement the CRUD operations.

Even though there are some other technologies we chose the REST style over the HTTP protocolo. It is widely used, there are many tools available for development and testing, and it is understood by programmers so it should be easy for a new developer with some experience in client-side applications to get to know with the ReCodEx API and develop a client application.

API server

The API server must handle HTTP requests and manage the state of the application in some kind of a database. It must also be able to communicate with the backend over ZeroMQ.

We considered several technologies which could be used:

• PHP + Apache -- one of the most widely used technologies for creating web servers. It is a suitable technology for this kind of a project. It has all the features we need when some additional extensions are installed (to support LDAP or ZeroMQ).
• ASP.NET (C#), JSP (Java) -- these technologies are very robust and are used to create server technologies in many big enterpises. Both can run on Windows and Linux servers (ASP.NET using the .NET Core).
• JavaScript (Node.js) -- it is a quite new technology and it is being used to create REST APIs lately. Applications running on Node.js are quite performant and the number of open-source libraries avialble on the Internet is very huge.

We chose PHP and Apache mainly because we were familiar with these technologies and we were able to develop all the features we needed without learning to use a new technology. Since the number of features was quite high and needed to meet a strict deadline. This does not mean that we would find all the other technologies superior to PHP in all other aspects - PHP 7 is a mature language with a huge comunity and a wide range of tools, libraries, and frameworks.

We decided to use an ORM framework to manage the database, namely the widely used PHP ORM Doctrine 2. This framework has a robust abstraction layer DBAL so the database engine is not very important and it can be changed without any need for changing the code. We chose an open-source database MariaDB.

To speed up the development process of the PHP server application we decided to use an MVC framework. After evaluating and trying several frameworks, such as Lumen, Laravel, and Symfony, we ended up using the framework Nette. This framework is very common in the Czech Republic -- its main developer is a well-known Czech programmer David Grudl -- and we were already familiar with the patterns used in this framework (e.g., dependency injection, authentication, routing). There is a good extension for the Nette framework which makes usage of Doctrine 2 very straighforward.

@todo: what database can be used, how it is mapped and used within code

@todo: authentication, some possibilities and describe used jwt

@todo: solution of forgotten password, why this in particular

@todo: rest api is used for report of backend state and errors, describe why and other possibilities (separate component)

@todo: what files are stored in api, why there are duplicates among api and fileserver

@todo: why are there instances and for which they can be used for, describe licences and its implementation

@todo: groups and hierarchy, describe arbitrary nesting which should be possible within instance and how it is implemented and how it could be implemented

@todo: where is stored which workers can be used by supervisors and which runtimes are available, describe possibilities and why is not implemented automatic solution

@todo: on demand loading of students submission, in-time loading of every other submission, why

Web-app

@todo: what technologies can be used on client frontend side, why react was used

@todo: please think about more stuff about api and web-app... thanks ;-)

# The Backend

The backend is the part which is hidden to the user and which has only one purpose: evaluate user’s solutions of their assignments.

@todo: describe the configuration inputs of the Backend

@todo: describe the outputs of the Backend

@todo: describe how the backend receives the inputs and how it communicates the results

Components

Whole backend is not just one service/component, it is quite complex system on its own.

@todo: describe the inner parts of the Backend (and refer to the Wiki for the technical description of the components)

Broker

@todo: gets stuff done, single point of failure and center point of ReCodEx universe

Fileserver

@todo: stores particular data from frontend and backend, hashing, HTTP API

Worker

@todo: describe a bit of internal structure in general

@todo: describe how jobs are generally executed

Monitor

@todo: not necessary component which can be ommited, proxy-like service

Backend internal communication

@todo: internal backend communication, what communicates with what and why

The Frontend

The frontend is the part which is visible to the user of ReCodEx and which holds the state of the system – the user accounts, their roles in the system, the database of exercises, the assignments of these exercises to groups of users (i.e., students), and the solutions and evaluations of them.

Frontend is split into three parts:

• the server-side REST API (“API”) which holds the business logic and keeps the state of the system consistent

• the relational database (“DB”) which persists the state of the system

• the client side application (“client”) which simplifies access to the API for the common users

The centerpiece of this architecture is the API. This component receives requests from the users and from the Backend, validates them and modifies the state of the system and persists this modified state in the DB.

We have created a web application which can communicate with the API server and present the information received from the server to the user in a convenient way. The client can be though any application, which can send HTTP requests and receive the HTTP responses. Users can use general applications like cURL, Postman, or create their own specific client for ReCodEx API.

Frontend capabilities

@todo: describe what the frontend is capable of and how it really works, what are the limitations and how it can be extended

Terminology

This project was created for the needs of a university and this fact is reflected into the terminology used throughout the Frontend. A list of important terms’ definitions follows to make the meaning unambiguous.

User and user roles

User is a person who uses the application. User is granted access to the application once he or she creates an account directly through the API or the web application. There are several types of user accounts depending on the set of permissions – a so called “role” – they have been granted. Each user receives only the most basic set of permissions after he or she creates an account and this role can be changed only by the administrators of the service:

• Student is the most basic role. Student can become member of a group and submit his solutions to his assignments.

• Supervisor can be entitled to manage a group of students. Supervisor can assign exercises to the students who are members of his groups and review their solutions submitted to these assignments.

• Super-admin is a user with unlimited rights. This user can perform any action in the system.

There are two implicit changes of roles:

• Once a student is added to a group as its supervisor, his role is upgraded to a supervisor role.

• Once a supervisor is removed from the lasts group where he is a supervisor then his role is downgraded to a student role.

These mechanisms do not prevent a single user being a supervisor of one group and student of a different group as supervisors’ permissions are superset of students’ permissions.

Login

Login is a set of user’s credentials he must submit to verify he can be allowed to access the system as a specific user. We distinguish two types of logins: local and external.

• Local login is user’s email address and a password he chooses during registration.

• External login is a mapping of a user profile to an account of some authentication service (e.g., CAS).

Instance

An instance of ReCodEx is in fact just a set of groups and user accounts. An instance should correspond to a real entity as a university, a high-school, an IT company or an HR agency. This approach enables the system to be shared by multiple independent organizations without interfering with each other.

Usage of the system by the users of an instance can be limited by possessing a valid license. It is up to the administrators of the system to determine the conditions under which they will assign licenses to the instances.

Group

Group corresponds to a school class or some other unit which gathers users who will be assigned the same set exercises. Each group can have multiple supervisors who can manage the students and the list of assignments.

Groups can form a tree hierarchy of arbitrary depth. This is inspired by the hierarchy of school classes belonging to the same subject over several school years. For example, there can be a top level group for a programming class that contains subgroups for every school year. These groups can then by divided into actual student groups with respect to lab attendance. Supervisors can create subgroups of their groups and further manage these subgroups.

Exercise

An exercise consists of textual assignment of a task and a definition of how a solution to this exercise should be processed and evaluated in a specific runtime environment (i.e., how to compile a submitted source code and how to test the correctness of the program). It is a template which can be instantiated as an assignment by a supervisor of a group.

Assignment

An assignment is an instance of an exercise assigned to a specific group. An assignment can modify the text of the task assignment and it has some additional information which is specific to the group (e.g., a deadline, the number of points gained for a correct solution, additional hints for the students in the assignment). The text of the assignment can be edited and supervisors can translate the assignment into another language.

Solution

A solution is a set of files which a user submits to a given assignment.

Submission

A submission corresponds to a solution being evaluated by the Backend. A single solution can be submitted repeatedly (e.g., when the Backend encounters an error or when the supervisor changes the assignment).

Evaluation

An evaluation is the processed report received from the Backend after a submission is processed. Evaluation contains points given to the user based on the quality of his solution measured by the Backend and the settings of the assignment. Supervisors can review the evaluation and add bonus points (both positive and negative) if the student deserves some.

Runtime environment

A runtime environment defines the used programming language or tools which are needed to process and evaluate a solution. Examples of a runtime environment can be:

• Linux + GCC
• Linux + Mono
• Windows + .NET 4
• Bison + Yacc

Limits

A correct solution of an assignment has to pass all specified tests (mostly checks that it yields the correct output for various inputs) and typically must also be effective in some sense. The Backend measures the time and memory consumption of the solution while running. This consumption of resources can be limited and the solution will receive fewer points if it exceeds the given limits in some test cases defined by the exercise.

User management

@todo: roles and their rights, adding/removing different users, how the role of a specific user changes

Instances and hierarchy of groups

@todo: What is an instance, how to create one, what are the licenses and how do they work. Why can the groups form hierarchies and what are the benefits – what it means to be an admin of a group, hierarchy of roles in the group hierarchy.

Exercises database

@todo: How the exercises are stored, accessed, who can edit what

Creating a new exercise

@todo Localized assignments, default settings

Runtime environments and hardware groups

@todo read this later and see if it still makes sense

ReCodEx is designed to utilize a rather diverse set of workers -- there can be differences in many aspects, such as the actual hardware running the worker (which impacts the results of measuring) or installed compilers, interpreters and other tools needed for evaluation. To address these two examples in particular, we assign runtime environments and hardware groups to exercises.

The purpose of runtime environments is to specify which tools (and often also operating system) are required to evaluate a solution of the exercise -- for example, a C# programming exercise can be evaluated on a Linux worker running Mono or a Windows worker with the .NET runtime. Such exercise would be assigned two runtime environments, Linux+Mono and Windows+.NET (the environment names are arbitrary strings configured by the administrator).

A hardware group is a set of workers that run on similar hardware (e.g. a particular quad-core processor model and a SSD hard drive). Workers are assigned to these groups by the administrator. If this is done correctly, performance measurements of a submission should yield the same results. Thanks to this fact, we can use the same resource limits on every worker in a hardware group. However, limits can differ between runtime environments -- formally speaking, limits are a function of three arguments: an assignment, a hardware group and a runtime environment.

Reference solutions

@todo: how to add one, how to evaluate it

The task of determining appropriate resource limits for exercises is difficult to do correctly. To aid exercise authors and group supervisors, ReCodEx supports assigning reference solutions to exercises. Those are example programs that should cover the main approaches to the implementation. For example, searching for an integer in an ordered array can be done with a linear search, or better, using a binary search.

Reference solutions can be evaluated on demand, using a selected hardware group. The evaluation results are stored and can be used later to determine limits. In our example problem, we could configure the limits so that the linear search-based program doesn't finish in time on larger inputs, but a binary search does.

Note that separate reference solutions should be supplied for all supported runtime environments.

Exercise assignments

@todo: Creating instances of an exercise for a specific group of users, capabilities of settings. Editing limits according to the reference solution.

Evaluation process

@todo: How the evaluation process works on the Frontend side.

Uploading files and file storage

@todo: One by one upload endpoint. Explain different types of the Uploaded files.

Automatic detection of the runtime environment

@todo: Users must submit correctly named files – assuming the RTE from the extensions.

REST API implementation

@todo: What is the REST API, what are the basic principles – GET, POST, Headers, JSON.

Authentication and authorization scopes

@todo: How authentication works – signed JWT, headers, expiration, refreshing. Token scopes usage.

HTTP requests handling

@todo: Router and routes with specific HTTP methods, preflight, required headers

HTTP responses format

@todo: Describe the JSON structure convention of success and error responses

Used technologies

@todo: PHP7 – how it is used for typehints, Nette framework – how it is used for routing, Presenters actions endpoints, exceptions and ErrorPresenter, Doctrine 2 – database abstraction, entities and repositories + conventions, Communication over ZMQ – describe the problem with the extension and how we reported it and how to treat it in the future when the bug is solved. Relational database – we use MariaDB, Doctine enables us to switch the engine to a different engine if needed

Data model

@todo: Describe the code-first approach using the Doctrine entities, how the entities map onto the database schema (refer to the attached schemas of entities and relational database models), describe the logical grouping of entities and how they are related:

• user + settings + logins + ACL
• instance + licenses + groups + group membership
• exercise + assignments + localized assignments + runtime environments + hardware groups
• submission + solution + reference solution + solution evaluation
• comment threads + comments

API endpoints

@todo: Tell the user about the generated API reference and how the Swagger UI can be used to access the API directly.

Web Application

@todo: What is the purpose of the web application and how it interacts with the REST API.

Used technologies

@todo: Briefly introduce the used technologies like React, Redux and the build process. For further details refer to the GitHub wiki

How to use the application

@todo: Describe the user documentation and the FAQ page.

Backend-Frontend communication protocol

@todo: describe the exact methods and respective commands for the communication

Initiation of a job evaluation

@todo: How does the Frontend initiate the evaluation and how the Backend can accept it or decline it

Job processing progress monitoring

When evaluating a job the worker sends progress messages on predefined points of evaluation chain. The sending place can be on very beginning of the job, when submit archive is downloaded or at the end of each simple task with its state (completed, failed, skipped). These messages are sent to broker through existing ZeroMQ connection. Detailed format of messages can be found on communication page.

Broker only resends received progress messages to the monitor component via ZeroMQ socket. The output message format is the same as the input format.

Monitor parses received messages to JSON format, which is easy to work with in JavaScript inside web application. All messages are cached (one queue per job) and can be obtained multiple times through WebSocket communication channel. The cache is cleared 5 minutes after receiving last message.

Publishing of the results

After job finish the worker packs results directory into single archive and uploads it to the fileserver through HTTP protocol. The target URL is obtained from API in headers on job initiation. Then "job done" notification request is performed to API via broker. Special submissions (reference or asynchronous submissions) are loaded immediately, other types are loaded on-demand on first results request.

Loading results means fetching archive from fileserver, parsing the main YAML file generated by worker and saving data to the database. Also, points are assigned by score calculator.

User documentation

Web Application

@todo: Describe different scenarios of the usage of the Web App

Terminology

@todo: Describe the terminology: Instance, User, Group, Student, Supervisor, Admin

Web application requirements

@todo: Describe the requirements of running the web application (modern web browser, enabled CSS, JavaScript, Cookies & Local storage)

Scenario #1: Becoming a user of ReCodEx

How to create a user account?

You can create an account if you click on the “Create account” menu item in the left sidebar. You can choose between two types of registration methods – by creating a local account with a specific password, or pairing your new account with an existing CAS UK account.

If you decide a new “local” account using the “Create ReCodEx account” form, you will have to provide your details and choose a password for your account. You will later sign in using your email address as your username and the password you select.

If you decide to use the CAS UK, then we will verify your credentials and access your name and email stored in the system and create your account based on this information. You can change your personal information or email later on the “Settings” page.

When crating your account both ways, you must select an instance your account will belong to by default. The instance you will select will be most likely your university or other organization you are a member of.

How to get into ReCodEx?

To log in, go to the homepage of ReCodEx and in the left sidebar choose the menu item “Sign in”. Then you must enter your credentials into one of the two forms – if you selected a password during registration, then you should sign with your email and password in the first form called “Sign into ReCodEx”. If you registered using the Charles University Authentication Service (CAS), you should put your student’s number and your CAS password into the second form called “Sign into ReCodEx using CAS UK”.

How do I sign out of ReCodEx?

If you don’t use ReCodEx for a whole day, you will be logged out automatically. However, we recommend you sign out of the application after you finished your interaction with it. The logout button is placed in the top section of the left sidebar right under your name. You will have to expand the sidebar with a button next to the “ReCodEx” title (shown in the picture below).

@todo: Simon's image

What to do when you cannot remember your password?

If you can’t remember your password and you don’t use CAS UK authentication, then you can reset your password. You will find a link saying “You cannot remember what your password was? Reset your password.” under the sign in form. After you click on this link, you will be asked to submit your email address. An email with a link containing a special token will be sent to the address you fill in. We make sure that the person who requested password resetting is really you. When you click on the link (or you copy & paste it into your web browser) you will be able to select a new password for your account. The token is valid only for a couple of minutes, so do not forget to reset the password as soon as possible, or you will have to request a new link with a valid token.

If you sign in through CAS UK, then please follow the instructions provided by the administrators of the service described on their website.

How to configure your account?

There are several options you have to edit your user account.

• changing your personal information (i.e., name)
• changing your credentials (email and password)
• updating your preferences (e.g., source code viewer/editor settings, default language)

You can access the settings page through the “Settings” button right under your name in the left sidebar.

Scenario #2: User is a student

@todo: describe what it means to be a “student” and what are the student’s rights

How to join a group for my class?

@todo: How to join a specific group

Which assignments do I have to solve?

@todo: Where the student can find the list of the assignment he is expected to solve, what is the first and second deadline.

Where can I see details of my classes’ group?

@todo: Where can the user see groups description and details, what information is available.

How to submit a solution of an assignment?

@todo: How does a student submit his solution through the web app

Where are the results of my solutions?

@todo: When the results are ready and what the results mean and what to do about them, when the user is convinced, that his solution is correct although the results say different

How can I discuss my solution with my teacher/group’s supervisor directly through the web application?

@todo: Describe the comments thread behavior (public/private comments), who else can see the comments, how notifications work (not implemented yet!).

Scenario #3: User is supervisor of a group

@todo: describe what it means to be a “supervisor” of a group and what are the supervisors rights

How do I become a supervisor of a group?

@todo: How does a user become a supervisor of a group?

How to add or remove a student to my group?

@todo: How to add a specific student to a given group

How do I add another supervisor to my group?

@todo: who can add another supervisor, what would be the rights of the second supervisor

How do I create a subgroup of my group?

@todo: What it means to create a subgroup and how to do it.

How do I assign an exercise to my students?

@todo: Describe how to access the database of the exercises and what are the possibilities of assignment setup – availability, deadlines, points, score configuration, limits

How do I configure the limits of an assignment and how to choose appropriate limits?

@todo: Describe the form and explain the concept of reference solutions. How to evaluate the reference solutions for the exercise right now (to get the up-to-date information).

How can I assign some exercises only to some students of the group?

@todo: Describe how to achieve this using subgroups

How can I see my students’ solutions?

@todo Describe where all the students’ solutions for a given assignment can be found, where to look for all solutions of a given student, how to see results of a specific student’s solution’s evaluation result.

Can I assign points to my students’ solutions manually instead of depending on automatic scoring?

@todo If and how to change the score of a solution – assignment settings, setting points, bonus points, accepting a solution (not implemented yet!). Describe how the student and supervisor will still be able to see the percentage received from the automatic scoring, but the awarded points will be overridden.

How can I discuss student’s solution with him/her directly through the web application?

@todo: Describe the comments thread behavior (public/private comments), who else can see the comments -- same as from the student perspective

Writing job configuration

To run and evaluate an exercise the backend needs to know the steps how to do that. This is different for each environment (operation system, programming language, etc.), so each of the environments needs to have separate configuration.

Backend works with a powerful, but quite low level description of simple connected tasks written in YAML syntax. More about the syntax and general task overview can be found on separate page. One of the planned features was user friendly configuration editor, but due to tight deadline and team composition it did not make it to the first release. However, writing configuration in the basic format will be always available and allows users to use the full expressive power of the system.

This section walks through creation of job configuration for hello world exercise. The goal is to compile file source.c and check if it prints Hello World! to the standard output. This is the only test case, let's call it A.

The problem can be split into several tasks:

• compile source.c into helloworld with /usr/bin/gcc
• run helloworld and save standard output into out.txt
• fetch predefined output (suppose it is already uploaded to fileserver) with hash a0b65939670bc2c010f4d5d6a0b3e4e4590fb92b to reference.txt
• compare out.txt and reference.txt by /usr/bin/diff

The absolute path of tools can be obtained from system administrator. However, /usr/bin/gcc is location, where the GCC binary is available almost everywhere, so location of some tools can be (professionally) guessed.

First, write header of the job to the configuration file.

submission:
    job-id: hello-word-job
    hw-groups:
        - group1

Basically it means, that the job hello-world-job needs to be run on workers that belong to the group_1 hardware group . Reference files are downloaded from the default location configured in API (such as http://localhost:9999/exercises) if not stated explicitly otherwise. Job execution log will not be saved to result archive.

Next the tasks have to be constructed under tasks section. In this demo job, every task depends only on previous one. The first task has input file source.c (if submitted by user) already available in working directory, so just call the GCC. Compilation is run in sandbox as any other external program and should have relaxed time and memory limits. In this scenario, worker defaults are used. If compilation fails, the whole job is immediately terminated (because the fatal-failure bit is set). Because bound-directories option in sandbox limits section is mostly shared between all tasks, it can be set in worker configuration instead of job configuration (suppose this for following tasks). For configuration of workers please contact your administrator.

- task-id: "compilation"
  type: "initiation"
  fatal-failure: true
  cmd:
      bin: "/usr/bin/gcc"
      args:
          - "source.c"
          - "-o"
          - "helloworld"
  sandbox:
      name: "isolate"
      limits:
          - hw-group-id: group1
            chdir: ${EVAL_DIR}
            bound-directories:
                - src: ${SOURCE_DIR}
                  dst: ${EVAL_DIR}
                  mode: RW

The compiled program is executed with time and memory limit set and the standard output is redirected to a file. This task depends on compilation task, because the program cannot be executed without being compiled first. It is important to mark this task with execution type, so exceeded limits will be reported in frontend.

Time and memory limits set directly for a task have higher priority than worker defaults. One important constraint is, that these limits cannot exceed limits set by workers. Worker defaults are present as a safety measure so that a malformed job configuration cannot block the worker forever. Worker default limits should be reasonably high, like a gigabyte of memory and several hours of execution time. For exact numbers please contact your administrator.

It is important to know that if the output of a program (both standard and error) is redirected to a file, the sandbox disk quotas apply to that file, as well as the files created directly by the program. In case the outputs are ignored, they are redirected to /dev/null, which means there is no limit on the output length (as long as the printing fits in the time limit).

- task-id: "execution_1"
  test-id: "A"
  type: "execution"
  dependencies:
      - compilation
  cmd:
      bin: "helloworld"
  sandbox:
      name: "isolate"
      stdout: ${EVAL_DIR}/out.txt
      limits:
          - hw-group-id: group1
            chdir: ${EVAL_DIR}
            time: 0.5
            memory: 8192

Fetch sample solution from file server. Base URL of file server is in the header of the job configuration, so only the name of required file (its sha1sum in our case) is necessary.

- task-id: "fetch_solution_1"
  test-id: "A"
  dependencies:
      - execution
  cmd:
      bin: "fetch"
      args:
          - "a0b65939670bc2c010f4d5d6a0b3e4e4590fb92b"
          - "${SOURCE_DIR}/reference.txt"

Comparison of results is quite straightforward. It is important to set the task type to evaluation, so that the return code is set to 0 if the program is correct and 1 otherwise. We do not set our own limits, so the default limits are used.

- task-id: "judge_1"
  test-id: "A"
  type: "evaluation"
  dependencies:
      - fetch_solution_1
  cmd:
      bin: "/usr/bin/diff"
      args:
          - "out.txt"
          - "reference.txt"
  sandbox:
      name: "isolate"
      limits:
          - hw-group-id: group1
            chdir: ${EVAL_DIR}